Northeast Fisheries Science Center Reference Document 02-02
The
2001 Assessment of the Gulf of Maine
Atlantic Cod Stock
A Report of the 33rd Northeast Regional Stock Assessment
Workshop (33rd SAW)
by R.K. Mayo, E.M. Thunberg, S.E. Wigley, and S.X. Cadrin
NOAA Fisheries NEFSC, 166 Water Street, Woods Hole MA 02543
Print
publication date March 2002;
web version posted March 25, 2002
Citation: Mayo, R.K.; Thunberg, E.M.; Wigley, S.E; Cadrin, S.X. 2002. The 2001 assessment of the Gulf of Maine Atlantic cod
stock. Northeast Fish. Sci. Cent. Ref. Doc. 02-02; 154 p.
Download complete PDF/print version
Abstract
The status of the Gulf of Maine cod (Gadus morhua)
stock is reviewed, and terminal year VPA estimates of 2000 fishing
mortality and spawning stock biomass and the survivors in 2001 are
presented. Precision estimates of the 2000 fishing mortality and
spawning stock biomass estimates for Gulf of Maine cod are also provided.
Short-term projections of 2002 catches and resulting 2003 spawning
stock biomass at various levels of 2002 fishing mortality are also
given. Long-term (25-yr) projections were conducted to evaluate relative
trajectories of stock biomass and catch under various fishing mortality
scenarios, and an age-structured production model was applied to
estimate MSY-based reference points.
The 2001 assessment is based on several sources of
information including: the age composition of USA commercial and
recreational landings, commercial fishing vessel trip reports (VTR),
Northeast Fisheries Science Center (NEFSC) sea sample data, MRFSS
estimates of recreational harvest, NEFSC and Massachusetts Division
of Marine Fisheries (DMF) spring and autumn research vessel survey
data, and standardized USA commercial fishing effort data. This assessment
updates the analyses presented in the 1998 assessment of the Gulf
of Maine cod stock (NEFSC 1998, Mayo et al. 1998) as well as those
prepared in 1999 and 2000 by the Northern Demersal Working Group
(NEFSC 2000, 2001).
Total landings of Gulf of Maine cod equaled
4,156 metric tons (mt) in 1998, declined to 1,636 mt in 1999, and increased
to 3,730 mt in 2000. The sharp decline in landings between 1998 and
1999 and the subsequent increase in 2000 likely reflects the imposition
of very low trip limits during 1999 and the subsequent relaxation of
these limits in early 2000. It is probable that the extent of discarding
increased sharply in 1999 in response to these reduced trip limits.
Commercial landings per unit of standardized effort declined
steadily between 1982 and 1987, increased during 1988-1990 but declined
sharply in 1992 and remained low in 1993. Fishery-independent spring
and autumn bottom trawl surveys conducted by the NEFSC have documented
a steady decline in total stock biomass since the 1960s; the largest
decreases occurred during the 1980s. Although the most recent indices
suggest a slight increase, overall, the Gulf of Maine cod stock biomass
remains low relative to the 1960s and 1970s. Except for the 1998 year
class, recent recruitment has been well below average.
Total stock biomass (ages 1+) declined from a maximum
of 41,900 mt in 1990 to 14,800 mt in 1998, but has since increased
to 20,400 mt in 2000. Spawning stock biomass (SSB) declined from over
24,200 mt in 1990 to a low of 9,900 mt in 1998, but increased to 13,100
mt in 2000. Mean biomass for ages 1+ declined from a maximum of 42,700
mt in 1989 to 14,800 mt in 1997 and 1998, but increased sharply between
1999 and 2000 to 25,900 mt, due, in part, to the impact of the 1998
year class. Bmsy is now estimated to be 90,300 mt (total
stock biomass, ages 1+) with a corresponding Fmsy of 0.23,
(fully recruited, ages 4+). With respect to the age-structured MSY-based
reference points, 2001 total stock biomass is slightly above 1/4 Bmsy
and 2000 F is over 3 times Fmsy. Fully recruited fishing mortality
appears to have declined slightly during 1998 - 2000 compared to pre-1998
fishing mortality rates, although F in 2000 (0.73) remained high relative
to fully recruited F reference points (F0.1 = 0.15; Fmsy =
0.23; Fmax = 0.27).
Introduction
Atlantic cod (Gadus morhua) in the Gulf of
Maine region have been commercially exploited since the 17th century,
and reliable landings statistics are available since 1893. Historically,
the Gulf of Maine fishery can be separated into four periods (Figure
1): (1) an early era from 1893-1915 in which record-high
landings (> 17,000 mt) in 1895 and 1906 were followed by about
10 years of sharply-reduced catches; (2) a later period from 1916-1940
in which annual landings were relatively stable, fluctuating between
5,000 and 11,500 mt, and averaging 8,300 mt per year; (3) a period
from 1941-1963 when landings sharply increased (1945: 14,500 mt)
and then rapidly decreased, reaching a record-low of 2,600 mt in
1957; and (4) the most recent period from 1964 onward during
which Gulf of Maine landings have generally increased but have declined
steadily since the early 1990s. Total landings doubled between 1964
and 1968, doubled again between 1968 and 1977, and averaged 12,200
mt per year during 1976-1985. Gulf of Maine cod landings subsequently
increased, reaching 17,800 mt in 1991, the highest level since the
early 1900s. Total landings declined sharply in 1992 to 10,891 mt,
and have since decreased steadily to 1,636 mt in 1999 before increasing
to 3,730 mt in 2000.
This report presents an updated and revised analytical
assessment of the Gulf of Maine cod stock (NAFO Division 5Y) for
the period 1982-2000 based on analyses of commercial, recreational
and and research vessel survey data through 2000. From the early
1960s through 1993, information on the catch quantity by market category
was derived from reports of landings transactions submitted voluntarily
by processors and dealers. More detailed data on fishing effort and
location of fishing activity were obtained for a subset of trips
from personal interviews of fishing captains conducted by port agents
in the major ports of the Northeast. Information acquired during
the course of these interviews was used to augment the total catch
information obtained from the dealer. Procedures for collecting and
processing commercial fishery data in the Northeast were revised
after 1993.
Beginning in 1994, data on number of hauls, average
haul time, and catch locale were obtained from logbooks submitted
to National Marine Fisheries Service (NMFS) by operators fishing
for groundfish in the Northeast under a mandatory reporting program.
Estimates of total catch by species and market category were derived
from mandatory dealer reports submitted on a trip basis to NMFS.
Catches (landed and discarded portions) by market category were allocated
to stock based on a matched subset of trips between the dealer and
logbook databases. Data in both databases were stratified by calendar
quarter, port group, and gear group to form a pool of observations
from which proportions of catch by stock could be allocated to market
category within the matched subset. The cross-products of the market
category by stock proportions derived from the matched subset were
employed to compute the total catch by stock, market category, calendar
quarter, port group, and gear group in the full dealer database.
A full description of the proration methodology and an evaluation
of the 1994-1996 logbook data is given in Wigley et al. (1998)
and DeLong et al. (MS 1997).
An initial analytical assessment of this stock was
presented at the Seventh NEFC Stock Assessment Workshop in November 1988
(NEFC 1989) and subsequent assessments were reviewed at the 12th,
15th, 19th, 24th and 27th Northeast
Regional Stock Assessment Workshops in June 1991, December 1992,
December 1994, June 1997 and June 1998 (NEFSC 1991, 1993, 1995, 1997,
1998; Mayo 1995, 1998; Mayo et al. 1993, 1998). Interim
assessments were reviewed by the Northern Demersal Working Group
in July 1999 (NEFSC 2000) and August 2000 (NEFSC 2001a). The present
assessment was reviewed at the 33rd Northeast Regional
Stock Assessment Workshop in June 2001 (NEFSC 2001b).
THE FISHERY
Management History
Fishing for Gulf of Maine cod had been managed under
international treaty prior to 1977 and by domestic management authority
since 1977 (Table 1). Annual Total Allowable
Catches (TACs) were first established under the International Commission
for the Northwest Atlantic Fisheries (ICNAF) for Division 5Y (i.e.,
the Gulf of Maine) cod in 1973. The TAC remained at 10,000 mt from
1973-1975; the 1976 TAC was reduced to 8,000 mt and the TAC proposed
for 1977 was reduced further to 5,000 mt.
Following implementation of the Magnuson Fishery Conservation
and Management Act (FCMA) in 1977, management of this stock fell
under the auspices of the New England Fishery Management Council.
TACs were carried forward for the first few years under the Fishery
Management Plan for Atlantic Groundfish, and were distributed among
vessel tonnage classes and quarters of the years until 1982 when
the "Interim" Plan for Atlantic groundfish was implemented. This
plan eliminated all direct catch controls (quotas) and established
mesh size and minimum landing size regulations as the primary regulatory
measures for cod, haddock and yellowtail flounder.
Management of the Gulf of Maine cod fishery has been
carried out since 1985 under the Northeast Multi-species Fishery
Management Plan (FMP). This plan and its Amendments 1 through 4 essentially
carried forward the regulatory measures originally implemented in
1982 under the "Interim" Plan. Beginning in 1994, with the implementation
of Amendment 5, the primary goal of the FMP became a reduction in
fishing mortality for 5 key monitoring stocks. This was to be achieved
through a combination of reductions in days at sea (DAS) usage and,
under Amendment 7, an additional series of seasonal and year-round
area closures oriented primarily towards Gulf of Maine stocks.
Commercial Fishery
Landings
Annual commercial landings data for Gulf of Maine cod
in years prior to 1994 were obtained from trip-level detailed landings
records contained in master data files maintained by the Northeast
Fisheries Science Center, Woods Hole, Massachusetts (1963-1993) and
from summary reports of the Bureau of Commercial Fisheries and its
predecessor the U.S. Fish Commission (1895-1962). Beginning in 1994,
landings estimates were derived from dealer reports prorated to stock
based on the distribution of reported landed catch contained in fishing
vessel logbooks as described above.
