Northeast Fisheries Science Center Reference Document 06-10
Assessment of the Georges Bank
Atlantic Cod Stock
for 2005
by Loretta O'Brien, Nina
Shepherd, and Laurel Col
National Marine Fisheries Service, 166 Water Street, Woods Hole MA 02543-1026
Print
publication date June 2006;
web version posted June 27, 2006
Citation: O’Brien L, Shepherd N, Col L. 2006.
Assessment of the Georges Bank Atlantic Cod Stock for 2005. Northeast
Fish. Sci. Cent. Ref. Doc. 06-10; 148 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 1978-2004 based on analysis of USA
and Canadian commercial landings data and research vessel survey data
through 2004. Estimates of 2004 fishing mortality and spawning
stock biomass, 2005 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 2004 were estimated
at a record low 4,583 mt, a 42% decrease from the 7,963 mt landed in
2003. The USA fleet landed 76% of the total, and the Canadian
fleet landed the remaining 24%. Fishery-independent surveys conducted
by the Northeast Fisheries Science Center show similar trends in both
biomass and numbers of cod since 1982. The 2004 indices remain
below the long term average. Autumn recruitment indices of age
1 cod have been below the time series average since 1989 and among
the lowest in the time series.
Spawning stock biomass declined from about 93,000 mt in 1980 to a
record low of 17,340 mt in 1995, and had increased to 30,000 mt by
2001 primarily due to the growth of the 1996 and 1998 year classes.
The 2004 estimate of SSB was 22,564 mt, a decrease of 25% from 2001.
Fishing mortality doubled between 1979 and 1985, increased to a record
high of 1.5 (72% exploitation rate) in 1994, and has since declined
to 0.24 (19% exploitation rate) in 2004. The 2004 year class
(10.4 million age 1 fish) was strong, but below the time series average
(14.7 million age 1 fish). The 2003 year class (21.2 million
age 1 fish), however, is the first above average year class since 1989. This
year class, and continued low exploitation with minimal discarding,
offer a realistic opportunity for the stock to recover toward the SSBMSY of
217,000 mt.
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 1978-2004 based on analysis of commercial landings
and research vessel survey data through 2004. This stock was last
assessed and peer reviewed in October 2002 (O’Brien et al. 2002)
. Landings were 12,769 mt in 2001 and fully recruited F (ages
4-8, unweighted average) was estimated to be 0.38 in 2001, the second
lowest F in the time series (1978-2001). Spawning stock biomass
was 29,170 mt in 2001 and continued the increasing trend from the record
low estimate of 17,375 mt in 1995. Since 1991, recruiting year
classes were all below the long term average and the 2000 and 2001 year
classes were the lowest in the time series. The NEFSC spring and
autumn bottom trawl survey indices continued to remain near record low
values. Autumn recruitment indices for age 2 fish from the 1994
through 1998 year classes were all below the time series (1963-2000)
average. The most recent above average autumn recruitment index occurred
in 1993.
The current assessment presented here is considered an update and the
methodology has remained the same as in the 2002 assessment (O’Brien et
al. 2002) . The life history of Georges Bank cod and the history
of the commercial fishery are described in O'Brien (1999) . An
outline of the history of management is provided in Appendix
1, Table 1 and Appendix 1, Table 2.
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) . 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 1997-2004 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). In
May 2004, dealers started submitting the dealer report electronically.
Total commercial landings of Georges Bank cod in 2004 were estimated
to be a record-low 4,583 mt, 42% lower than in 2003 (Table
1, Figure 1). The USA fleet landed 76%
(3,471 mt) of the total, and the Canadian fleet landed the remaining
24% (1,112 mt).
USA cod landings are generally highest in the second calendar quarter
(April-June) and are taken predominantly from the western part (statistical
areas [SA] 521-522, 525-526, 537-539, and Subarea 6) of Georges Bank
throughout the year (Figure 2 and Figure 3). The
majority of the landings from the eastern part (SA 561-562) of Georges
Bank are taken in the first and second calendar quarter (January to June; Table
2). 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 (69%) and gillnet gear (16%) during
2000-2003. In 2004, otter trawl gear
accounted for the majority (80%) of the USA landings (Table
3). Canadian
landings were taken primarily by the longline (53%) and otter trawl (34%)
fisheries during 2000-2003. In 2004, otter trawl gear accounted
for 55% and longline gear accounted for 30% of the Canadian landings
(Hunt et al. 2006).
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 (63%) and number (69%) in 2000 (Table
4). Historically,
'market' cod have accounted for 37-67% of the landings by weight.
