Northeast Fisheries Science Center Reference Document 06-02
The 2005 Assessment
of the Gulf of Maine
Atlantic Cod Stock
by by Ralph K. Mayo and Laurel A. Col
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St.,
Woods
Hole, MA 02543
Print
publication date March 2006;
web version posted March 14, 2006
Citation: Mayo RK, Col LA. 2006. The 2005 assessment of the Gulf of Maine Atlantic cod stock. US Dep
Commer, Northeast Fish Sci Cent Ref Doc. 06-02; 109 p.
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ABSTRACT: The status of the Gulf of Maine Atlantic cod (Gadus morhua)
stock is reviewed, and terminal year VPA estimates of 2004 fishing
mortality, spawning stock biomass and the survivors in 2005 are presented. Precision
estimates of the 2004 fishing mortality and spawning stock biomass
estimates for Gulf of Maine cod are also provided.
The 2005 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 2001 assessment of the Gulf of Maine cod stock reviewed at SAW
33 (NEFSC 2001a, b, Mayo et al. 2002) and those reviewed in
2002 at the Groundfish Assessment Review Meeting (GARM) (NEFSC 2002b). The
analyses presented herein were recently reviewed at the 2005 GARM (NEFSC
2005).
Total landings of Gulf of Maine cod decreased from 4,156 metric tons
(mt) in 1998 to 1,636 mt in 1999, increased to 3,730 mt in 2000 and have
since remained between 3,800 and 4,400 mt. The sharp decline in
landings between 1998 and 1999 and the subsequent increase in 2000 likely
reflect 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 the 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. CPUE estimates have not been
included in the assessment model since 1994 because of uncertainty in
the effort units between the interview-based estimates and the VTR-based
estimates as well as recent management initiatives including trip limits
and closed areas imposed to control fishing mortality. 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 modest increase since the early 1990s, the Gulf of Maine cod
stock biomass remains low compared to the 1960s and 1970s. Except
for the 1998 year class, recruitment during the 1990s has been well below
the long-term mean. The 1999 and 2000 year classes are weak but
there are signs that the 2003 year class may be well above average.
Total stock biomass (ages 1+) declined from a peak of 41,966 mt in 1990
to 15,867 mt in 1997, but increased to 29,000 mt in 2001 and has remained
at about that level through 2004. Spawning stock biomass (SSB)
declined from over 24,200 mt in 1990 to a low of 11,128 mt in 1997; SSB
increased to 25,369 mt in 2002 but declined to 20,549 mt in 2004 due
to very low abundance of the poor 1999 and 2000 year classes. Fully
recruited instantaneous fishing mortality (F, ages 4-5) remained close
to or above 1.0 between 1983 and 1997, but declined to 0.35 by 2002 and
has since increased to 0.58 in 2004. SSBmsy is now estimated
to be 82,830 mt with a corresponding Fmsy of 0.23, (fully
recruited, ages 4+) (NEFSC 2002a). With respect to the age-structured
MSY-based reference points, 2004 spawning stock biomass is well below ½ SSBmsy, and
2004 F is 2.5 times Fmsy.
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 commercial
landings doubled between 1964 and 1968, doubled again between 1968 and
1977, and averaged 12,200 mt per year during 1976-1985 (Table
1). 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. Total commercial
landings have since fluctuated between 3,800 and 4,400 mt. Landed
cod from the recreational sector have represented between 6 and 39 percent
of the combined commercial and recreational harvest.
This report presents an updated and revised analytical assessment of
the Gulf of Maine cod stock (NAFO Division 5Y) for the period 1982-2004
based on analyses of commercial and recreational data through 2004 and
research vessel survey data through spring 2005. 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 the 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 (Serchuk and Wigley 1986)
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 most recent peer review
of this assessment (Mayo et al. 2002) occurred at the 33rd Northeast
Regional Stock Assessment Workshop in June 2001 (NEFSC 2001b) and an
updated assessment through 2001 was reviewed at the Groundfish Assessment
Review Meeting in October 2002 (NEFSC 2002b, Mayo and Col 2002). The
present assessment was reviewed at the second Groundfish Assessment Review
Meeting in August 2005 (NEFSC 2005, Mayo and Col 2005).