Total commercial landings in 2000 were 3,730 mt, approximately
130% greater than in 1999 but 10% less than in 1998 (Table 2, Figure
1). Since 1977, the USA fishery has accounted for all of the
commercial catch. Canadian landings reported as Gulf of Maine catch
during 1977-1990 are believed by Canadian scientists to be misreported
catches from the Scotian Shelf stock (Campana and Simon 1985; Campana
and Hamel 1990). Although otter trawl catches account for most of
the landings (54% by weight in 2000), the otter trawl percentage
has declined considerably compared to the period prior to 1993. Most
of this change can be attributed to an increase in the percentage
of cod taken by sink gillnets since 1993, although the percentage
from combined handline and line trawls also increased substantially
during the 1990s (Table 3).
Commercial Fishery
Discards
Discard rates have been routinely calculated for Gulf
of Maine cod by quarter and gear from NEFSC sea sampling data collected
since 1989 (Table 4). Discard and kept components
of the catch were summed for all observed tows, within each gear
type, occurring in Division 5Y, and the ratio of the discarded- to-kept
quantity was applied to landings for the corresponding quarter and
gear type within each year. Data were available for otter trawls,
shrimp trawls, and sink gillnets.
For otter trawl gear, discard-to-kept ratios (D/K)
and absolute quantities of discarded cod declined from relatively
high values in 1989 and 1990 to relatively low levels from 1991 through1998
as D/K ratios generally fluctuated between 0.002 and 0.155. In the
shrimp trawl fishery, D/K ratios remained high throughout 1989-1991,
but declined substantially in 1992 and remained negligible in 1993.
Sea sampling data for 1994-2000 were minimal; therefore, landings
by this gear component were not distinguished from all other otter
trawls in the proration scheme employed to derive the landings by
stock for the present assessment. Consequently, discard estimates
from both otter trawl and shrimp trawl gear were combined for the
1994-2000 period. D/K ratios from the sink gill net fishery remained
relatively low between 1989 and 1998, generally in the range of 0.05
or so. In 1999, discard ratios increased sharply for otter trawl
and sink gill nets during the second and third quarters, declined
from these peak levels in the fourth quarter, but continued to remain
relatively high through all of 2000 compared to pre-1999 ratios.
Discards of Gulf of Maine cod ranged from 139 mt in
1998 to 3,598 mt in 1990 (Table 4). Discards exceeded 1,000 mt in
each year between 1989 and 1991 before declining steadily since 1992.
The relatively high discard rates calculated for otter trawl and
shrimp trawl gear during 1989-1991 coincide with recruitment of the
strong 1987 year class to the small mesh shrimp trawl gear and then
the large mesh general otter trawl gear. Available length composition
data for these gear types suggest that most of the discarded cod
were about 30-50 cm with a mode around 40 cm. Discards emanating
from these two gears are the likely result of minimum size regulations.
In contrast, the relatively low, but persistent, discards of cod
in the gillnet fishery comprised fish of all lengths, up to 125 cm.
The larger size range reflects discarding resulting from minimum
size regulations as well as poor fish quality (in the case of the
larger, marketable cod). Discards in 1999 were estimated to be 2,630
mt, one of the highest in the data series, due to the imposition
of low trip limits. Estimated discards declined to 1,170 mt in 2000
as trip limits were relaxed to 400 lbs/day in early 2000.
To further evaluate discarding in 1999 and 2000 when
low trip limits were imposed, all available vessel trip report (VTR)
records were examined from trips fishing in the Gulf of Maine and
reporting some catch of cod. All trips from vessels which never reported
any discard were excluded from the discard analyses. The VTR data
were treated in the same manner as the sea sample data except that
the discard-to-kept ratios and subsequent estimates of absolute discard
were derived on a monthly basis rather than a quarterly basis. This
increased temporal resolution, available due to the greater quantity
of VTR records, afforded a means of comparing the seasonal progression
of discarding with the evolution of trip limits in 1999 and 2000.
Analysis of the VTR data (Figure 2) generally
confirms the seasonal patterns as well as the magnitude of the discard
estimates derived from the sea sample data in 1999 and 2000 (Appendix
1: Figures 1-3). The estimated total discards of Gulf of Maine
cod derived from the monthly VTR discard-to-kept ratios equaled 2,822
mt in 1999 (Table 5a) and 2,246 mt in 2000
(Table 5b).
A third approach to estimating the magnitude of 1999
and 2000 discards of Gulf of Maine cod was based on a predictive
model by imposing 1999 and 2000 trip limits on 1996 and 1997 VTR
data at the appropriate times of the year. Given the manner in which
fishery conditions change from year to year (number of trips taken
and catch rates) as well as regulatory changes over time, the primary
objective was to estimate a discard-to-kept ratio rather than a direct
estimate of discards. The resulting discard-to-kept ratios were then
applied to observed 1999 and 2000 calendar year Gulf of Maine cod
landings to provide an estimate of total discards in those years.
The predictive model incorporated information about
total trip income and fishing costs, including operating costs and
payments to labor, to determine which trips may no longer be profitable
as a result of the trip limit. Trips that were no longer profitable
were assumed to be abandoned while the remaining trips were assumed
to occur while incurring discards of all cod in excess of the trip
limit. That is, if the cod value (Pcod*Qcod)
plus income earned from all component catch (PiQi) exceeds the cost
of paying crew (Ccrew) plus operating the vessel (Coperating):
| (1) |
 |
the trip was assumed to be taken as observed. Otherwise
the trip was assumed to be abandoned. Given that prices and landings
are generally known, the economic relationship described in (1) will
be sensitive to assumptions about crew and operating costs. Estimated
operating costs for principal gear types (otter trawl, gillnet, and
hook) were based on cost surveys (Georgianna and Cass 1998, Lallemand et
al. 1998, Lallemand et al. 1999). Since payments to
crew are based on a share system, crew income will be affected by
trip limits. Thus, some minimum return to crew was assumed to be
required to enable a vessel to make a trip.
The minimum crew payment was estimated using two different
methods; a minimum share and a minimum payment. The minimum share
method is consistent with the manner in which crew are remunerated
which reflects some risk sharing between the crew and owner but could
result in unrealistically low residual payments to labor. By contrast,
the minimum payment approach provides an income floor below which
the vessel owner may be assumed to be unable to recruit crew because
they could earn more income by taking a job elsewhere. This income
floor was assumed to be equal to the average wage rate for blue-collar
occupations in New England ($13 per hour). Three sensitivity trials
were used for the minimum share (50%, 25%, and 10%) and one minimum
payment trial ($13 per hour x 8 hours or $104 per crew per day) was
conducted to test the sensitivity of the discard-to-kept ratios to
crew payment assumptions.
The predictive model was applied to VTR records for
calendar years 1996 and 1997 to infer what landings and discards
would have been had the trip limits been implemented in those calendar
years. Since these data come from observed trips the trip limit model
provides an estimate of landings and total discards (discards due
to the trip limit plus recorded VTR discards for other reasons).
The 1996 and 1997 calendar years were selected for analysis because
they represent a time period over which the Gulf of Maine cod fishery
was least affected by trip limits (there were no trip limits in 1996
and the trip limits for 1997 were not binding on most occasions).
By contrast, the 1998 trip limits, as well as the rolling closures,
make use of data from that calendar year problematic.
The trip limit model was run separately for each of
the 1996 and 1997 calendar year data and the four different sensitivity
runs yielding 8 estimates for each of the 1999 and 2000 discard-to-keep
ratios (Table 6a). Note that as the assumed
payment necessary to attract labor to the fishery declines, formerly
marginal trips become profitable resulting in higher estimated landings
and discarding hence the increasing discard-to-kept ratios. Overall,
the minimum payment trial results in an intermediate discard-to-kept
estimate. The estimated Gulf of Maine cod discard-to-kept ratios
ranged from 1.80 to 2.47 with a median value of 2.15 for calendar
year 1999. Due to higher trip limits, the discard-to-kept ratios
ranged between 0.72 and 0.99 with a median value of 0.83 for calendar
year 2000. Applying the estimated discard-to-kept ratios to the observed
landings results in a median estimate of 3,524 metric tons of discards
of Gulf of Maine cod in 1999. Similarly, the median estimate of calendar
year 2000 Gulf of Maine cod discards was 3,081 metric tons (Table
6b).
The estimates of discard of Gulf of Maine cod derived
by each of the 3 methods are reasonably close to each other, within
the range of 2,600-3,500 mt for 1999 and 1,200-3,100 mt for 2000.
Each method has advantages and limitations. The sea sample data are
less subjective since they are based on consistent interpretation
by a small group of individuals. But these data are rather sparse,
leading to considerable imprecision. The 1999 VTR data provide considerably
more observations, which may increase precision, but these data may
have been influenced by possible reporting bias in response to severe
management actions in 1999. The third method uses VTR data from years
prior to the imposition of severe trip limits, and presumably is
less affected by reporting bias. However, this method relies on several
assumptions regarding constancy of effort and catch rates.
While there is, at present, no objective basis to select
one method over any other, all 3 suggest that minimum estimates of
total discards were in the range of approximately 2,500 mt in 1999
and 1,000 mt in 2000. When these discards are added to the reported
landings, the resulting total commercial catch is estimated to be
4,136 mt in 1999 (1,636 mt + 2,500 mt) and 4,730 mt in 2000 (3,730
mt + 1,000 mt). These results provide expansion factors of 2.53 in
1999 (4,136 mt/1,636 mt) and 1.27 in 2000 (4,730 mt/3,730 mt) to
convert commercial landings to commercial catch.
Commercial Fishery
Sampling Intensity
A summary of USA length frequency and age sampling
of Gulf of Maine cod landings during 1982-2000 is presented in Table
7. USA length frequency sampling averaged one sample per 155-200
mt landed during 1983-1987 but the sampling intensity was reduced
in 1990 (1 sample per 387 mt) and 1993 (1 sample per 360 mt),
and the absolute level of sampling was extremely low in 1993. Overall
sampling improved slightly in 1994 and 1995, but the seasonal distribution
was uneven and poorly matched to the landings. Sampling improved
substantially in 1996 and remained equally high in 1997, reaching
all-time highs in terms of both absolute number of samples and samples
per ton landed in both years.
Most of the USA samples have been taken from otter
trawl landings, but sampling and the estimation of length composition
is stratified by market category (scrod, market, and large). Although
the length composition of cod differs among gear types (primarily
between otter trawl and gillnet), the length composition of cod landings
within each market category is virtually identical among gear types.