Commercial Discards
Preliminary estimates of the weight of fish discarded on otter trawl
and gillnet trips were derived for 1989-2004 using the Sea Sampling Database. Discard
ratios were estimated as the amount of cod discarded to the amount kept
for catch taken for all of Georges Bank (SA 521-522, 525-526, 537-539,
561-562). Discard to kept ratios ranged from 0.001 to 0.12 in the
otter trawl fishery and from 0.001 to 0.193 in the gillnet fishery, but
were predominantly less than 0.10 in both fisheries (Appendix
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 300 and 8,600
mt, accounting for 1-19% of the total landings (Table
5). Recreational
landings were 346 mt in 2004 and accounted for 2.6% of the total (total
commercial + recreational) landings.
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) there was a 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 (Northeast
Fisheries Science Center 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 6 and Table 7. Sampling intensity was very
high in 2004 with 1 sample per 27 mt for the USA (Table
7) and 1 sample
per 9 mt for the Canadian fishery. The average number in each length
sample was 70 fish for the USA and 141 fish for Canada during 2004. 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 'scrod' market category, and the third and fourth
quarter for the ‘medium’ and ‘large’ market category.
Recreational Landings
During 1981-2003, the number of fish sampled ranged from 0.01 to 0.06%
of the total number landed. In 2004, 0.11% of the fish landed were
sampled (a total of 190 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 landed at age. Annual estimates of landings at age were
obtained by summing values over market category and quarter (Table
8). 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 disaggregated
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 1997-2004 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 gillnet samples were pooled, and the
handline and longline samples were pooled and then applied to the corresponding
combined landings. The 1994-2004 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
8). The USA eastern Georges Bank landings-at-age was included in the Canadian assessment
of cod in area 5Zj,m (Hunt et al. 2006).
Canadian landings-at-age data (Table 9) from the Northeast Peak of Georges
Bank (SA 551-552) were provided by J. Hunt (DFO, St. Andrews, NB, pers.
comm) for 2004. Canadian and USA data were combined to produce
a total landings-at-age matrix for 1978-2004 (Table
10). The USA fishery
accounted for 75% and 76% of the total landings by number and weight,
respectively in 2004.
Total commercial landings and USA landings in 2004 were dominated in
numbers by age 3 fish from the 2001 year class and in weight by age 6
fish from the 1998 year class (Table
11, Figure
4). In the Canadian
fishery the landings were dominated by the 1998 year class in both weight
and numbers of fish (Table
11, 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 8-10. 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 2003 and 2004 are the highest
in the time series and may be due to the increase in mesh size in 2002
and the high sampling intensity. Variability in mean weight of the older
fish in recent years may be due to fewer older fish (age 10+) being landed. Beginning
year stock mean weights at age, derived from catch mean weights at age
(Rivard 1980) are presented in Table 12 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. Total standardized or "raised" effort
was calculated by dividing total USA landings by the standardized LPUE
(O'Brien and Munroe 2001).
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, effort data collected since 1994 may no longer be equivalent
to the historic 1978-1993 effort series. Additionally, the effort
estimated since 1994 has been derived from provisional data. Given these
uncertainties the effort data was not analyzed for 2001-2004. The
LPUE series has not been used as an index of abundance in the calibration
of the VPA since 1993.
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 1963-2004 (Table 13). All
surveys were conducted with a Yankee #36 trawl except for spring surveys
during 1973-1981, when a Yankee #41 trawl was employed. No adjustments
were made for gear changes; 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 (Northeast Fisheries Science
Center 1991) . The entire time series for both spring and autumn
was re-estimated in 2001 (O'Brien and Munroe 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
3, Table 1.
NEFSC spring and autumn catch-per-tow biomass and abundance indices
show similar trends throughout the time series (Table
13, Figure 6 and
Figure 7). Survey biomass indices were relatively stable between 1963
and 1971, 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 indices have improved slightly since 1994
and have fluctuated with no trend. Both the spring and autumn biomass
and abundance indices remained below average in 2004. 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 3, Table 2; Figure
8 and Figure 9). The 2003 year class
at age 1 is the strongest year class since the 1992 year class (Figure
10). The magnitude of an above-average year class has been
declining over time, particularly noticeable in the recruits at age 1
(Figure
10).
Canadian
Surveys
Canadian research vessel bottom trawl surveys have been conducted during
February on Georges Bank since 1986. Survey abundance indices for
age 1 indicate a strong year class in 1996 and above average year classes
in 1998 and 2003. Age 2 indices have fluctuated and generally declined
during 1990-2003 (Appendix 3, Table 2 ; Figure
11). Indices for
2005 were not available due to vessel problems encountered during the
survey (J. Hunt. pers. comm.). Abundance indices for ages 1 and
2 indicate above-average recruitment of the 1985, 1988, and 1990 year
classes (Figure 11). In 1993, 1994, and 2005,
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
3, Table
1 and Appendix 3, Table 2). Total mortality was estimated using Heincke=s method
(Ricker 1975) with spring data as:
ln (Σ age 4+ for years i to j/ Σ age 5+ for
years i+1 to j+1) ,
and with autumn data as:
ln (Σ age 3+ for years i-1 to j-1/ Σ age 4+
for years i to j).