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
(Appendix 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 (Serchuk et al. 1994). 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 Multispecies 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
(Appendix 1). 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. Amendment 13, implemented
in May 2004, added additional restrictions on Days at Sea usage and
further defined the use of A DAS and B DAS to allow fishing on stocks
in relatively good condition while still restricting effort on stocks
of concern (including Gulf of Maine cod).
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 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 landed catch reported in fishing
vessel logbooks as described above.
Total commercial landings in 2004 were 3,798 mt, slightly below those
from 2001-2003 but approximately 132% greater than in 1999 (Table 1,
Figure 1). Since 1977, the USA fishery has accounted for all
of the commercial catch. Canadian landings reported as Gulf of
Maine catch after 1977 are believed by Canadian scientists to have
been misreported catches from the Scotian Shelf stock (Campana and
Simon 1985; Campana and Hamel 1990) and have thus been excluded. Although
otter trawl catches account for most of the landings (averaging between
50 and 59% between 1993 and 2003), 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
2). The percentage landed by otter trawls declined further
in 2004 to 44%.
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 3). 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 (through 1993 only),
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 during 1989-1991, but declined substantially
in 1992 and remained negligible in 1993. Shrimp trawl sampling
data for 1994-2004 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 1994-2004 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 2000-early 2004 compared
to pre-1999 ratios. Ratios declined in the second quarter of
2004 after trip limits were relaxed.
Discards of Gulf of Maine cod ranged from 154 mt in 1998 to 3,598
mt in 1990 (Table 3). 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, and fluctuated between 1,500 and 2,000 mt
between 2001 and 2003 before declining to about 575 mt in 2004 when
the trip limit was increased to 800 lbs/day in the second quarter (Table
3).
To further evaluate discarding during 1999 - 2004 when low trip limits
were imposed, all available vessel trip report (VTR) records were examined
from trips reporting some catch of cod in the Gulf of Maine. 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. Comparisons between discard estimates
based on Sea Sample and VTR data are provided in Tables
4 and 5.
The discard estimates of Gulf of Maine cod derived from the two data
sets have been reasonably close to each other, with annual differences
of 3-18 percent on the estimates of total commercial catch. Each
method and data set 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 have been sparse
in some years, leading to considerable imprecision. The 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 beginning in 1999. Note
that the discards estimated from the VTR data exceed the estimates
derived from the Sea Sample data in 1999 and 2000, while the opposite
is true from 2001-2004 (Tables 4 and 5).
There is no objective basis to select the results obtained from either
data set. Thus, the SAW 33 SARC Panel concluded that both estimates
could be used to derive annual estimates to the nearest 500 mt increment. This
approach has continued in the present assessment and the results are
given in Table 4 for the 1999-2004 period. While it is acknowledged
that this approach is subjective, it has been accepted by the SAW33
SARC Panel and the 2 meetings of the GARM in 2002 (NEFSC 2002b) and
2005 (NEFSC 2005).
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
6. 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 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 was 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 have
been somewhat compromised. Sampling improved considerably in
2001, especially in the case of large market category cod as a result
of augmented sampling effort from the Massachusetts Division of Marine
Fisheries (DMF) samplers, and has remained high (less than 100 mt per
sample).
Of the 201 samples collected in 2004, 46 were scrod samples (23%),
65 were market (32%), and 90 were large (45%). Compared with
the 2004 market category landings distribution by weight (scrod: 3%;
market: 41%; large: 53%) (Table 7), sampling in 2004 over-represented
the scrod category and well represented the market and large categories.
As well, the seasonal distribution of samples became skewed for several
years 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.
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 8a).
Age composition of landings from 1994 through 2004 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 6). 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. Quarterly
stratification resumed in 2001 and continued through 2004.