Beginning in 1998, the quality of commercial port sampling
for Gulf of Maine cod has declined considerably. The total number
of samples taken declined sharply in 1998 and again in 1999, a possible
outcome of the very low trip limits imposed in 1999. Although the
number of samples collected increased in 2000, the distribution by
market category has been out of phase with actual landings. In particular,
the number of 'Large' market category cod samples has diminished
to the point that the representation of the older age groups may
be somewhat compromised in recent years.
Of the 61 samples collected in 2000, 24 were scrod
samples (39%), 36 were market (59%), and 1 was large (2%). Compared
with the 2000 market category landings distribution by weight (scrod:
9%; market: 59%; large: 30%) (Table 8),
sampling in 2000 over-represented the scrod category and severely
under-represented the large category.
As well, the seasonal distribution of samples has become
skewed such that, although there appears to have been sufficient
numbers of samples taken, there has been insufficient sampling in
some quarters and half-years, requiring pooling of samples on an
annual basis. This approach was necessary in 1999 and 2000.
Commercial Landings
Age Composition
The age composition of landings during 1982-1993 was
estimated, by market category, from monthly length frequency and
age samples, pooled by calendar quarter. Quarterly mean weights,
by market category, were obtained by applying the NEFSC research
vessel survey length-weight equation for cod:
ln Weight (kg,live) =
-11.7231 + 3.0521 ln Length (cm)
to the quarterly market category sample length frequencies.
Computed mean weights were then divided into quarterly market category
landed weight to derive estimated numbers landed by quarter, by market
category. Quarterly age/length keys were applied to the quarterly
market category numbers at length distributions to provide numbers
at age. These results were summed over market categories and quarters
to derive the annual landings-at-age matrix (Table
9a).
Age composition of landings from 1994 through 2000
was estimated in a manner similar to that employed for the 1982-1993
estimates except that samples and landings were, at times, pooled
to semi-annual or annual resolution because of the uneven distribution
of length and age samples by quarter (Table
7). Semi-annual pooling was required for the 1st and 2nd quarters
of 1994 because of incomplete sampling coverage of scrod and large
cod landings; in 1995, samples were pooled in both semi-annual periods
due to the absence of large cod samples and the sparse coverage of
market cod in quarters 1 and 3. Quarterly allocation of samples to
landings was achieved for all market categories in 1996 and 1997,
but semi-annual and annual pooling was required in 1998 and annual
pooling was required in 1999 and 2000.
Biological sampling in 2000 was especially problematic
for 'Large' category cod. As only one sample was taken throughout
the year, the entire representation of older age groups depended
on this sample with a maximum length at just over 100 cm. To achieve
greater representation of larger fish, the 'Large' category commercial
port sample was augmented with length measurements of > 100 cm
cod obtained from Gulf of Maine sea sample trips. The resulting 2000
age compositions obtained from the original and the augmented length
data are presented in Table 9a and Table 10a.
It was the consensus of the SARC that the 2000 age composition based
on the original port sample data be used for further analyses.
Gulf of Maine cod landings have been generally dominated
by age 3 and 4 fish in numbers and by ages 3, 4, and 5 in weight.
Cod from the strong 1987 year class predominated from 1990 through
1992 but, by 1993, fish from the 1990 year class accounted for the
greatest proportion of the total number landed. In terms of weight,
the 1993 landings were equally distributed between the 1987 and 1990
year classes. In 1993 these two year classes accounted for approximately
70% of the total number and weight landed. From 1994 through 1996,
landings were dominated by age 4 cod in both number and weight. In
1997 age 5 fish were dominant in terms of both number and weight,
reflecting the higher abundance of the 1992 year class. Although
traditionally low in terms of their contribution to the total landings,
age 10 and 11+ fish were completely absent in 1993 and 1996, and
numbers of age 8 and 9 fish have also been unusually low (Table
9a). Although this pattern may be partly a result of the poor
sampling of 'Large' category cod, especially in recent years, a trend
towards fewer older fish in the landings has been apparent since
1991. As well, the contribution of age 2 fish to the landings has
decreased in recent years.
Adjustment of the 1999
and 2000 Commercial Landings at Age
The fishery for Gulf of Maine cod was affected by management
actions which occurred in 1999 and have continued into 2000. Primarily,
the imposition of extremely low trip limits in 1999 are likely to
have precipitated a substantial increase in the amount of cod discarded
compared to previous years, as noted above. Consequently, the 1999
and 2000 estimated commercial landings at age presented in Tables
9 and 10 do not reflect the full extent
of removals from the stock by the fishery. Therefore, prior to inclusion
in the VPA, the 1999 and 2000 landings estimates must be adjusted
upwards at each age by the ratio of total estimated catch biomass
(landings+discard) to the landed catch biomass.
This approach assumes that the age composition of the
discarded component of the catch is the same as the landed component.
In most discarding cases, where discards generally occur in response
to mesh selectivity which is out of phase with minimum landing size
regulations, it is necessary to estimate the size and age composition
of the discarded component separate from the landed component. In
general, the discards comprise the smaller, younger fish compared
to those that are landed. However, in this case , where regulatory
discards were generated as a result of extremely low trip limits,
it is presumed that cod of all sizes and ages were discarded without
prejudice. An examination of the 1998, 1999 and 2000 kept and discarded
length composition samples from the NEFSC Sea Sample database supports
this assumption. The sizes of discarded cod in 1998, when trip limits
were considerably higher, were primarily below the 48 cm minimum
landing size and the sizes of retained cod were approximately the
same as those observed in the commercial port samples. In 1999 and
2000, however, the sizes of discarded and retained cod were generally
the same, well above the minimum landing size and similar to those
observed in the 1999 commercial port samples. Therefore the 1999
and 2000 commercial landings at age estimates from Table
10 were multiplied by discard adjustment factors of 2.53 and
1.27, respectively, before inclusion in the VPA catch at age matrix
(see page 5).
Commercial Landings
Mean Weights at Age
Mean weights at age in the catch for ages 1-11+ during
1982-2000 are given in Table 9b and,
based on landings patterns, are considered mid-year values. Mean
weights of age 2 and 3 cod have risen since about 1992, reflecting
decreased partial recruitment of younger fish to the fishery, while
those for intermediate aged fish have fluctuated without any particular
trend. Mean weights for ages 9 and older fluctuate considerably and
are particularly sensitive to sampling variability. Thus, it is unlikely
that the apparent increases in mean weight at age for ages 10 and
11+ since the late 1980s would indicate a shift in growth or an increase
in older fish in the plus group.
In 1990, mean weights at age for ages 2 and 4 were
the lowest in the 9-year time series, while mean weights for ages
6, 7, and 9 were among the highest. These changes, however, may be
artifacts of low sampling levels in 1990. Mean weights at ages 8
and 9 in 1993 and at ages 5 and 6 in 1995 were the highest in the
series, but these anomalies are also the likely result of poor sampling.
However, the generally higher mean weights at ages 2 through 4 since
1996 may be related to the required use of 152 mm (6 in.) mesh in
the otter trawl fishery. Catch at age and recalculated mean weights
at age for the 7+ group which are used in the VPA are given in Tables
10a and 10b.
Recreational Fishery
Catches
Estimates of the recreational cod catch were derived
from the Marine Recreational Fishery Statistics Survey (MRFSS) conducted
annually since 1979. The Gulf of Maine cod catch was estimated assuming
that catches of cod recorded by that portion of the intercept survey
were removed from the ocean in statistical areas adjacent to the
state or county of landing. The MRFSS database has been recently
revised, resulting in adjusted catch estimates for the years 1981
through 1997. Estimates of the total Gulf of Maine cod recreational
catch as well as the portion of the catch excluding those caught
and released through 2000 are provided in Table
11. Information on the catch prior to 1981, which has not been
revised, is included in Table 11 to provide
a longer-term perspective. Further information on the details of
the allocation scheme and sampling intensity are given in NEFSC (1992).
The quantity of cod retained generally exceeded 75%
of the total recreational catch from 1979 through 1991, but has averaged
less than 50% since 1993. The estimated total cod catch (including
those caught and released) declined from over 5,000 mt in 1980 and
1981 to less than 2,000 mt between 1983 and 1986, increased to over
3,500 mt in 1990 and 1991, then fluctuated between 1,100 and 2,600
mt between 1992 and 1996 before declining sharply to 671 mt in 1997.
The total catch has since increased to 2,853 mt in 2000 of which
1,147 mt was retained. The proportion of the total landings (commercial
and recreational) taken by the recreational sector increased to 34
and 24 percent in 1999 and 2000, respectively. The reported total
catch and retained cod from party/charter vessel VTR reports is also
provided in Table 11 since 1995.
Recreational Fishery
Sampling Intensity
Information on the length frequency sampling levels
of Gulf of Maine cod taken in the recreational fishery is provided
in Table 11. An examination of the available
length frequency sampling coverage was conducted to evaluate the
potential utility of these data in estimating the overall length
composition of the recreational removals from the stock. Overall,
sampling for cod taken by recreational gear is poor, averaging less
than 1 sample per 1,000 mt removed (Table 11).
Sampling of the recreational fishery improved in 1994-1996, but has
been relatively low in recent years. The age composition of the 1982-1996
recreational landings was derived for the 1997 assessment (Mayo 1998)
but, given the highly variable sampling, these data were not formally
included in the VPA conducted in 1997 (NEFSC 1997; Mayo 1998). However,
given the recent increase in the proportion of the total landings
accounted by the retained recreational catch, the 1997-2000 age composition
of the recreational landings was estimated for the current assessment
and the 1982-2000 estimates were incorporated into the total catch
at age.
Recreational Fishery
Landings Age Composition
Given the limited sampling coverage in this sector
of the fishery, estimation of numbers caught by length and age required
that samples be pooled on an annual basis. The low inter-seasonal
variability displayed by the sample length composition data supports
this approach. Differences between the party/charter and private/rental
fishing modes are also minimal. Therefore, estimates of the age composition
of cod retained by the recreational sector were derived from the
length composition data applied to the retained numbers of cod based
on pooled annual length frequency samples from Gulf of Maine trips.
