Northeast Fisheries Science Center Reference Document 01-10
Assessment
of the Georges Bank Atlantic Cod Stock
for 2001
by Loretta
O'Brien and Nancy J. Munroe
NOAA/NMFS/NEFSC, 166 Water Street, Woods Hole MA 02543
Print
publication date July 2001;
web version posted July 25, 2001
Citation: O’Brien, L.; Munroe, N.J. 2001. Assessment of the Georges Bank Atlantic cod stock for 2001. Northeast Fish. Sci. Cent.
Ref. Doc. 01-10; 126 p.
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Abstract
This report presents an updated analytical assessment of the status
of the Georges Bank Atlantic cod Gadus morhua stock (NAFO Division
5Z and Subarea 6) for the period 1978-2000 based on analysis of USA
and Canadian commercial landings and effort data and research vessel
survey data through 2000. Estimates of 2000 fishing mortality and
spawning stock biomass, 2001 beginning year stock size, and the precision
of the fishing mortality and spawning stock biomass estimates are
presented.
Total commercial landings of Georges Bank cod in 2000 were estimated
at 9,189 mt, a 7% decrease from the 9,880 mt landed in 1999. The
USA fleet landed 83% of the total, and the Canadian fleet landed
the remaining 17%. Commercial landings per unit of standardized effort
(LPUE) declined to a record low in 1995, then increased in 1996 and
1997 and have remained relatively stable through 2000. Fishery-independent
surveys, conducted by the Northeast Fisheries Science Center, show
similar trends in both biomass and numbers of cod since 1982. The
2000 indices remain well below the long term average. Since 1989
recruitment indices of age 1 cod have been below the time series
average and among the lowest in the time series.
Spawning stock biomass declined from about 93,000
mt in the early 1980s to a record low of 18,000 mt in 1995 and has
since increased to 29,000 mt in 2000. Mean biomass exhibits similar
trends. Fishing mortality doubled between 1979 and 1985, increased
to a record high of 1.47 (71% exploitation rate) in 1994 and has since
declined to 0.22 (18% exploitation rate) in 2000. However, recruiting
year classes have been well below the long term average (16 million
fish) since 1991. Recent increases in stock biomass are attributed
to increased survival of adults rather than improved recruitment.
INTRODUCTION
This report presents an updated analytical assessment
of the Georges Bank cod Gadus morhua stock (NAFO Division 5Z and Subarea
6) for the period 1978-2000 based on analysis of commercial landings
and research vessel survey data through 2000. The life history of Georges
Bank cod and the history of the commercial fishery is described in
O'Brien (1999). An outline of the history of management is provided
in Table 1.
THE
FISHERY
Commercial Landings
The collecting and processing of the commercial fishery and landings
data has been conducted using two methods during the time series. Prior
to 1994, information of 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 from the interview
was used to augment the total catch information obtained from the dealer.
In 1994, a mandatory reporting system was initiated requiring anyone
fishing for or purchasing regulated groundfish in the Northeast to
submit either vessel trip reports (logbooks) or dealer reports, respectively
(Power et al. 1997 WP). Information on fishing effort (number of hauls,
average haul time) and catch location were now obtained from logbooks
submitted to NMFS by vessel captains instead of personal interviews.
Estimates of total catch by species and market category were derived
from mandatory dealer reports submitted on a trip basis to NMFS. Catches
by market category were allocated to stock based on a matched subset
of trips between the dealer and logbook databases. Both databases were
stratified by calendar quarter, port group and gear group to form a
pool of observations from which proportion of catch, by stock, could
be allocated to market category with 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. The USA landings for Atlantic cod for 1994-1996 were
derived for Eastern Georges Bank (statistical areas 560, 561, 562,
551, 552) and Western Georges Bank (statistical areas 520-526, 530,
537-539, 600-639) using the proration methodology described above.
The 1997-2000 data were also prorated using the same methodology, however,
the criteria for matching the data were modified and resulted in a
larger data set being available for proration (Wigley et al. 1998).
Total commercial landings of Georges Bank cod in 2000 were estimated
to be 9,189 mt, 7% lower than in 1999 (Table 2, Figures
1a and 1b). The USA fleets landed 83% (7,617 mt) of the total,
and the Canadian fleets landed the remaining17% (1,572 mt).