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. Representation
of age 2 cod was relatively high in the early 1980s but, in response
to a series of minimum mesh size increases during the 1990s, age 2
fish have gradually all but disappeared from the landings. 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 absent for several years during the 1990s, and numbers
of age 8 and 9 fish have also been unusually low (Table 8a). 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 began in 1991.
More recently, the 1998 year class has dominated the landings at ages
3 through 6 in 2001 through 2004, respectively. As well, the
proportion of cod older than age 7 has begun to increase. In
2004 ages 8 and older represented 15% of the landed weight, more than
the 7-13% contribution during 1982-1984 and the very low 1% contribution
in 2000. Although the fraction of age 8 and older fish has begun
to increase, the period of low representation during the 1990s precludes
the use of these older fish in the assessment model. Therefore,
the age 7+ group was continued in the present assessment model (Table
9a and 9b).
Adjustment of the 1999 - 2004 Commercial Landings at Age
The fishery for Gulf of Maine cod was affected by management actions
that began in 1999 and have continued into 2004. The implementation
of extremely low trip limits in 1999 likely precipitated a substantial
increase in the amount of cod discarded compared to previous years,
as noted above. While these trip limits were relaxed to some
extent in subsequent years, a substantial portion of the total catch
continues to be discarded. Consequently, the 1999-2004 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 1999-2004 landings estimates had
to 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 cases
where discards occur because the mesh selectivity in the fishery is
not consistent 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,
where regulatory discards are generated as a result of extremely low
trip limits (as occurred during 1999-2004), it is presumed that cod
of all sizes and ages are discarded without prejudice. Examination
of the 1998, 1999 and 2000 kept and discarded length composition samples
from the NEFSC Sea Sample database support 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 -2004 commercial
landings at age estimates in Table 9 were multiplied by the discard
adjustment factors in Table 4 before inclusion in the VPA catch at
age matrix (see pages 3 and 4 for discard estimation procedures).
Commercial Landings Mean Weights at Age
Mean weights at age in the landings during 1982-2004 are given in
Table 8b for ages 1-11+ and in Table
9b and Figure 2 for ages 2-7+.
Based on landings patterns these are considered mid-year values. Mean
weights of age 2 and 3 cod have increased since about 1992 and mean
weights of age 4 cod have increased since 2000, reflecting reduced
partial recruitment of younger fish to the fishery, while the average
weights for age 5 and 6 cod have fluctuated without trend. Mean
weights for ages 9 and older fluctuate considerably and are particularly
sensitive to sampling variability. However, when the older ages
are aggregated into a single age 7+ group, a marked decline is evident
in mean weights during the 1990s, followed by an increase beginning
in 2001. This likely reflects the decline of older fish in the
landings during the 1990s and recent increases in the proportion of
the stock ages 7 and older.
The generally higher mean weights of age 2 and 3 cod since the mid
1990s may be related to an increase in minimum codend mesh size from
140 mm (5.5 in.) to 152 mm (6 in.) in 1994, while the increase in mean
weights of age 4 cod occurred after an increase in the minimum codend
square mesh to 165 mm (6.5 in.) in May, 1999.
Recreational Fishery Catches
Estimates of the recreational cod catch were derived from the Marine
Recreational Fishery Statistics Survey (MRFSS), which has been conducted
annually since 1979. Gulf of Maine cod catches were estimated
assuming that catches of cod recorded in the intercept survey were
removed from the ocean in statistical areas adjacent to the reported
state or county of landing. The MRFSS database has been revised,
resulting in adjusted catch estimates for the years 1981 through 1997. In
addition, the Gulf of Maine cod catch from 1994 to 2004 were re-estimated
using a revised port stratification scheme to better reflect sampling
allocation in New England ports. Information on the catch prior to
1981, which has not been revised, is included in Table
10 to provide
a longer-term perspective. Further information on the details
of the allocation scheme and sampling intensity are given in NEFSC
(1992). Estimates of the total Gulf of Maine cod recreational
catch as well as the retained portion of the catch (i.e., excluding
those caught and released) are provided in Table
10.