Only the retained numbers of cod were included because the intercept
sampling may not accurately reflect the size composition of the released
cod. Age-length keys obtained from sampling the commercial landings,
augmented by age samples from NEFSC bottom trawl surveys for cod
less than 40 cm, were applied to the numbers retained at length on
an annual basis to derive the numbers retained at age (Table
12a).
During the 1980s, Gulf of Maine cod recreational landings
in numbers were dominated by age 3 fish with age 2 fish next in importance.
Following the increases in minimum retention size in 1989 and again
in 1996, the proportion of age 2 cod declined, and the age composition
of the landings from this sector now resembles that from the commercial
fishery with ages 3, 4 and 5 predominant (Tables
10a and 12a). The strong 1987 year
class dominated the recreational catch in 1990, 1991 and 1992, and
the 1992 year class can also be tracked in the estimated catch at
age between 1995 and 1999. Ages 3 and 4 cod generally predominate
in terms of weight caught, although the 1987 and 1992 year classes
predominated at age 5 in 1992 and 1997, respectively.
Recreational Landings
Mean Weights at Age
Mean weights at age were obtained by applying the NEFSC
research vessel survey length-weight equation for cod to the numbers
retained at age on an annual basis:
ln Weight (kg,live) =
-11.7231 + 3.0521 ln Length (cm)
Mean lengths and weights at age of cod landed by the
recreational sector (Table 12b) are
consistently lower than those taken in the commercial fishery. This
pattern persists through age 5, but for ages 6 and older, mean weights
are highly variable due to the relatively poor sampling of fish at
the larger sizes combined with the lack of market category stratification.
Despite this variability, patterns present in the commercial landings
mean weights are also evident in the recreational landings, e.g.,
low mean weights in 1990 and higher mean weights at age 2 in 1995
and 1996.
Total Landings Age
Composition
Estimates of the age composition of total cod landings
(Table 13a) were derived by combining the
separate age composition estimates obtained for the commercial (Table
10a) and recreational sectors (Table 12a).
Given the general similarities between the age compositions estimated
for the commercial and recreational sectors, the total age composition
reflects the same dominant year classes and age structure over time.
In general, ages 3, 4 and 5 have predominated; the 1987 year class
dominated the total landings in 1990, 1991 and 1992, and the 1992
year class can also be tracked between 1995 and 1999.
Total Landings Mean
Weights at Age
Mean lengths and weights at age of cod landed by the
combined commercial and recreational sectors (Table
13b) are intermediate to those obtained from the individual sectors.
Mean weights at age are highly variable for the older ages due to
the relatively poor sampling of fish at the larger sizes. Mean weights
at age for calculating stock biomass at the beginning of the year
are provided in Table 14. These values
were derived from the landings mean weight at age data (Tables
9b and 13b) using procedures described
by Rivard (1982).
STOCK ABUNDANCE and
BIOMASS INDICES
Commercial Catch Rates
Trends in commercial landings per unit effort (LPUE)
and fishing effort for the period 1965-1993 and 1994-1996 have been
recently reported by Mayo (1998). Given the uncertainty in reported
fishing effort since 1994, the 1994-1997 LPUE data were not formally
included in the VPA conducted in 1998 (NEFSC 1998; Mayo et al.1998).
Recent management actions, including imposition of trip limits and
rolling closures also make interpretation of 1997-2000 LPUE inconsistent
with previous years. Until effort units are resolved in the commercial
fishery database, no further treatment of the LPUE series after 1993
will be performed. Trends in commercial LPUE through 1996 are illustrated
in Figure 3.
The 1982-1993 age composition of the landings corresponding
to the effort sub-fleet as presented by Mayo et al. (1994)
was used with the updated standardized effort estimates to calculate
a revised LPUE-at-age index. Numbers landed at age were estimated
by applying quarterly commercial age-length keys to quarterly commercial
numbers landed at length by market category. The LPUE-at-age indices
were derived by dividing the estimated numbers landed at age by corresponding
1982 through 1993 standardized fishing effort. Further details regarding
data selection, preparation and estimation procedures are provided
in Mayo et al. (1994).
Research Vessel Survey
Indices
Indices of cod abundance (stratified mean catch per
tow in numbers) and biomass (stratified mean weight per tow in kilograms),
developed from NEFSC and Commonwealth of Massachusetts Division of
Marine Fisheries (MADMF) research vessel bottom trawl survey data,
have been used to monitor changes and assess trends in population
size and recruitment of cod populations off New England. Offshore
(> 27 m) stratified random NEFSC surveys have been conducted annually
in the Gulf of Maine in autumn since 1963 and in spring since 1968.
Inshore areas of the Gulf of Maine (< 27 m) have been sampled
during spring and autumn NEFSC and MADMF inshore bottom trawl surveys
since 1978. For the NEFSC surveys, a "36 Yankee" trawl has been the
standard sampling gear except during spring 1973-1981 when a modified "41
Yankee" trawl was used.
Prior to 1985, BMV oval doors (550 kg) were used in
all NEFSC surveys; since 1985, Portuguese polyvalent doors (450 kg)
have been used. Details on NEFSC survey sampling design and procedures
are provided in Azarovitz (1981) and Clark (1981). The MADMF inshore
bottom trawl sampling program is described in Howe et al.
(1981). No adjustments in the survey catch-per-tow data for cod have
been made for any of the trawl differences, but vessel and door coefficients
have been applied to adjust the stratified means (number and weight
per tow) as described in Table 15. Standardized
catch-per-tow-at-age (number) indices are listed in Appendix 2: Table
2 and catch-per-tow-at-age indices from DMF spring and autumn surveys
are listed in Appendix 2: Table 3.
NEFSC spring and autumn offshore catch per tow indices
for Gulf of Maine cod have generally exhibited similar trends throughout
the survey time series (Table 15, Figure
4). Number-per-tow indices declined during the mid- and late
1960s, but since 1972-1973 have fluctuated as a result of a series
of recruitment pulses. Sharp increases in the number per tow indices
reflect above-average recruitment of the 1971, 1973, 1977-1980, 1983,
and 1985-1987 year classes at ages 1 and 2 (Appendix
2: Table 2, Figure 5). The sequential
dominance of these cohorts at older ages can be discerned from number-per-tow-at-age
values in both spring and autumn NEFSC surveys (Appendix
2: Table 2). The recent increases in the autumn 1994-1995
and spring 1996-1997 biomass indices may be attributed to somatic
growth of fish from the 1992 year class which was the largest within
the recent series of poor year classes.
Spring NEFSC number-per-tow indices have remained relatively
low since 1985, below the 1981-1984 average (Table
15); spring weight-per-tow indices have also remained relatively
low through 1991, but the index increased substantially in 1992,
and remained relatively high in 1993, due to a large contribution
from the 1987 year class (Appendix
2: Table 2). The index declined markedly in 1994, remained
low in 1995, increased moderately in 1996 and remained essentially
unchanged in 1997. Spring weight-per-tow indices have since declined
through 2000 (Figure 4).
Autumn number- and weight-per-tow indices declined
sharply in 1991 to unprecedented lows; weight-per-tow indices continued
to decline to record low levels through 1993 and remained extremely
low through 1998 (Figure 4), but increases
were evident in 1999 and 2000. The increased abundance in 1988 and
1989, resulting from recruitment of the 1986 and 1987 year classes,
became depleted by 1991, resulting in the sharp declines in the overall
index. This reduction, combined with a general paucity of large fish
in the surveys in recent years (Appendix
2: Table 2), resulted in the decline in the weight-per-tow
indices after 1991. The recent increase in the autumn abundance and
biomass indices in 1994 and 1995 reflected recruitment of the 1992
year class, but these indices had already begun to decline by 1996.
Although the autumn biomass indices increased in 1999 and 2000, they
still remain relatively low compared to earlier periods (Figure
4).
Overall, the 1987 year class appears to have been one
of the strongest ever produced; catch-per-tow indices for this cohort
at ages 1-3 in the NEFSC autumn surveys and at ages 0 and
1 in the MADMF autumn inshore surveys were nearly all record-high
values (Appendix 2:
Tables 2 and 3). Based on MADMF and NEFSC survey catch per
tow indices, the 1992 and 1998 year classes appear to have been of
moderate strength; the intervening year classes of Gulf of Maine
cod, particularly the 1993, 1994, 1995, and 1996 year classes have
been well below average (Figures 5 and 6).
Inshore/Offshore Biomass
Comparisons
To examine changes in the distribution of cod biomass
in the Gulf of Maine, the NEFSC autumn survey data were partitioned
into an inshore strata set (strata: 26 and 27; area: 1,734 square
miles) and an offshore strata set (strata: 28-30, 36-40; area: 16,158
square miles). The inshore strata set approximates the area in the
vicinity of Massachusetts Bay up to Jeffreys Ledge which represents
the core area where cod presently occur in greatest concentrations.
When two or more strata sets of unequal area are compared in this
manner, the stratified mean catch per tow indices must be considered
to represent the density of fish (index of number per unit area)
rather than actual abundance or biomass (index of population size).
To compare trends in actual abundance and biomass between
regions, the indices must be weighted by the area of each strata
set. This provides an index of population size within each strata
set which can be directly compared on the same basis by taking account
of the area of the two regions (in this case, the inshore and offshore
strata sets). Trends in the autumn NEFSC survey stratified mean weight-per-tow
indices are illustrated in Figure 7 for
each region and for the combined strata set (as in Figure
4). Stratified mean biomass indices from the inshore Gulf of
Maine are considerably higher (generally between 20 and 60 kg/tow)
than those for the offshore region (generally less than 20 kg/tow),
simply indicating greater densities of cod in the two inshore strata.
When area is taken into account, an opposite pattern is evident (Figure
8).
When compared in this manner, it is more readily apparent
that, while biomass has declined since the 1960s and 1970s in both
the inshore and the offshore regions of the Gulf of Maine, the decline
has been most severe in the offshore region. This trend is also evident
when trends in the proportion of total biomass from each region are
compared (Figure 9). During the 1960s and
1970s, between 70 and 80 percent of the cod biomass in the Gulf of
Maine was distributed in the offshore region. The offshore proportion
began to decline during the early 1980s, culminating in an approximately
50:50 split during the 1990s. Since then, the proportion of cod in
the offshore region appears to have increased slightly.