USA cod landings are generally highest in the second calender quarter
(April-June) and are taken predominantly from the western part (statistical
areas 521-522, 525-526, 537-539, and Subarea 6) of Georges Bank throughout
the year (Figure 2 and Figure 3).
Historically, landings from the eastern part (SA 561-562) of Georges
Bank were taken in the first and second calender quarter (January to
June). Since 1993, the contribution of landings from the eastern part
of Georges Bank has declined by more than 70% (Table
3). The Canadian fishery for Georges Bank cod opens in June, and
the majority of the landings are taken in the third calendar quarter
(July-September).
USA landings were taken primarily by otter trawl gear (62%)and line
trawl gear (18%) during 1994-1999. In 2000, otter trawl gear accounted
for the majority (62%) of the USA landings (Table
4). Canadian landings were taken primarily by the otter trawl (36%)
and long line (48%) fisheries during 1994-1999. In 2000, otter trawl
gear accounted for 34% and long line gear accounted for 51% of the
Canadian landings (Hunt and Hatt, 2001).
Cod landings from Georges Bank, categorized by size as ‘scrod'
(small) , ‘market' (medium), and ‘large', continued to be
dominated by ‘market' cod in both weight (60%) and number (54%)
in 2000 (Table 5). Historically, ‘market'
cod have accounted for 35-60% of the landings by weight.
Commercial Discards
Preliminary estimates of the weight of fish discarded on otter trawl
and gill net trips were derived for 1989-2000 using the Sea Sampling
Database. Discard ratios were estimated as the amount of cod discarded
to the amount kept for catch taken in the western part and the eastern
part of Georges Bank. In the otter trawl fishery discard ratios ranged
from 0.0 to 0.10 with less discarding occurring in the eastern part
than in the western part of Georges Bank (Appendix
1, Table 1a). In the gill net fishery discard ratios ranged from
0.0 to 0.19 but were predominantly less than 0.10 (Appendix
1, Table 1b).
Estimates of discards in the otter trawl and gill net fisheries were
also derived using the Vessel Trip Report (VTR) database for 1994-2000.
Discard ratios were estimated from a subset of trips (VTRs) that had
a history of recording discarded fish of any species (Appendix
1, Table 2). A trip limit of 2000 lb/day or 20,000 lb/trip was
implemented in August 1999 and remains in effect at the current time.
Comparisons to discard rates reported on VTRs prior to 1999 indicate
that discarding did increase in 1999 and 2000. Quarterly discard rates
prior to 1999 had been about 1% or less and in 1999 increased to as
much as 9% in the otter trawl fishery and 5% in the gill net fishery.
In 2000, discard rates were about 2-3 % for both fisheries (Appendix
1, Table 2).
Discard estimates were not included in the assessment due primarily
to the lack of data for 1978-1988. In addition, the available Sea Sampling
data from 1989-2000 are limited by both inadequate coverage of trips
and few biological samples.
Recreational Landings
Recreational cod landings during 1981-2000 ranged between 400 to 9,000
mt, accounting for 1-19% of the total landings (Table
6). Recreational landings were 1,056 mt in 2000, almost three times
the 356 mt landed in 1999. The 2000 recreational landings account for
10.4% of the total (total commercial + recreational) landings, the
highest since 1995.
A previous assessment that incorporated recreational landings in the
catch at age resulted in slightly elevated stock sizes with little
change in fishing mortality or spawning stock biomass estimates from
the VPA (O'Brien 1999). The 24th SARC recommended that recreational
catches not be included in the assessment analysis at that time because
1) the recreational catch at age is based on very few length samples
and may not fully characterize the recreational landings, 2) including
the recreational catch at age would require excluding the first three
years of the time series given the lack of recreational landings data
for 1978-1980, and 3) the minimal difference in estimates of fishing
mortality and spawning stock biomass observed in the terminal year
from comparable ADAPT formulations that had commercial catch at age
only vs. commercial plus recreational catch at age (NEFSC 1997) .