The quantity of cod retained generally exceeded 80% of the total recreational
catch during 1981 through 1991, but has averaged less than 40% since
1992. 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, and fluctuated between 1,200 and 3,300 mt between 1992 and
1999. The total catch increased sharply beginning in 2000, reaching
7,700 mt in 2003 before declining to 3,500 mt in 2004. Trends
in the weight of cod retained were similar to the total catch, but
the magnitude of the removals is considerably less, especially since
1992.
The fraction of the total landings (commercial and recreational) taken
by the recreational sector (retained cod) generally ranged from 6 to
20% between 1981 and 1998. The recreational component increased
to 35 percent in 1999 and has remained between 25 and 39 percent through
2004.
Recreational Fishery Sampling Intensity
Information on the length frequency sampling levels of Gulf of Maine
cod taken in the recreational fishery is also provided in Table
10. Overall,
sampling of cod taken by recreational gear is poor, averaging less
than 1 sample per 1,000 mt removed. Sampling of the recreational
fishery improved during the 1990s, 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 age composition of the recreational
landings in 1997-2004 was estimated for the current assessment and
the 1982-2004 recreational landings at age 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,
the estimation of the number of Gulf of Maine cod 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 supported 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 11a).
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
recreational landings now resembles that from the commercial fishery
with ages 3, 4 and 5 predominant (Tables 9a and 11a). The strong
1987 year class dominated the recreational catch in 1990, 1991 and
1992. The 1992 year class can also be tracked in the estimated catch
at age between 1995 and 1999 and the 1998 year class predominates after
2000. Ages 3 and 4 cod generally predominate in terms of weight
caught, although the 1987, 1992, and 1998 year classes predominated
at age 5 in 1992, 1997 and 2003 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 11b) 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, i.e., an increase in the mean weight of age 2 and 3 cod beginning
in the mid-to-late 1990s, apparent stability of mean weights of age
4, 5 and 6 cod, and an indication of a similar decline and recent increase
in the mean weight of the 7+ group fish.
Total Landings Age Composition
Estimates of the age composition of total cod landings (Table
12a)
were derived by combining the separate age composition estimates obtained
for the commercial (Table 9a) and recreational fisheries (Table
11a). 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, the 1992 year class can
also be tracked between 1995 and 1999, and the 1998 year class dominates
the period after 2000.
Total Landings Mean Weights at Age
Mean lengths and weights at age of cod landed by the
combined commercial and recreational sectors (Table
12b) 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 Tables
13 and 14 for ages 1-11+ and 1-7+, respectively. The age 11+ values
were derived from the commercial landings mean weight at age data (Table
9b) and the age 7+ values were derived from the total (commercial and
recreational) landings mean weight at age data (Table
12b) using procedures
described by Rivard (1980, 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 were 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 1997-2004 LPUE inconsistent with previous years. Until effort units
are resolved in the commercial fishery database, no further treatment
of the LPUE series after 1993 is meaningful.
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 mean catch-per-tow-at-age (number) indices
are listed in Appendix 2: Tables 1 and 2, and catch-per-tow-at-age indices
from DMF spring and autumn surveys are listed in Appendix
2: Table 3. The
entire series of spring and autumn abundance and biomass indices was
re-estimated for this assessment to better account for vessel effects
between RV Albatross IV and RV Delaware II. Although the only major
difference during the 1982-2004 assessment period occurred in 1987, many
minor changes to the indices occurred in most years. Therefore,
the indices listed in Table 15 and Appendix 2: Tables 1 and 2 may differ
slightly from those provided in previous assessments.
NEFSC spring and autumn offshore stratified mean catch
per tow indices for Gulf of Maine cod have generally exhibited similar
trends throughout the survey time series (Table 15, Figure
3). Number-per-tow indices
declined during the mid- and late 1960s, but between 1972 and 1985 have
fluctuated as a result of a series of recruitment pulses. Sharp
increases in the autumn number per tow indices reflect above-average
recruitment of the 1971, 1973, 1977-1980, and 1985-1987 year classes
at ages 1 and 2 (Appendix 2: Table 2; Figure
4). The sequential
dominance of these cohorts at older ages is evident from number-per-tow-at-age
values in both spring and autumn NEFSC surveys (Appendix
2: Table 2). 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 a series of poor year classes.