Concentration Indices
The Lorenz curve is an econometrics method developed
to study the distribution of income among individuals (Lorenz 1905,
Dagum 1985). Thompson (1976) applied the Lorenz curve in a study
of the distribution of fish caught by a population of fishermen (i.e.,
was it true that 90 percent of the fish were caught by 10 percent
of the fishermen?). Myers and Cadigan (1995) applied this method
to northern cod biomass off Newfoundland using 76 strata from a 12
year research survey time series. When the technique is applied to
fish distributions, the Lorenz curve simultaneously takes into account
biomass and area and puts them on a comparable basis. The Lorenz
curve method used by Myers and Cadigan does not fully account for
strata of unequal size. Since the NEFSC survey has a wide range of
strata sizes, Wigley (1996) modified the method to account for strata
of unequal size.
A Lorenz curve is calculated as follows: for a set
of n strata, let xi be the biomass and ai be
the area of stratum i, i=1,2...n, ranked by mean weight per tow.
The Lorenz curve is the polygon joining the points (Ah/An,
Lh/Ln), h=(0,1,2 ... n) where L0 =
0 and Lh = 
hi=1 xi is
the total biomass in the h strata with the lowest biomass, and A0 =
0 and Ah = hi=1 ai is
the total area of the h strata with the lowest biomass. The x-axis
of the Lorenz curve represents the cumulative percentage of area,
while the y-axis depicts the cumulative percentage of biomass. If
fish are evenly distributed among strata the Lorenz curve would be
an identity function. If fish are unevenly distributed (i.e., concentrated)
the Lorenz curve bows downward and to the right . The concentration
index is derived by doubling the area between the identity function
and the Lorenz curve (Dagum 1985).
The Lorenz curve method was applied to NEFSC research
vessel survey data to examined the distribution of cod biomass as
estimated from NEFSC autumn bottom trawl surveys in the Gulf of Maine
region over a 38 year period. Lorenz curves were calculated for each
NEFSC autumn bottom trawl survey between 1963 and 2000. The strata
set used corresponded to that used in the stock assessment, strata
26-30, 36-40. Biomass values used in the analysis were estimates
of minimum swept area biomass (kg) calculated for each stratum in
each year. Cod biomass values were adjusted for differences in fishing
power of the Albatross IV and the Delaware II,
and for differences in the catchability of BMV doors and the polyvalent
doors introduced to the survey in 1985.
Annual Lorenz curve plots (Figure
10) indicate that cod distribution in the Gulf of Maine became
increasingly more evenly distributed between 1963 and the early
1980's, as indicated by the general declining trend in the concentration
indices (Figure 11). However, in the
second half of the time series, the concentration indices generally
increase, indicating that cod biomass has become more concentrated
in recent years. The 1982 concentration index is highly influenced
by a one tow of cod in stratum 26.
Overall, patterns in cod distribution and concentration
are consistent with the notion that, in recent years, the Gulf of
Maine cod population has been primarily distributed in the inner,
western regions of the Gulf of Maine. Thus, a higher proportion of
the stock is now found within a relatively small area compared to
earlier periods. This contraction in the overall distribution of
the stock may have implications on catchability in the fishery.
MORTALITY
Total Mortality Estimates
Pooled estimates of instantaneous total mortality (Z)
were calculated for 7 time periods encompassed by the NEFSC spring
and autumn offshore surveys: 1964-1967, 1968-1976, 1977-1982, 1983-1987,
1988-1992, 1993-1997, and 1998-1999 (Table
16). Total mortality was calculated from NEFSC survey catch per
tow at age data (Appendix 2: Table 2) for fully recruited age groups
(ages 4+) by the loge ratio of the pooled age 3+/age 4+
indices in the autumn surveys, and the pooled age 4+/age 5+ indices
in the spring surveys. For example, the 1983-1987 values were derived
from:
Spring: ln ( age
4+ for 1983-87/ age 5+
for 1984-88)
Autumn: ln ( age 3+ for 1982-86/ age
4+ for 1983-87)
Different age groups were used in the spring and autumn
analyses so that Z could be evaluated over the same year classes
within each time period.
Values of Z derived from the spring surveys are generally
comparable to those calculated from the autumn data. Rather than
selecting one survey series over the other, total mortality was calculated
by taking a geometric mean of the spring and autumn estimates in
each time period. The pooled estimates indicate that total mortality
was relatively low (Z <= 0.50) between 1964 and 1982, but increased
significantly thereafter to approximately 1.0 during 1983-1997, with
an indication of a slight decline after 1997.
Estimates of total mortality were also derived on an
annual basis from the spring and autumn survey data (Figure
12). These values of Z exhibit considerable inter-annual variability
due primarily to year effects in the surveys. When smoothed, however,
the annual estimates suggest the same pattern of increasing mortality
during the1980s as indicated by the pooled analysis presented in
Table 16.
Natural Mortality
Instantaneous natural mortality (M) for Gulf of Maine
cod is assumed to be 0.20, the conventional value of M used for all
Northwest Atlantic cod stocks (Paloheimo and Koehler 1968, Pinhorn
1975, Minet 1978).
ESTIMATION of FISHING
MORTALITY RATES and STOCK SIZE
Virtual Population Analysis
Calibration
The ADAPT calibration method (Parrack 1986, Gavaris
1988, Conser and Powers 1990) was used to derive estimates of terminal
fishing mortality (F) in 2000. As in previous assessments, age-disaggregated
analyses were performed. Several comparative ADAPT calibrations were
performed, each using the same NEFSC spring and autumn (ages 2-6)
and MADMF spring (ages 2-4) and autumn (age 2) survey series. Due
to uncertainty in the interpretation of effort units in the 1994-1997
VTR data, USA commercial LPUE abundance indices for ages 2-6 were
included only through 1993. This change effectively removed the influence
of the LPUE indices on the terminal year outcome of the calibration,
while preserving the historic relationship employed in the previous
assessment. As in the previous assessments (see Mayo et al. 1998),
the USA commercial LPUE indices from 1982 through 1993 were derived
from the catch at age corresponding to the effort sub-fleet used
in the estimation of standardized fishing effort as described by
Mayo et al. (1994). The NEFSC and MADMF autumn indices were
lagged forward by one age and one year whereby age 1-6 indices were
related to age 2-7 stock sizes in the subsequent year for corresponding
cohorts. All NEFSC and MADMF indices were related to January 1 stock
sizes, and USA commercial LPUE indices were related to mid-year stock
sizes.
The 1982-2000 commercial landings at age as provided
in Table 9a include true ages 2-10 as well
as the 11+ group. In recent years, however, fish beyond age 7 have
been poorly represented. As reported by Mayo (1995), a calibration
run employing an extended age complement (true ages 2-9) produced
high coefficients of variation (CV) on the terminal year stock size
estimates and variable estimates of F on ages 7-9 in most years prior
to the terminal year. Therefore, as in previous assessments of this
stock (Mayo et al. 1993; Mayo 1995, Mayo 1998, Mayo et
al. 1998, NEFSC 2000, NEFSC 2001), all VPA formulations employed
a reduced age range (ages 2-6 and 7+).
Impact of 1999 and 2000
Discards
The VPA for the current assessment includes commercial
landings from1982-2000 (Table 10), commercial discards from 1999
and 2000, and recreational landings from 1982-2000 (Table 12). The
final catch at age used in the VPA is listed in Table 13, including
the discard adjustment to the 1999 and 2000 commercial landings at
age. Comparative ADAPT calibrations were performed to evaluate the
impact of a range of discard estimates in 1999 and 2000 on terminal
year fishing mortality. A summary of each of three VPA runs (lower,
middle, and upper range of discard estimates in 1999 and 2000) is
provided in Table 17.
Very little difference in the overall model fit is
evident among the three runs. The total sums of squares and the mean
square residuals are almost identical under all scenarios, although
there is a slight degradation in the coefficients of variation (CV)
of the stock size estimates (2001 Ns) under the upper end discards
scenario (Table 17). The major impact of
the various discard scenarios occurs in the estimation of terminal
year F. The effects on stock size estimates is relatively minor.
Differences in fishing mortality between the lower and middle range
scenarios are minor, but the estimate of the 2000 fully recruited
fishing mortality is substantially greater under the upper end discards
scenario.
Impact of Including
Recreational Landings
The VPA formulation presented above was employed in
an additional analysis to evaluate the specific impact of including
(or excluding) recreational landings in the VPA. In general, inclusion
of the recreational landings served to marginally increase the estimates
of fully recruited F, and to substantially revise upwards the estimates
of stock size. The CVs on estimates of stock size in 2001 were almost
identical to those obtained from the commercial-only base formulation.
The retrospective pattern, evident in the commercial-only run, remains
in the commercial/recreational run. Overall, inclusion of recreational
landings does not alter our perception of current stock status.
Final VPA Formulation
The ADAPT formulation employed in the final VPA calibration
was the same as that used in the previous assessments (Mayo et
al. 1998, NEFSC 2000, NEFSC 2001) except for the inclusion of
1982-2000 recreational landings at age. This analysis provided direct
stock size estimates for ages 2 through 6 in 2001 and corresponding
estimates of F on ages 1 through 5 in 2000. Since the age at full
recruitment was defined as 4 years in the input partial recruitment
vector, the terminal year F on age 6 was estimated as the mean of
the age 4 and 5 Fs; age 6 is also the oldest true age in the terminal
year. In all years prior to the terminal year, F on the oldest true
age (age 6) was determined from weighted estimates of Z for ages
4 through 6. In all years, the age 6 F was applied to the 7+ group.
Spawning stock biomass (SSB) was calculated at spawning time (March
1) by applying a series of period-specific maturity ogives. The present
analysis used a maturity schedule which reflected earlier maturation
beginning in 1994.
Residuals of the observed and predicted indices derived
from the final VPA formulation (Figure 13)
do not indicate any consistent trends over the period of the VPA,
except for the MADMF age 2 autumn index.