Sampling Intensity
Commercial Landings
The numbers of samples taken to characterize the length and age composition
of the USA and Canadian commercial cod landings from Georges Bank are
summarized in Table 7. Sampling intensity
was high in 2000 with 1 sample per 49 mt for the USA (Table
8) and 1 sample per 15 mt for the Canadian fishery. The average
number in each length sample was 79 fish for the USA and 194 fish for
Canada during 2000. Although overall sampling intensity was high, the
spatial and temporal pattern of sampling for USA landings resulted
in semi-annual pooling of quarterly samples for the ‘large' market
category. The sampling for USA landings from the eastern part of Georges
Bank (SA 561 and 562) was minimal in 2000 with a total of 2 ‘scrod'
samples. The distribution of sampling by market category (large:4%,
market:41%, scrod:54%) does not approximate the distribution of the
2000 landings (by number) as well as in previous years (Table
5).
Recreational Landings
Since 1981, 0.02% of the total recreational landings have been sampled
for both weight and length (0.1% of the USA commercial landings were
sampled for the same time period). During 1981-2000, the number of
fish sampled ranged from 0.01 to 0.06% of the total number landed.
In 2000, 0.01% of the fish landed were sampled, a sample of 15 fish.
Commercial Landings
at Age
The age composition of the 1978-1993 USA landings was estimated, by
market category, from length frequency and age samples pooled by calendar
quarter. Landed mean weights were estimated by applying the length-weight
equation:
ln Weight (kg,live) = -11.7231 + 3.0521 ln Length
(cm) ,
to the quarterly length frequency samples, by market category. Numbers
landed, by quarter, were estimated by dividing the mean weight into
the quarterly landings, by market category, and prorating the total
numbers by the corresponding market category sample length frequency.
Quarterly age-length keys were then applied to the numbers-at-length
to estimate numbers caught at age. Annual estimates of landings at
age were obtained by summing values over market category and quarter
(Table 9). Derivation of landings by quarter,
rather than by month, was performed since not all months had at least
two length frequency samples per market category (i.e., minimum desired
for monthly catch estimates).
The age composition of the 1994-1996 USA landings was also estimated,
by market category, from quarterly length frequency and age samples,
but in some years samples were pooled semi-annually due to an insufficient
number of samples within a quarter. The landings were dis-aggregated
into eastern (SA 561-562) and western Georges Bank (SA 521-522, 525-526,
537-539). The age composition of the USA landings from eastern Georges
Bank was estimated by applying USA length frequencies and combined
USA and Canadian age samples, while the age composition of the USA
landings from western Georges Bank was estimated by applying USA length
frequencies and age samples.
The age composition of the 1997-2000 USA landings was estimated in
a similar manner, however, due to the lack of length samples from eastern
Georges Bank, combined length frequencies were applied. The assumption
was made that length frequencies from eastern and western Georges Bank
would be similar, therefore, all length frequencies were combined to
characterize the eastern component of landings. In addition, for 2000
only, the otter trawl and gill net samples were pooled, and the handline
and longline samples were pooled and then applied to the corresponding
combined landings. The 1994-2000 landings-at-age was then derived as
described above for the 1978-1993 landings-at-age. The eastern and
western Georges Bank landings-at-age were combined to obtain the landings-at-age
matrix for USA Georges Bank cod landings for 2000 (Table
9). The USA eastern Georges Bank landings-at-age was included in
the Canadian assessment of cod in area 5Zj,m (Hunt and Hatt 2000).
Canadian landings-at-age data (Table 10)
from the Northeast Peak of Georges Bank (SA 551-552) were provided
by J. Hunt (DFO, St. Andrews, NB, pers. comm) for 2000. Canadian and
USA data were combined to produce a total landings-at-age matrix for
1978-2000 (Table 11). The USA fishery accounted
for 85% and 83% of the total landings by number and weight, respectively
in 2000.
Total commercial landings and USA landings in 2000 were dominated
by age 4 fish from the 1996 year class in both numbers and weight and
by the age 2 fish from the 1998 year class in numbers of fish (Table
12 and Figure 4). In the Canadian fishery
the landings were dominated by the 1996 year class in both weight and
numbers of fish (Table 12 and Figure
4).
Commercial Mean Weights
at Age
Mean lengths and weights at age for ages 1-10+ are summarized for
USA, Canadian, and total landings in Tables 9, 10 and 11.
There does not appear to be a consistent trend in mean weight by age
during the 23-year time series. The mean weight for age 2 fish in 2000
is the highest in the time series and may be due to the record high
number of samples taken for the ‘scrod' category. Variability
in mean weight of the older fish in recent years may be due to poorer
sampling in these years. Beginning year stock mean weights at age,
derived from catch mean weights at age (Rivard 1980), are presented
in Table 13 and Figure
5.