A three-year moving average was fit to each survey series (Figure
12a-12b)
and also to the sequential spring and autumn mortality estimates (Figure
12c). 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 in a Virtual Population Analysis (VPA) to derive
estimates of instantaneous fishing mortality (F) in 2004 and beginning-year
stock sizes in 2005. The landings at age data used in the VPA consisted
of combined USA and Canadian commercial landings from 1978-2004 for ages
1-9 with a 10+ age group (Table 10). The indices of abundance used
to calibrate the VPA included the NEFSC 1978-2005 spring survey indices
for ages 1-8, the Canadian 1986-1992 and 1995-2004 spring survey indices
for ages 1-8, and the NEFSC 1977-2004 autumn survey indices for ages
0-6 (Appendix 3). The NEFSC spring survey was disaggregated 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 1982-2005 for the #36 trawl. The autumn survey indices
were shifted forward one age and one year to match cohorts in the spring
survey in the subsequent year. The transformed (ln) observed survey
indices, standardized to the mean, generally show similar trends between
surveys (Figure 13).
Comparative VPA Calibrations
Since the last assessment in 2002 (O’Brien et al. 2002)
, modifications have been made to the VPA software. In addition,
revisions occurred in the USA 2000 and 2001 age samples and the Canadian
commercial catch at age for 2001. The VPA from the 2002 assessment (O’Brien et
al.2002) was recalibrated with each of these modifications to determine
any effects on the estimates of F and stock size in 2001. The
VPA was first run using the new NFT software, then each change was sequentially
added to the next VPA calibration (Table 13). Comparison of the
new NFT software results vs. FACT software results did not show any substantial
change in the original estimates, and neither did the upgraded age data,
or the revised Canadian catch-at-age data (Table 14). Based on
these results, the revised NFT software and the updated data were used
for the final VPA calibration for terminal year 2004.
Final VPA
The base ADAPT formulation provided stock size estimates for ages 1-8
in 2005 and corresponding F estimates for ages 1-7 in 2004. Assuming
full recruitment at age 4, the F on ages 8 and 9 in the terminal year
was estimated as the average of the F on ages 4-7. The F on age
9 in all years prior to the terminal year was derived from weighted estimates
of Z for ages 4-8. For all years, the 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,
1990-1994, 1995-1997, 1998-2000, 2001-2002, and 2003-2004 (Table
15)
derived from NEFSC spring research survey data using methodology described
in O'Brien (1990) . Due to insufficient numbers of fish at age
in annual 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
4 for
estimates of F, stock size, and SSB at age, and are summarized
in Table 15. Estimates of stock size were more precise for ages
2-8, with CVs ranging from 0.29 to 0.46, than for age 1 (CV=0.57). The
residual patterns of the indices did not show any strong trends for the
four surveys (Figure 14).
Average fully-recruited fishing mortality (ages 4-8) in 2004 was estimated
at 0.24 (19% exploitation), a decrease from the 2003 estimate of 0.39
(30% exploitation; Table 15, Figure
15). In the terminal year,
fishing mortality estimates from the VPA do not indicate a pronounced
domed partial recruitment as observed in previous assessments (O’Brien et
al. 2002 , O'Brien and Munroe 2001) . The pattern of domed
partial recruitment that had started in 1994 in previous assessments
is not as apparent in the present assessment. This may be due to
the improved sampling of the ‘large’ market category during
2001-2004 compared to the minimal samples taken during 1994-1999 (Table
7). The lack of samples during 1994-1999 may have caused a
biased characterization of the landings to the smaller "market" and "scrod" fish
in the catch at age.
Spawning stock biomass declined from about 93,000 mt in 1980 to a record
low of 17,340 mt in 1995. SSB slowly increased to 30,000 mt in
2001 primarily due to the growth of the 1996 and 1998 year classes.
The 2004 estimate of SSB was 22,564 mt, a decrease of 25% from 2001
-- the last year the 1996 year class dominated the SSB. The 2004 estimate
is an increase, however, of 30% from the record low 1995 estimate (Table
15, Figure 16).