Virtual Population Analysis
Results
A complete listing of the final ADAPT VPA calibration
is given in Appendix 3, and summary results, including age-specific
estimates of instantaneous fishing mortality (F), stock size, mean
biomass and spawning stock biomass, are presented in Table 18. All
parameter estimates were significant (Appendix
3). Coefficients of variation on the stock size estimates
ranged from 0.29 (age 4) to 0.53 (age 6), while CVs on the estimates
of q were between 0.15 and 0.20. Slopes of the abundance index-stock
size relationships increased with age through age 6 for the NEFSC
spring and autumn surveys and the USA commercial LPUE indices. The
MADMF spring indices exhibited an increasing trend in q between ages
2 and 4 (Appendix 3).
Average (ages 4-5, unweighted) fishing mortality in
2000 was estimated to be 0.73 (Table 18, Figure
14), a slight decrease from 1999. The spawning stock biomass
of age 1 and older cod declined from 23,900 mt in 1982 to 15,300
mt in 1987. Following the recruitment and maturation of the strong
1987 year class, SSB increased to 24,200 mt in 1990 but declined
to 11,400 mt in 1993, a 3-year reduction of 53% (Table
18, Figure 15). SSB increased to 14,600
mt in 1995 due to the growth and maturation of the 1992 year class,
but declined again in 1996 and reached a record-low of 9,900 mt in
1998. SSB is estimated to have increased gradually between 1998 and
2000 (Table 18). Total stock size (ages
1+) has also declined sharply in recent years from 44.6 million fish
in 1988 to an average of 12.4 million fish during 1996-1998 (Table
18), a decrease of 72%, but is estimated to have increased to
about 18-19 million fish in 1999 and 2000 due in large part to recruitment
of the 1998 year class.
Since 1982, recruitment at age 1 has ranged from less
than 3.5 million fish (1993, 1994, and 1995 year classes) to 25.2
million fish (1987 year class). Over the 1982-2000 period, geometric
mean recruitment for the 1981-1999 year classes was 6.6 million fish.
The 1987 year class is the highest in the 1982-2000 series and about
twice the size of the next strongest year class. The1992 year class
was of moderate strength, and the 1998 year class appears to be comparable
(Table 18, Figure
15).
Precision of F and SSB
A bootstrap procedure (Efron 1982) was used to evaluate
the precision of terminal year estimates, by generating 600 estimates
of the 2000 fully recruited fishing mortality rate and spawning stock
biomass. Summary statistics for the bootstrap analyses are provided
in Appendix 4, and the distributions of the bootstrap estimates and
the corresponding cumulative probability curves are shown in Figures
16 and 17. The cumulative probability expresses the likelihood that
the fishing mortality rate was greater than a given level (Figure
16) or the likelihood that spawning stock biomass was less than
a given level (Figure 17), when measurement
error is considered.
Coefficients of variation for the 2001 stock size (numbers)
estimates ranged from 0.29 (age 4) to 0.51 (age 2), and CVs for qs
among all indices ranged from 0.14 to 0.18 (Appendix
4). The fully-recruited fishing mortality in 2000 for
ages 4+ was reasonably well estimated (CV = 0.30). The mean bootstrap
estimate of F (0.76) was slightly higher than the point estimate
(0.73) from the VPA, and ranged from 0.41 to 2.36. The 80% probability
interval ranges from 0.58 to 0.96 (Figure
16).
Although the abundance estimates for individual ages
in 2001 had wide variances (CV = 0.29 to 0.51), the estimates of
2000 spawning stock biomass and mean biomass were robust (CV = 0.17
and 0.13, respectively). The bootstrap means were 2.9 - 4.6% higher
than the VPA point estimates (Appendix
4). The 80% probability interval for SSB ranges from 11,200
mt to 15,600 mt (Figure 17). Despite
this variability, current spawning stock biomass is estimated to
have increased substantially from recent record lows. In general,
estimates of stock size and fishing mortality in the present assessment
are estimated with about the same precision as in the previous assessment
of this stock (Mayo et al.1998).
Retrospective Analysis
The previous retrospective analysis for this stock
was reported by Mayo et al. (1998). Although the formulation
used in the present assessment is the same as in the previous assessment,
changes in management measures for this stock during 1997-2000 may
have imposed additional uncertainty in the interpretation of current
stock status. Therefore, the retrospective analyses were conducted
again, and the tabular results are given in Appendix 3.
Retrospective patterns with respect to terminal F are
evident for Gulf of Maine cod in the most recent years (Figure
18). Mean F (ages 4-5, unweighted) in the terminal year had been
generally under-estimated between 1994 and 1997 by the ADAPT calibration.
The previous retrospective analysis by Mayo et al. (1998)
indicated the same pattern, but was able to detect the opposite pattern
(slight over-estimate of F) prior to 1994. Convergence of estimates
is generally evident within 3 years, and often within 2 years, prior
to any given terminal year. The retrospective analysis provides additional
evidence that current fishing mortality on this stock, although somewhat
lower than in previous years, remains relatively high. The retrospective
pattern for age 1 recruits suggests that recruitment has generally
been under-estimated over the past 6 years. The estimates of SSB
have been relatively stable, although there was a slight tendency
to under-estimate spawning biomass.
Spawning Stock and Recruitment
The relationship between spawning stock biomass and
recruitment for Gulf of Maine cod was examined from two perspectives.
First, a traditional spawning stock-recruitment scatterplot (Figure
19a) was constructed over the period covering the 1982-1999 year
classes. In addition, a survival ratio, expressed as recruits per
unit of SSB (R/SSB) was also calculated for each year class (Figure
19b). The stock-recruitment trajectory indicates the position
of the most recent levels of SSB and recruitment in the lower left
corner of the plot. The 1993-1997 year classes are all below average
and the 1993-1995 year classes are the lowest in the series.
Survival ratios of pre-recruits up to age 1 are highest
for the 1987, 1992 and 1998 year classes, the first two emerging
from about average SSB and the 1998 year class from low SSB. Survival
ratios were generally higher during the early-to-mid 1980s prior
to the emergence of the large 1987 year class. Survival declined
after the 1992 year class appeared, but increased in 1997 and 1998.
Hind-cast VPA Total
Biomass Estimates
The 1982-2000 total stock biomass estimates derived
from the VPA were extended back through time to 1963 utilizing NEFSC
autumn research vessel survey biomass (kg/tow) indices. Estimates
of the catchability coefficient (q), defined as the ratio between
the survey index of total biomass and the VPA estimate of age 1+
stock biomass, were computed annually from 1982-2000. The average
of these ratios was then applied to the entire 1963-2000 series of
survey biomass indices to derive scaled estimates of total stock
biomass. Results suggest that the total biomass of Gulf of Maine
cod was likely to have been well over 100,000 mt during the 1960s
and 1970s (Figure 20), and that VPA estimates
beginning in 1982 may represent the condition of the stock following
sharp declines in the late 1970s and early 1980s.
BIOLOGICAL REFERENCE
POINTS
Yield and Spawning Stock
Biomass per Recruit
Yield, total stock biomass, and spawning stock biomass
per recruit analyses were performed using the method of Thompson
and Bell (1934). Mean weights at age for application to the yield
per recruit analysis were computed as a 17-year arithmetic average
of total catch mean weights at age (Table
13b) over the 1982-1998 period. Mean weights at age for application
to the SSB per recruit analysis were computed as a 17-year arithmetic
average of stock mean weights at age (Table
14) over the 1982-1998 period. The 1999 and 2000 mean weights
at age were excluded due to poor sampling of commercial landings
during these years. The maturation ogive was the same as used in
computing SSB during the 1994-2000 period in the VPA. To obtain the
exploitation pattern for these analyses, a two-year geometric mean
F at age was first computed over 1999 and 2000 from the final converged
VPA results. These years were chosen specifically to encompass the
period since enactment of the most recent increase in the minimum
allowable mesh (165 mm). A smoothed exploitation pattern was then
obtained by dividing the F at age by the mean unweighted F for ages
4-5, adjusted to the average partial recruitment for ages 4 and 5.
The final exploitation pattern is:
Age 1 0.000, Age 2 0.0134, Age 3 0.2867, Age 4 0.9889,
Ages 5+ 1.000
This pattern is similar to that used in the 1998 assessment
(Mayo et al.1998) for ages 1 through 3, but indicates increased
selection of age 4 fish (from about 80% to 100%) compared to the
1998 assessment, possibly reflecting the inclusion of recreational
data in the catch at age employed in the VPA. This partial recruitment
pattern was used in yield and SSB per recruit calculations. Input
data and results of the yield and SSB per recruit calculations are
listed in Table 19 and are illustrated
in Figure 21. The yield per recruit analyses
indicate that F0.1 = 0.15 and Fmax = 0.27,
and SSB per recruit calculations indicate that F20% =
0.36. The yield per recruit reference points (F0.1 and
Fmax), and the SSB per recruit reference point (F20%)
are slightly lower than those reported in the 1998 assessment (Mayo et
al.1998).
MSY-Based Reference
Points
The existing estimates of Bmsy and Fmsy for
Gulf of Maine cod were derived in 1998 from a biomass dynamics model
(ASPIC; Prager 1994, 1995) integrating landings and relative biomass
indices over the period 1963-1997 (Anon.1998). The biomass dynamics
model analysis was conditioned on the relationship between age 1+
mean biomass derived from the 1997 VPA and biomass indices from the
NEFSC spring and autumn surveys and the MADMF spring survey. Estimates
of q, expressed as the ratio of the survey index to the age 1+ mean
biomass, were fixed for each of the 3 surveys used to calibrate the
production model. The analysis conditioned on age 1+ VPA mean biomass
suggested that Bmsy for Gulf of Maine cod was in the range
of 33,000 mt and that the corresponding age 1+ Fmsy was
0.31 (Fwb).
Because Gulf of Maine cod do not recruit to the fishery
until age 2, the biomass dynamics model was re-run, conditioned on
the relationship between age 2+ mean biomass derived from the current
VPA and the same survey biomass indices updated through 2000. The
revised analysis suggests that age 2+ Bmsy for Gulf of
Maine cod is in the range of 26,000 mt and that the corresponding
age 2+ Fmsy is 0.41 (Fwb). The modeling results indicate
that stock biomass was above Bmsy from the 1960s to the
early 1980s but, as F exceeded Fmsy in the early 1980s,
stock biomass declined to low levels in the 1990s. The model further
suggests that stock biomass increased sharply in 1999 and 2000, approaching
Bmsy as F declined below Fmsy.