STOCK
ABUNDANCE AND BIOMASS INDICES
Commercial
Catch Rates
A general linear model (GLM) was applied to all USA interviewed otter
trawl trips landing cod from Georges Bank and South during 1978-1993
to derive standardized fishing effort and commercial landings-per-unit-effort
(LPUE) (O'Brien 1999; Mayo et al. 1994). Standardized fishing effort
and LPUE during 1994-2000 were estimated by applying the re-transformed
GLM coefficients (area, quarter, tonnage class, and depth) to the effort
estimate of all trips reporting cod landings in the Vessel Trip Reporting
(VTR) database (Table 14). Total standardized
or ‘raised' effort was calculated by dividing total USA landings
by the standardized LPUE (Table 15).
Nominal and standardized LPUE exhibit similar trends and, since 1985,
are almost equivalent (Table 15, Figure
6). Standardized LPUE peaked in 1980 at 2.9 mt/day fished and declined
steadily from 1982 to1987. LPUE increased slightly until 1990 and then
declined steadily until 1995. LPUE increased slightly in 1996 and 1997
and has remained relatively stable through 2000. LPUE is estimated
to be about 0.5 mt/day fished in 2000. Standardized raised effort and
nominal effort have similar trends in general, although effort trends
did diverge in 1989, 1991, and 1995 (Figure 7).
Raised effort more than doubled from 1978 to 1985, declined in 1986,
and then increased to historic high levels until 1991. Standardized
raised effort has since declined and in 2000 is similar to estimates
for the early 1980s.
Under the current management restrictions of days at sea (DAS), greater
mesh sizes, closed areas since December of 1994, mandatory logbooks
for collection of effort data, implemented in May 1994, and other management
measures, the 1994-2000 effort data may no longer be equivalent to
the historic 1978-1993 effort series. Additionally, the effort estimates
for 1994-2000 were derived from provisional data. The LPUE series was,
therefore, not used as an index of abundance in the subsequent calibration
of the VPA.
Research Vessel Survey
Indices
USA Surveys
NEFSC spring and autumn research vessel bottom trawl surveys have
been conducted off the Northeast coast of the USA since 1968 and 1963,
respectively (Azarovitz 1981). Indices of abundance (stratified mean
number per tow) and biomass (stratified mean weight per tow (kg)) were
estimated from both the spring and autumn surveys for Georges Bank
cod (strata 13-25) during 1963-2000 (Table 16).
All surveys were conducted with a ‘36 Yankee' trawl except for
spring surveys during 1973-1981 when a ‘41 Yankee' trawl was employed.
No adjustments were made for gear chnages, however, the indices were
adjusted for differences in fishing power of the Albatross IV and the
Delaware II, and for differences between catchability of BMV and polyvalent
doors, introduced in 1985. The fishing power coefficients of 0.79 and
0.67 and the door conversion coefficients of 1.56 and 1.62 were applied
to abundance and biomass indices, respectively (NEFSC 1991). The entire
time series for both spring and autumn was re- estimated in 2001 to
include any large tows that had previously been excluded, and these
indices were used in the calibration of the VPA. Standardized catch
per tow at age in number for NEFSC spring and autumn surveys and the
catch per tow at age for Canadian spring surveys are presented in Appendix
2: Tables 1 and 2.
NEFSC spring and autumn catch per tow biomass and abundance indices
show similar trends throughout the time series (Table
16, Figures 8-9). Survey biomass indices
were stable between 1963 and 1971 and then increased to a record high
in 1973. Biomass indices generally declined over the next two decades,
reaching record low levels between 1991 and 1994. The index increased
in 1995, and has fluctuated in recent years. The spring and autumn
indices indicate an opposite trend in biomass and a similar increasing
trend in numbers, in 2000. Both the spring and autumn biomass and abundance
indices remained well below average in 2000. Autumn survey abundance
indices for both ages 1 and 2 indicate above-average recruitment of
the 1965, 1966, 1971, 1975, 1977, 1980, 1985, and 1988 year classes
(Appendix 2: Table 1; Figure
10 and Figure 11). As 2 year old fish, the 1993 year class was above average.
The magnitude of an above-average year class has been declining over
time, particularly noticeable in the recruits at age 1 (Figure
12).