More recently, autumn biomass indices have shown a modest increase,
but the large value in 2002 is the result of a single very large tow
that unduly influenced the calculation of the mean. The 1998 year
class is equivalent to the 1992 year class, and the 2003 year class appears
to be the strongest since the 1987 year class. Biomass indices in 2003
and 2004 suggest the population biomass remains above the low level of
the early 1990s.
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 1). 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 increased in 2001 and 2002 and remained high in
2003; however, the 2004 and 2005 indices suggest that current biomass
remains relatively low (Figure 3).
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
1 and 3). Based on MADMF and NEFSC survey catch per tow indices,
the 1992, 1998 and 2003 year classes appear to be of moderate strength;
the intervening year classes of Gulf of Maine cod, particularly the 1993,
1994, 1995, 1996 and 2000 year classes have been well below average (Figures
4 and 5).
MORTALITY
Total Mortality Estimates
Estimates of instantaneous total mortality (Z) were calculated on an
annual basis from 1964 through 2004. Total mortality was calculated
from NEFSC survey catch per tow at age data (Appendix
2: Tables 1 and
2) for fully recruited age groups 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
estimates were derived from:
Spring: ln
(
age 4+ for 1983/ age 5+ for 1984)
Autumn: ln
( age 3+ for
1982/ age 4+
for 1983)
Different age groups were used in the spring and autumn analyses so
that Z could be evaluated over the same year class within each year.
Values of Z derived from the spring surveys are generally comparable
to those calculated from the autumn data (Figure 6). These values
of Z exhibit considerable inter-annual variability due primarily to year
effects in the surveys. When smoothed, however, the annual estimates
suggest a pattern of increasing mortality during the1980s, with mortality
remaining close to 1.0 through the mid 1990s. Total mortality declined
during the late 1990s, but the most recent estimates suggest an increase.
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 2004. 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 1982-2004
commercial landings at age as provided in Table 8a 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, Mayo et al. 2001, Mayo and
Col, 2002, 2005), all VPA formulations employed a reduced age range (ages
2-6 and 7+) (Table 9a).
Comparative VPA Calibrations
Recent changes to the VPA software and some revisions to the commercial
and MRFSS data components were made in the intervening years between
the current assessment and the previous assessment reviewed at the 2002
Groundfish Assessment Review Meeting (Mayo and Col 2002). These
include:
- A change in the software from the FACT
package to the NFT package. The major change is in the estimation
of F on the oldest age.
- The discovery
of additional commercial age samples for the years 2000 and 2001
- Revision
to the MRFSS catch estimates for the period back to 1994
- Re-calculation
of the NEFSC bottom trawl survey indices to better account for vessel
effects between RV Albatross IV and RV Delaware II. Only minor
differences occurred during the period of the VPA assessment.
To provide a bridge to the previous (2002) assessment, key results from
six calibration runs with 2001 as the terminal year are compared in Table
16. The first set of results was taken directly from the 2002 assessment
(Mayo and Col 2002) based on the FACT software package. The second
set of results is based on running the NFT software on the 2002 assessment
input file. The next 3 sets of results are also based on the NFT
VPA software reflecting the following data changes: 1) re-estimated commercial
catch at age for 2000 and 2001, 2) re-estimated recreational landings
at age for 1994-2001, and 3) re-estimated NEFSC bottom trawl indices. The
final set of results, also based on the NFT VPA software, incorporates
all three data changes noted above.
In general, differences across all runs are minor (Table
16). The
change in software did not produce a substantial change in the aggregate
results (RSS, F and SSB), but there was a small difference in the estimated
F at the fully recruited ages (4 and 5). Results from the remaining
calibration runs incorporating the various data changes were very similar
to the initial calibration run using the NFT software. Had these
changes in software and data been in place in 2002, the results would
have been very similar to what was obtained from the 2002 assessment. Based
on these conclusions, the NFT software and the data revisions were carried
forward with the updated catch and survey data for 2002-2004.