The rapid increase in biomass estimated by the biomass
dynamics model is consistent with the recent increase in mean biomass
derived from the VPA. However, the age-structured information provided
by the VPA suggests that a considerable portion of the recent increase
in mean biomass can be attributable to the recruitment of the 1998
year class. This effect is also reflected in the survey biomass indices
which were incorporated into the production model analysis.
Age-Structured Production
Model
As an alternative to the ASPIC biomass dynamics model,
an age-structured production model (Sissenwine and Shepherd 1987)
was developed using stock and recruitment observations from VPA and
yield and biomass per recruit results. Age-structured production
models are more informative than biomass dynamics models and can
determine Fmsy in the form of fully-recruited F, and can
estimate SSBmsy as an alternative to Bmsy.
As concluded by the SAW Methods Working Group (Section D of this
report), fully-recruited Fmsy and SSBmsy are
less sensitive to transient conditions and are directly comparable
to VPA estimates of fully-recruited F and SSB. Comparison of current
VPA results with reference points derived from the biomass dynamics
model in Anon. (1998) is no longer appropriate, because the revised
VPA includes recreational catch (1982-2000), and historical recreational
catch is not available for a revised ASPIC analysis.
Age-Structured Production
Model Results
A Beverton-Holt (1957) stock- recruit function was
fit to the VPA estimates of SSB (in thousand mt) and age-1 recruitment
(in millions) assuming a lognormal error structure:
(1) R=(9.87SSB)/(7.55+SSB)
Estimates of yield, total biomass, and spawning biomass
per recruit (YPR, BPR, and SPR) were derived from the Thompson-Bell
(1934) dynamic pool model over a range of fully-recruited fishing
mortality rates (Table 19, Figure
21). Equilibrium SSB (SSB*) was then calculated at various levels
of fully-recruited fishing mortality to scale the dynamic pool estimates
of SSB per recruit to absolute values:
(2) SSB*=(9.87SSB per recruit)-7.55
Equilibrium recruitment (R*) was calculated as a function
of SSB*, using equation 1, and equilibrium yield was calculated as
the product of yield per recruit and R*.
Fmsy was determined as the F that produced
the maximum equilibrium yield (MSY), SSBmsy was the SSB*
at Fmsy, and Bmsy was calculated as the product
of yield per recruit and R* at Fmsy. F on total biomass
was also approximated as YPR/BPR for comparison to biomass dynamics
results. Estimates of yield, F, SSB, and B from VPA were plotted
with equilibrium calculations for comparison (Figure
22).
Results indicate that MSY=16,100 mt, fully-recruited
Fmsy=0.23, Bmsy=90,300 mt, and that SSBmsy=78,000
mt (Figure 22). Alternative stock recruit
decisions were considered for sensitivity analyses, including the
use of hindcasted SSB and R observations (Brodziak et al.
2001) and assuming geometric mean recruitment. Estimates of FMSY appeared
to be robust to stock-recruit decisions, ranging from 0.23-0.27.
However, MSY and Bmsy were more sensitive to alternative
stock recruit assumptions and were proportional to the estimate of
maximum R. For comparison, Fmsy on biomass (0.18) is substantially
less than the estimate from the ASPIC biomass dynamics model, and
Bmsy is substantially greater than that from ASPIC. However,
fully-recruited Fmsy is only slightly less than Fmax,
which was the previous overfishing definition.
Difference Between Old
and New Reference Points
There are many factors contributing to differences
between the existing Fmsy and Bmsy reference
points derived from the biomass dynamics model and those derived
from the present analysis based on the age-structured production
model. First, the age structured approach better accounts for the
productivity of the stock by specifically incorporating past and
present information on the relationship between spawning stock and
recruitment. In addition, the age structured approach is predicated
on the yield and biomass per recruit analyses which incorporate age-specific
growth and maturity information and the most appropriate exploitation
pattern from the fishery. The age-aggregated approach employed in
the biomass dynamics model subsumes all of the age-specific information
into an estimate of a single parameter (r), the intrinsic rate of
growth of the stock. This rate of increase may not always reflect
the current growth potential of the stock. As noted above, the age-structured
model is consistent with the assessment model because it is based
on the SSB and recruitment from the current VPA, which includes recreational
catch and recent discards. It is not currently possible to develop
a long time series of recreational catch for a revised ASPIC analysis
that could be comparable to the VPA.
The ASPIC approach was adopted by the Overfishing Definition
Review Panel (Anon. 1998) as a means of applying a consistent method
across as many stocks as possible, including those for which information
on age structure was not yet available. In the case of the Gulf of
Maine cod analysis, it was necessary to condition the biomass dynamics
model (i.e., fix the estimates of q) based on the relationship between
the NEFSC survey biomass indices and the corresponding VPA estimates
of mean biomass in order to obtain a significant fit. This may have
imposed constraints on the subsequent estimates of Bmsy and
Fmsy.
Long-term projections, reported below, confirmed the
results from the age-structured production model. The projection
results indicate that long-term yield at the revised estimate of
FMSY (0.23) is significantly greater than the previous
estimate of MSY (10,000 mt, Anon.1998) and is near the revised estimate
of MSY (16,100 mt). Similarly, projected total stock biomass is significantly
greater than the previous estimate of Bmsy (33,000 mt)
and close to the revised estimate of Bmsy (90,300 mt).
Furthermore, historical survey observations indicate that stock biomass
exceeded the revised estimate of BMSY during most of the
1960s and 1970s (Figure 20). Therefore,
it appears that the previous estimates of MSY and Bmsy were
greatly underestimated (conversely it appears that Fmsy was
over-estimated), and revised reference point estimates are more consistent
with long-term projections and historical observations.
CATCH and STOCK BIOMASS
PROJECTIONS
Stochastic age-based projections (Brodziak and Rago
MS1994) were performed over a 25-year time horizon to evaluate relative
trajectories of stock biomass and catch under various fishing mortality
scenarios. Recruitment was derived from the Beverton-Holt spawning
stock-recruitment relationship employed in the age structured production
model. Stock and catch mean weights at age, the maturity at age schedule,
and the partial recruitment at age vector are the same as those employed
in the yield and SSB per recruit analyses presented above. The 2001
survivors derived from 600 bootstrap iterations of the final VPA
formulation were employed as the initial population vector. The projection
was performed at four fishing mortality rates: F0.1 (0.15),
Fmsy (0.23), Fmax (0.27) and Fsq (0.73).
Fully recruited fishing mortality in 2001 was assumed equal to that
in 2000 (0.73) under all F scenarios. Short-term forecasts of 2002
catch and corresponding 2003 SSB were derived from the first two
years of the long-term projections. All input data are provided in Table
20.
Short-Term Projection
Results
The forecast for 2002 and 2003 is summarized in Table
20 and Figure 23. The results suggest
that if the current fishing mortality rate is reduced to Fmax or
less in 2002, SSB will continue to increase in 2003. However, if
F in 2002 remains at or near the 2000 F, SSB in 2003 will not increase
beyond that projected for 2002.
Long-Term Projection
Results
The long-term projections (Table
21; Figures 24 and 25)
suggest that fishing at Fmsy (0.23) will result in the
total stock biomass stabilizing at about 92,000 mt providing total
catches of about 15,000 mt per year. If F is not reduced from the
current level (0.73), neither total stock biomass nor spawning
stock biomass are likely to increase appreciably above the existing
level. Because the spawning stock-recruit relationship for this
stock is relatively flat across most observed levels of SSB (Figure
22), recruitment is estimated to be only slightly impaired at this
high fishing mortality rate. Given the recent trends in observed
recruitment at low SSB, however, this outcome is both unlikely
and optimistic.
CONCLUSIONS
The Gulf of Maine cod stock remains at a low biomass
level, although there are indications of a recent increase in total
biomass and spawning stock biomass in 1999 and 2000 . Fully recruited
fishing mortality appears to have declined only slightly in 2000
(0.73), indicating that F continues to remain very high relative
to fully recruited F reference points (F0.1 = 0.15; Fmsy =
0.23; Fmax = 0.27). Spawning stock biomass (SSB) declined
from over 24,000 mt in 1990 to a low of 9,900 mt in 1998, but increased
to 13,100 mt in 2000.
The 1987 year class has been the strongest in the VPA
assessment period (1982-2000), but research vessel survey results
suggest that even stronger year classes occurred during the 1970s.
Year classes subsequent to 1987 have been poor except for those from
1992 and 1998. The 1993, 1994, and 1995 year classes are among the
poorest in the VPA time series. Survival ratios (R/SSB) declined
through 1998 but now appear to be increasing.
Total (age 1+) stock biomass in 2001 is slightly above
1/4 of the revised Bmsy reference point (90,300 mt) and
fully recruited F in 2000 is about 3 times greater than the revised
Fmsy reference point (0.23).
A substantial retrospective pattern has existed in
the VPA results for this stock whereby fully recruited F has generally
been underestimated in the terminal year since 1994. In the retrospective
analysis of the present assessment, F1998 and F1999 appear
to have been slightly overestimated, while terminal Fs from 1994-1997
were underestimated.
ACKNOWLEDGEMENTS
We are indebted to members of the Northern Demersal
Working Group who provided a thorough, constructive review of the
initial version of this assessment, and to several members of the
Stock Assessment Review Committee review panel of the 33rd Stock
Assessment Workshop who contributed additional analyses not provided
in the initial assessment.
LITERATURE CITED
Anon. 1998. Evaluation of existing overfishing definitions
and recommendations for new overfishing definitions to comply with
the Sustainable Fisheries Act. Final Report. Overfishing Definition
Review Panel. June 17, 1998.
Azarovitz, T.R. 1981. A brief historical review of
the Woods Hole Laboratory trawl survey time series, p. 62-67. IN:
Doubleday, W. G., and D. Rivard (eds.), Bottom Trawl Surveys. Can.
Spec. Publ. Fish. Aquat. Sci. 58: 273 p.