Canadian Surveys
Canadian research vessel bottom trawl surveys have been conducted
in the spring on Georges Bank since 1986. Survey abundance indices
have fluctuated and have generally declined during 1990-2001 (Appendix
2: Table 2, Figure 9). Abundance indices
for ages 1 and 2 indicate above average recruitment of the 1985, 1988,
and 1990 year classes and below average recruitment for the 1991 -
1998 year classes (Figure 13). In 1993 and
1994, the Canadian survey did not sample the western part of Georges
Bank (Canadian strata 5Z5 - 5Z7), therefore, the indices of stratified
mean number per tow at age in those years were not used in the calibration
of the VPA.
MORTALITY
Natural Mortality
Instantaneous natural mortality (M) of Georges Bank cod is assumed
to be 0.2 , the conventional value of M used for all Northwest Atlantic
cod stocks (Paloheimo and Koehler 1968, Pinhorn 1975, Minet 1978)
Total Mortality
Estimates of instantaneous total mortality (Z) were derived from both
NEFSC spring and autumn survey catch per tow indices (Appendix 2: Table
1). Total mortality was estimated with spring data using Heincke's
method ( Ricker 1975) as:
ln (
age 4+
for years i to j/ age 5+ for
years i+1 to j+1).
Total mortality was estimated with autumn data as :
ln ( age 3+
for years i-1 to j-1/ age 4+
for years i to j).
A three year moving average was fit to each survey series (Figure
14a-14b) and also to the sequential spring and autumn mortality
estimates (Figure 14c). The estimates are
highly variable throughout the time series, although there appears
to be a trend of increasing Z from the mid-1970s to the mid-1990s.
ESTIMATES
OF STOCK SIZE AND FISHING MORTALITY
Virtual Population Analysis
Calibration
The ADAPT calibration method (Parrack 1986, Gavaris 1988, Conser and
Powers 1990) was used to derive estimates of instantaneous fishing
mortality (F) in 2000 and beginning-year stock sizes in 2001. The landings
at age data used in the VPA consisted of combined USA and Canadian
commercial landings from 1978-2000 for ages 1-9 with a 10+ age group
(Table 11). The indices of abundance used
to calibrate the VPA included the NEFSC 1978-2000 spring survey indices
for ages 1-8, the Canadian 1986-1992 and 1995-2001 spring survey indices
for ages 1-8, and the NEFSC 1977-2000 autumn survey indices for ages
0-6 (Appendix 2: Tables 1 and 2). The NEFSC
spring survey was dis-aggregated into two series based on the use of
the Yankee #36 or #41 trawls. The NEFSC employed the #41 trawl during
1973 to 1981. The spring indices were split into a index series for
1978-1981 for the #41 trawl and a series for 1982-2000 for the #36
trawl. The autumn survey indices were lagged forward one age and one
year to match cohorts in the subsequent year. The transformed (ln)
observed survey indices, standardized to the mean, generally show similar
trends between surveys (Figure 15).
The base ADAPT formulation provided stock size estimates for ages
1-8 in 2001 and corresponding unweighted F estimates for ages 1-7 in
2000. Assuming full recruitment at age 4, the unweighted F on ages
8 and 9 in the terminal year was estimated as the average of the F
on ages 4-8. The unweighted F on age 9 in all years prior to the terminal
year was derived from weighted estimates of Z for ages 4-9. For all
years, the unweighted F on age 9 was applied to the 10+ age group.
Spawning stock size estimates were derived by applying pooled maturity
ogives for 1978-1981, 1982-1985, 1986-1989, 1990-1993, 1994-1996, and
1997-2000 (Table 17) derived from NEFSC spring
research survey data using methodology described in O'Brien (1990).
Due to the insufficiency of the annual number of samples, data for
adjacent years that had similar annual median maturity at length and
age were pooled to derive a more representative ogive.
The final ADAPT calibration results are presented in Appendix
3 for estimates of F, stock size, and SSB at age and are summarized
in Table 17. Estimates of stock size were
more precise for ages 2-8, with CVs ranging from 0.31 to 0.37, than
for age 1 (CV=0.76). The residual patterns of the indices did not
show any strong trends for the four surveys (Figure
16).
Average fishing mortality (ages 4-8) in 2000 was estimated at 0.22
(18% exploitation), a decrease from the 1999 estimate of 0.45 (33%
exploitation) (Table 17, Figure
17a). The 2000 estimate of SSB was 29,000 mt, an increase of about
6% from the 1999 estimate (Table 17, Figure
18).