Final VPA Formulation
The ADAPT formulation employed in the final VPA calibration was the
same as that used in the 2002 assessment except for the revisions to
the software and data as noted above. This analysis provided direct
stock size estimates for ages 2 through 6 in 2005 and corresponding estimates
of F on ages 1 through 5 in 2004. 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 (by age group abundance, in numbers) 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 7) 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 17. All parameter
estimates were significant (Appendix 3). Coefficients of variation
on the stock size estimates at the beginning of 2005 ranged from 0.23
(age 4) to 0.36 (age 5), while CVs on the estimates of survey qs were
between 0.03 and 0.17. 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 a decreasing trend in q between ages 2 and 4 (Appendix
3).
Average (ages 4-5, unweighted) fishing mortality fluctuated around 1.0
during the 1980s and early 1990s, but steadily declined thereafter to
less than 0.4 in 2002 (Table 17, Figure
8). Fishing mortality in 2004
was estimated to be 0.58, an increase from 2002 and 2003. The increase
in F in 2004 is notable because the two ages upon which the fully recruited
average F is based (ages 4 and 5) represent the below-average 1999 and
the very weak 2000 year classes. The spawning stock biomass of
age 1 and older cod declined from 23,987 mt in 1982 to 15,302 mt in 1987. Following
the recruitment and maturation of the strong 1987 year class, SSB increased
to 24,261 mt in 1990 but declined to 10,797 mt in 1993, a 3-year reduction
of 55% (Table 17, Figure 9). SSB increased to 14,996 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 11,128 mt in 1997. SSB
increased again beginning in 1998, reaching 25,369 mt in 2002, based
primarily on growth and maturation of the 1998 year class (Table
17). Spawning
stock biomass has since declined to 20,549 mt in 2004. Total stock size
(ages 1+) has also declined sharply in recent years from 44.6 million
fish in 1988 to an average of 14.0 million fish during 1996-1998 (Table
17), a decrease of 72%, but is estimated to have increased to about 32.2
million fish in 2004 due in large part to an initial estimate of the
strength of the 2003 year class.
Since 1982, recruitment at age 1 has ranged from less than 4 million
fish (1993, 1994, 1995, 2000 and 2002 year classes) to 25.2 million fish
(1987 year class). Over the 1982-2004 period, geometric mean recruitment
for the 1981-2003 year classes was 6.3 million fish. The 1987 year
class is the highest in the 1982-2004 series and about twice the size
of the next strongest year class. The 1992, 1998 and 2001 year
classes were of moderate strength. (Table 17, Figure
9), while the 1993-1995,
1999-2000 and 2002 year classes were weak. The initial estimate
of the strength of the 2003 year class (22 million) is very close to
that of the 1987 year class.
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 2004 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 Figure 10. The cumulative probability
expresses the likelihood that the fishing mortality rate was greater
than a given level (Figure 10a) or the likelihood that spawning stock
biomass was less than a given level (Figure 10b), when measurement error
is considered.
Coefficients of variation for the 2005 stock size (numbers) estimates
ranged from 0.24 (age 4) to 0.49 (age 2), and CVs for qs among all indices
ranged from 0.03 to 0.31 (Appendix 4). The fully-recruited fishing
mortality in 2004 for ages 4+ was reasonably well estimated (CV = 0.20). The
mean bootstrap estimate of F (0.60) was slightly higher than the point
estimate (0.58) from the VPA. The 80% probability interval ranges
from 0.46 to 0.76 (Figure 10a).
Although the abundance estimates for individual ages in 2005 had wide
variances (CV = 0.24 to 0.49, the estimate of 2004 spawning stock biomass
was robust (CV = 0.14). The bootstrap mean was 2.9% higher than
the VPA point estimates (Appendix 4). The 80% probability interval
for SSB ranges from 17,800 mt to 24,800 mt (Figure
10b). Despite
this variability, current spawning stock biomass is estimated to have
increased substantially from earlier 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 and Col 2002).