Beverton, R.J.H. and S.J. Holt. 1957. On the dynamics
of exploited fish populations. U.K. Minist. Agric. Fish. Fish. Invest
(Ser.2) 19.
Brodziak, J. and P.J. Rago. MS 1994. A general approach
for short-term stochastic projections in age-structured fisheries
assessment models. Working Paper No. 4, 18th SARC Assessment Methods
Subcommittee: 27 p.
Brodziak, J.K.T., W.J. Overholtz and P.J. Rago. 2001.
Does spawning stock affect recruitment of New England groundfish?
Can. J. Fish. Aquat. Sci. 58: 306-318.
Campana, S., and J. Hamel. 1990. Assessment of the
1989 4X cod fishery. CAFSAC Res. Doc. 90/44: 46 p.
Campana, S., and J. Simon. 1985. An analytical assessment
of the 4X cod fishery. CAFSAC Res.Doc. 85/32: 40 p.
Clark, S.H. 1981. Use of trawl survey data in assessments,
p. 82-92. IN: Doubleday, W. G., and D. Rivard (eds.),
Bottom Trawl Surveys. Can. Spec. Publ. Fish. Aquat. Sci. 58: 273
p.
Conser, R.J. and J.E. Powers. 1990. Extension of the
ADAPT VPA tuning method designed to facilitate assessment work on
tuna and swordfish stocks. ICCAT, Coll. Vol. Sel Pap. 32:461-467.
Dagum, C. 1985. Lorenz curve. p. 156-161. In: S.
Kotz and N.L. Johnson [ed.] Encyclopedia of statistical sciences.
Vol 5. Wiley and Sons, New York.
DeLong, A., K. Sosebee and S. Cadrin. MS 1997. Evaluation
of vessel logbook data for discard and CPUE estimates. SAW24/SARC
Working Paper Gen 5.
Efron, B. 1982. The jacknife, the bootstrap and other
resampling plans. Phila. Soc. for Ind. and Appl. Math. 34: 92p.
Gavaris, S. 1988. An adaptive framework for the estimation
of population size. CAFSAC Res. Doc. 88/29: 12p.
Georgianna, D. and A. Cass. 1998. "The Cost of Hook
Fishing for Groundfish in Northeastern United States. University
of Massachusetts Dartmouth. North Dartmouth, MA.
Howe, A.B., F.J. Germano, J.L. Buckley, D. Jimenez,
and B.T. Estrella. 1981. Fishery resource assessment, coastal Massachusetts.
Completion Rept., Mass. Div. Mar. Fish., Comm. Fish. Rev. Div. Proj.
3-287-R-3: 32 p.
Lallemand, P., J. M. Gates, J. Dirlam, and J. Cho.
1998. "The Costs of Small Trawlers in the Northeast." Department
of Environmental and Natural Resource Economics, University of Rhode
Island, Kingston, RI.
Lallemand, P., J. M. Gates, J. Dirlam, and J. Cho.
1999. "The Costs of Large Trawlers in the Northeast." Department
of Environmental and Natural Resource Economics, University of Rhode
Island, Kingston, RI.
Lorenz, M.C. 1905. Methods of measuring the concentration
of wealth. J. Amer. Stat. Assoc. 9, 209-219.
Mayo, R.K. 1995. Assessment of the Gulf of Maine Cod
Stock for 1993. NMFS, NEFSC, Woods Hole Lab. Ref. Doc. 95-02: 74
p.
Mayo, R.K. 1998. Assessment of the Gulf of Maine Cod
Stock for 1997. NMFS, NEFSC, Northeast Fisheries Science Center Ref.
Doc. 98-08: 88 p.
Mayo, R.K., L. O'Brien, and F.M. Serchuk. 1993. Assessment
of the Gulf of Maine Cod Stock for 1992. NMFS, NEFSC, Woods Hole
Lab. Ref. Doc. 94-04: 54 p.
Mayo, R.K., T.E. Helser, L. O'Brien, K.A. Sosebee,
B.F. Figuerido and D.B. Hayes. 1994. Estimation of standardized otter
trawl effort, landings per unit effort, and landings at age for Gulf
of Maine and Georges Bank cod. NMFS, NEFSC, Woods Hole Lab. Ref.
Doc. 94-12: 17 p.
Mayo, R.K., L. O'Brien, and S.E.Wigley. 1998. Assessment
of the Gulf of Maine Cod Stock for 1998. NMFS, NEFSC, Northeast Fisheries
Science Center Ref. Doc. 98-13: 88 p.
Minet, J.P. 1978. Dynamics and yield assessment of
the northeastern Gulf of St. Lawrence cod stock. Int. Comm. Northw.
Atlant. Fish., Selected Papers 3: 7-16.
Myers, R.A. and N.G. Cadigan. 1995. Was an increase
in natural mortality responsible for the collapse of northern cod?
Can. J. Fish. Aquat. Sci. Vol. 52, 1274-1285.
NEFC (Northeast Fisheries Center). 1989. Report of
the Seventh NEFC Stock Assessment Workshop (Seventh SAW). NMFS, NEFC,
Northeast Fisheries Center. Ref. Doc. 89-04: 108p.
NEFSC (Northeast Fisheries Science Center). 1991. Report
of the Twelfth Northeast Regional Stock Assessment Workshop (12th
SAW). NMFS, NEFSC, Northeast Fisheries Science Center. Ref. Doc.
91-03: 187 p.
NEFSC (Northeast Fisheries Science Center). 1992. Report
of the Thirteenth Northeast Regional Stock Assessment Workshop (13th
SAW). NMFS, NEFSC, Northeast Fisheries Science Center. Ref. Doc.
92-02: 183 p.
NEFSC (Northeast Fisheries Science Center). 1993. Report
of the Fifteenth Northeast Regional Stock Assessment Workshop (15th
SAW). Stock Assessment Review Committee (SARC) Consensus Summary
of Assessments. NMFS, NEFSC, Northeast Fisheries Science Center.
Ref. Doc. 93-06: 108 p.
NEFSC (Northeast Fisheries Science Center). 1995.
Report of the Nineteenth Northeast Regional Stock Assessment Workshop
(19th SAW). Stock Assessment Review Committee (SARC) Consensus Summary
of Assessments. NMFS, NEFSC, Northeast Fisheries Science Center.
Ref. Doc. 95-08: 221 p.
NEFSC (Northeast Fisheries Science Center). 1997. Report
of the Twenty-fourth Northeast Regional Stock Assessment Workshop
(24th SAW). Stock Assessment Review Committee (SARC) Consensus Summary
of Assessments. NMFS, NEFSC, Northeast Fisheries Science Center.
Ref. Doc. 97-12: 291 p.
NEFSC (Northeast Fisheries Science Center). 1998. Report
of the Twenty-seventh Northeast Regional Stock Assessment Workshop
(27th SAW). Stock Assessment Review Committee (SARC) Consensus Summary
of Assessments. NMFS, NEFSC, Northeast Fisheries Science Center Ref.
Doc. 98-15: 350 p.
NEFSC (Northeast Fisheries Science Center). 2000.
Assessment of 11 Northeast Groundfish stocks through 1999. Report
of the Northern Demersal Working Group, Northeast Regional Stock
Assessment Workshop. NMFS, NEFSC, Northeast Fisheries Science Center
Ref. Doc. 00-05:173 p.
NEFSC (Northeast Fisheries Science Center). 2001a.
Assessment of 19 Northeast Groundfish stocks through 1999. Report
of the Northern Demersal Working Group, Northeast Regional Stock
Assessment Workshop. NMFS, NEFSC, Northeast Fisheries Science Center
Ref. Doc. 01-20: 217 p.
NEFSC (Northeast Fisheries Science Center). 2001b.
Report of the Thirty-third Northeast Regional Stock Assessment Workshop
(33rd SAW). Stock Assessment Review Committee (SARC) Consensus Summary
of Assessments. NMFS, NEFSC, Northeast Fisheries Science Center Ref.
Doc. 01-18: 281 p.
Paloheimo, J.E., and A.C. Koehler. 1968. Analysis
of the southern Gulf of St. Lawrence cod populations. J. Fish. Res.
Board Can. 25(3): 555-578.
Parrack, M.L. 1986. A method of analyzing catches and
abundance indices from a fishery. Int Comm. Conserv. Atlantic Tunas,
Coll. Vol. Sci. Pap. 24:209-221.
Pinhorn, A.T. 1975. Estimates of natural mortality
for the cod stock complex in ICNAF Division 2J, 3K and L. Int. Comm.
Northw. Atlant. Fish. Res. Bull. 11: 31-36.
Power, G., K. Wilhelm, K. McGrath and T. Theriault.
MS 1997. Commercial fisheries dependent data collection in the Northeastern
United States. SAW24/SARC Working Paper Gen 3.
Prager, M.H. 1994. A suite of extensions to a non-equilibrium
surplus-production model. Fish. Bull. 92: 374-389.
Prager, M.H. 1995. Users's manual for ASPIC: A Stock
Production Model Incorporating Covariates, program version 3.6x.
Miami Laboratory Document MIA-92/93-55, National Marine Fisheries
Service. 29p.
Rivard, D. 1982. APL programs for stock assessment
(revised). Can. Tech. Rep. Fish. Aquat. Sci. 1091: 146 p.
Sissenwine, M.P. and J.G. Shepherd. 1987. An alternative
perspective on recruitment overfishing and biological reference points.
Can. J. Fish. Aquat. Sci. 44: 913-918.
Thompson, Jr. W.A. 1976. Fisherman's Luck. Biometrics.
32, 265-271.
Thompson, W.F., and F.H. Bell. 1934. Biological statistics
of the Pacific halibut fishery. 2. Effect of changes in intensity
upon total yield and yield per unit of gear. Rep. Int. Fish. (Pacific
Halibut) Comm. 8: 49 p.
Wigley, S.E. 1996. The Lorenz curve Method applied
to NEFSC bottom trawl survey data. Northeast Fisheries Science Center
Ref. Doc. 9605f. 11 p.
Wigley, S.E., M. Terceiro, A. DeLong and K. Sosebee.
1998. Proration of 1994-1996 commercial landings of cod, haddock
and yellowtail flounder. NMFS, NEFSC, Woods Hole Lab. Ref. Doc. 98-02:
32 p.