Since 1978, recruitment at age 1 has ranged from 3 million (1997 year
class) to 43 million (1985 year class). The 2000 year class is estimated
to be about 1.7 million fish at age 1, well below the long term average
of 16 million fish and the lowest in the time series. The most recent
above average year class occurred in 1990 (18 million age 1 fish).
The 1996 year class (10 million age 1 fish), although below average,
is the strongest since 1990. The 1994 and 1997 year classes are the
poorest of the 23-year time series (Table 17, Figure
18). The survival ratio of recruits to spawning stock biomass indicates
a trend similar to the estimates of recruits at age 1, however, after
1991 the relationship is not as apparent (Figure
19). In the last decade, low recruitment at age 1 has been associated
with low SSB in contrast to the earlier time period (1978-1990) when
higher recruitment was realized from higher SSB (Figure
20).
In the terminal year, fishing mortality estimates from the VPA indicate
a pronounced domed partial recruitment with the highest F occurring
on age 3 fish, the 1997 year class. This pattern of domed partial recruitment.
although not as strong and more variable, appears to have occurred
since 1994. Alternative ADAPT runs that estimated age 9 as well as
ages 1-8 did not influence this pattern. The pattern in the terminal
year may be influenced by having only two surveys (DFO spring ages
1-9 and NEFSC autumn ages 1-6 ) in the calibration instead of three,
and only one index for the larger age 7 and age 8 fish.
The variable domed partial recruitment pattern since 1994 may be influenced
by both a shift in fishing pattern and by a shift in the catch at age
matrix. Since 1994 , year round closures have been in effect for both
Area I and Area II. Fishing patterns may have shifted such that the
older fish are no longer as available to the fishery as they would
have been before the closure. Also, since 1994 sampling for larger
fish has not been adequate (Table 8) and the
lack of samples may have caused a biased characterization of the landings
to the smaller ‘market' and ‘scrod' fish in the catch at
age.
In addition to the estimate of unweighted F for ages 4-8 , an alternative
F weighted by population size for ages 3-8 is also presented (Figure
17b).
Precision of F and Stock
Biomass Estimates
A conditional non-parametric bootstrap procedure (Efron 1982) was
used to evaluate the uncertainty associated with the estimates of fishing
mortality and spawning stock biomass from the final VPA. One thousand
bootstrap iterations were performed to estimate standard errors, coefficients
of variation (CVs), and bias for age 1-8 stock size estimates at the
start of 2001, the catchability estimates (q) for each index of abundance
used in calibrating the VPA, and the F at ages 1-7 in 2000 (Appendix
4).
The bootstrap results indicate that stock sizes were well estimated
for ages 2-8 with coefficients of variation (CVs) varying between 0.25
and 0.28. Stock size for age 1 was not well estimated with a CV of
0.83. The CVs for the catchability coefficients for all indices ranged
between 0.11 and 0.31. The fully recruited F for ages 4+ was well estimated
with a CV=0.13 . The bootstrap estimate was almost equivalent to the
non-linear least squares (NLLS) estimate (Appendix
4). The distribution of the 2000 F estimates, derived from 1,000
bootstrap iterations, ranged from 0.16 to 0.39. There is an 80% probability
that the F in 2000 is between 0.18 and 0.25 (Figure
21).
The spawning stock biomass was reasonably well estimated (CV=0.10)
and slightly higher than the NLLS estimate of 29,000 mt (Appendix
4). The distribution of the 2000 spawning stock biomass estimates,
derived from the 1000 bootstrap iterations, ranged from 22,000 to 40,000
mt (Figure 22). There is an 80% probability that the 2000 SSB is between
25,000 and 32,000 mt (Figure 22).
The distribution of the 2000 mean biomass estimates, derived from
1000 bootstrap iterations, ranged from 30,000 to 54,000 mt (Figure
23). There is a 80% probability that the mean biomass in 2000 was
between 33,000 mt and 43,000 mt.