Retrospective Analysis
The previous retrospective analysis for this stock was reported by Mayo et
al. (2002). Although the formulation used in the present
assessment is the same as in the previous assessment, changes in management
measures for this stock during 2002-2004 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 11), but the patterns are
not consistent through time. Mean F (ages 4-5, unweighted) in the terminal
year was under-estimated in 1995 and 1996 by the ADAPT calibration. The
retrospective analysis presented 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. The retrospective pattern in
F reversed between 1997 and 1998 revealing a tendency to over-estimate
F in 2000 and 2001. Most recently, there appears to be a slight
tendency to again under-estimate F in 2002 and 2003. 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 the 1980s and 1990s, is showing a tendency
to increase again. The retrospective patterns for SSB and
age 1 recruits show the opposite patterns, with an indication of over-estimation
in 2002 and 2003.
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
12a) was constructed
over the period covering the 1982-2003 year classes. In addition,
a survival ratio, expressed as recruits per unit of SSB (R/SSB) was also
calculated for each year class (Figure 12b). The stock-recruitment
trajectory indicates the position of the most recent levels of SSB and
recruitment in the upper right corner of the plot illustrating the emergence
of the 2003 year class from a relatively high spawning stock.
Survival ratios of pre-recruits up to age 1 are highest for the 1987,
1992, 1998 and 2003 year classes, the first two emerging from about average
SSB, the 1998 year class from low SSB and the 2003 year class from relatively
high 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 1996 and
1998, declined thereafter and increased again in 2003.
BIOLOGICAL REFERENCE
POINTS
Yield and Spawning Stock Biomass per Recruit Reference Points
Yield and spawning stock biomass biological reference points were not
revised during the preparation of the current assessment. The following
reference points were taken from Mayo et al. (2002) and are the
same as those applied by the Working Group on Biological Reference Points
(NEFSC 2002a) and in the 2002 and 2005 GARM assessments (NEFSC 2002b,
2005; Mayo and Col 2002, 2005). These are:
F0.1 = 0.15
Fmax = 0.27
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).
Age-Structured Production Model Reference Points
MSY-based reference points were not revised during the preparation of
the current assessment. An age-structured production model (Sissenwine
and Shepherd 1987) was developed and applied by the Working Group on
Biological Reference Points (NEFSC 2002a) using data and results from
the 2001 assessment (Mayo et al. 2002). A complete description
of the approach is given in Mayo et al. (2002) and NEFSC (2002a). The
MSY-based reference points calculated using this method are:
MSY = 16,600 mt
SSBMSY = 82, 830 mt
FMSY =
0.23
CONCLUSIONS
Gulf of Maine cod spawning stock biomass has increased since the late
1990s from 15,900 mt in 1997 to 20,500 mt in 2004, but the stock remains
low relative to SSBMSY (82,830 mt). Fully recruited fishing
mortality declined to about 0.35 in 2000 and 2001, but has since increased
to 0.58 in 2004, 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). Thus, the stock remains in an overfished
condition and overfishing continues to occur.
The 1987 year class is the strongest in the VPA assessment period (1982-2004). Subsequent
year classes subsequent have been poor-to-average, except for the 1992,
1998, 2001 and 2003 cohorts. The 1993-1995, 1999-2000 and 2002
year classes are among the poorest in the VPA time series. Survival
ratios (R/SSB) declined after 1998 but survival increased substantially
with the appearance of the strong 2003 year class.
A retrospective pattern has existed in the VPA results for this stock,
but the pattern has reversed several times over the past decade. At present,
it appears that there is a slight tendency to under estimate fully recruited
F and over estimate biomass in the terminal year.
ACKNOWLEDGMENTS
We are indebted to the Groundfish Assessment Review Meeting (GARM II)
participants who provided a thorough, constructive review of this assessment.
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