Retrospective Analysis
A retrospective analysis was performed to evaluate how well the current
ADAPT calibration would estimate recruits at age 1, spawning stock
biomass, and fishing mortality for the six years prior to the current
assessment, 1996-2000. Convergence of the estimates generally occurs
after about six years (Figures 24A-C). With
the exception of the1998 value, the retrospective analysis indicates
a pattern of underestimating the recruits at age 1 (Figure
24A). Estimates of SSB are consistently overestimated, (Figure
24B) and estimates of fishing mortality (F) are consistently underestimated
(Figure 24C). Factors influencing the retrospective
pattern may include mis-reporting of catch, immigration or emigration,
an unrepresentative estimate of natural mortality, and mis-specification
of the model.
Fishing mortality in 2000 was projected to be 0.22 ( = status quo
F in 1999) and landings were projected to be 7,658 mt (NDWG, NESAW
2001). The current assessment estimated F in 2000 to be 0.22 and total
landings were 9,189 mt (USA landings were 7,617 mt).
BIOLOGICAL
REFERENCE POINTS
Yield and Spawning Stock
Biomass per Recruit
Yield, total stock biomass, and spawning stock biomass per recruit
were estimated using methodology of Thompson and Bell (1934). The input
data and the results presented were derived in the 1998 assessment
(O'Brien and Cadrin 1999). Estimates were based on arithmetic means
of the 1995-1997 catch mean weight at age and stock mean weight at
age and the 1994-1997 maturity ogive. A partial recruitment (PR) vector
was calculated as the geometric mean of the 1994-1997 F estimates from
the final VPA to reflect the change in mesh regulations in 1994. The
final exploitation pattern was derived by dividing the PR by the geometric
mean of the unweighted F for ages 4-8 and smoothed by applying full
exploitation at ages 4 and older.
Input values for the yield-per-recruit analysis are provided in Table
18, and results of the analysis are provided in Table
18 and Figure 25. The resulting biological
reference points were F0.1= 0.18 and Fmax = 0.34.
Projections
Short term projections will not be presented in this assessment. These
analyses will be performed at a later date by the Multispecies Monitoring
Committee (MMC) of the New England Fisheries Management Council.
SFA Control Rule
The Sustainable Fisheries Act (SFA) control rule for Georges Bank
cod is based on BMSY (108,000 mt) and states that when the stock biomass
is between 1/4 and ½ BMSY (27,000-54,000 mt), the threshold
mortality rate is defined by a five year rebuilding time period, and
if the stock is between 1/2 BMSY and BMSY the rebuilding time period
is10 years. In 2000, mean biomass is estimated to have been about 38,500
mt, less than ½ BMSY. Applying the 2000 mean biomass to the
target control rule indicates that the stock should be fished at a
biomass weighted F of about 0.11 (Figure 26).
CONCLUSIONS
The Georges Bank cod stock remains at a low biomass level. Biomass
indices derived from research surveys indicate that the stock remains
below the long term average of the 37 year time series. Fishing mortality
(ages 4-8) declined from record-high levels in 1993 and 1994 (1.1,
1.4) to 0.22 in 2000. Spawning stock biomass declined from about 93,000
mt in the early 1980s and reached a record-low of 18,000 mt in 1995.
As fishing mortality has declined, the SSB has gradually increased,
primarily due to somatic growth, but was still near record-low size
(29,000 mt) in 2000. Trends in mean biomass have been similar to the
trends in SSB. Recruiting year classes have been well below the long
term average (16 million fish) since 1991. The 2000 year class is estimated
to be about 2 million fish, about 13% of the long-term average.
Accounting for the estimation uncertainty associated with SSB (29,000
mt), mean biomass (39,000 mt), and F (0.22) estimates, there is an
80% probability that SSB was between 25,000 and 33,000 mt, that mean
biomass was between 33,000 mt and 43,000 mt, and that F was between
0.18 and 0.25 in 2000. Retrospective analysis indicates a pattern of
inconsistencies in which estimates of SSB in the last year of the VPA
are greater than the converged estimates of SSB. Similarly, F estimates
in the last year of the VPA are less than the converged estimates of
F.
Recovery of the stock will depend on further reductions in fishing
mortality as well as improved recruitment.
ACKNOWLEDGMENTS
I appreciate the constructive review of all the members of the Transboundary
Assessment Working Group: R. Brown, S. Correia, S. Gavaris, B. Hatt,
J.J. Hunt, R. Mayo, B.Overholtz, P. Perley, K. Stone, L. Van Eeckhaute,
Susan Wigley and of all participants of the TRAC . I also thank J.J
Hunt and B.Hatt for providing statistics for the Canadian fishery.
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