Since 1978, recruitment at age 1 has ranged from 2 million (2002 year
class) to 43 million (1985 year class). The 2004 year class is
estimated to be about 10.4 million fish at age 1, below the long term
(1978-2004) average of 14.7 million age 1 fish. The 2003 year class,
however, is estimated to be 21.2 million age 1 fish, well above the long
term average. The previous above average year class occurred in
1990 (17.8 million age 1 fish). The 1998 year class (12.8 million
age 1 fish), although below average, was the strongest since 1990. The
1994, 2001, and 2002 year classes are the poorest of the 28-year time
series (Table 15, Figure 16).
The survival ratio of recruits to spawning stock biomass initially indicates
a trend similar to the estimates of recruits at age 1; however, after
1991 the relationship is not as apparent (Figure 17). In
the last decade, low recruitment at age 1
has generally been associated with low SSB in contrast to the earlier
time period (1978-1990) when higher recruitment was realized from higher
SSB (Figure 18).
Precision of F and Stock Biomass Estimates
A conditional non-parametric bootstrap procedure (Efron 1983) 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 2005, the catchability estimates (q) for each index of abundance used
in calibrating the VPA, and the F at ages 1-7 in 2004 (Appendix
5).
The bootstrap results indicate that stock sizes were well estimated
for ages 2-8 with coefficients of variation (CVs) varying between 0.27
and 0.45. Stock size for age 1 was not well estimated with a CV
of 0.90. The CVs for the catchability coefficients for all indices
ranged between 0.09 and 0.83. The fully recruited F for ages 4+ was well
estimated with a CV=0.18. The distribution of the 2004 F estimates, derived
from 1,000 bootstrap iterations, ranged from 0.13 to 0.35 (Figure
19). The
spawning stock biomass was reasonably well estimated (CV=0.13) and slightly
higher than the NLLS estimate of 22,564 mt (Appendix
5). The distribution
of the 2004 spawning stock biomass estimates, derived from the 1000 bootstrap
iterations, ranged from 15,000 mt to 33,000 mt (Figure
19).
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 five years prior to the current assessment,
1999-2003. Convergence of the estimates generally occurs after
about six years (Figures 20a-c). The retrospective analysis indicates
a pattern of overestimating the recruits at age 1 in the recent years,
and then underestimating the recruits in the earlier years (Figure
20a). There
is no retrospective pattern for SSB estimates in the recent years, but
there was a pattern of overestimation for 1996-1999 (Figure
20b). Estimates
of fishing mortality (F) are consistently underestimated (Figure
20c). Factors
influencing the retrospective pattern may include mis-reporting of catch,
immigration or emigration; an unrepresentative estimate of natural mortality;
mis-specification of the model; and inadequate sampling that does not
properly characterize the landings.
BIOLOGICAL
REFERENCE POINTS
Yield and Spawning Stock
Biomass per Recruit Biological Reference Points
Yield and spawning stock biomass biological reference points (BRPs)
were not re-estimated during this assessment. The Working Group
on Re-Evaluation of Biological Reference Points did, however, re-estimate
the BRPs with input data (Table 16) derived in the 2001 assessment using
the methods of Thompson and Bell (1934) (NEFSC 2002). Input
data for catch weights (ages 1-10+) and stock weights (ages 1-9) were
derived from the long term average weight during 1978-2000 (O’Brien
and Munroe 2001). Stock mean weights for ages 10+ were derived
from an expanded age structure out to age 18 (oldest age observed in
survey) at F = F40% = 0.167 and M= 0.2. The mean
weights for ages 10 to 18 were estimated from the length- weight equation
(O’Brien and Munroe 2001): ln Weight (kg, live) = -11.7231 + 3.0521
ln Length (cm). The mean length at ages 10-18 were derived from
the linear regression of length vs. ln (age) using the 1978-1997 commercial
length sample data. The partial recruitment (PR) was based on a
normalized geometric mean of 1996-1999 fishing mortality and the maturity
ogive was from the 2001 assessment (O'Brien and Munroe 2001) .
The newly estimated biological reference points for F0.1=0.169,
Fmax = 0.331, and F40% = 0.167 (Table
16, Figure 21) are slightly lower than those reported in O’Brien and Munroe
(2001). These BRPs were applied in the 2002 GARM assessment (O’Brien et
al. 2002) , and in the current assessment.
Age-structured Production
Model Reference Points
Maximum sustainable yield (MSY) reference points were derived from an
age-structured production model (Sissenwine and Shepherd 1987) using
data and results from the 2001 assessment (O'Brien and Munroe 2001) . The
Working Group on Re-Evaluation of Biological Reference Points developed
the reference points using a Beverton-Holt stock recruit relationship
with an assumed prior for the unfished recruitment and yield- and spawning
stock-per-recruit information (NEFSC 2002.) . The model provided
the following reference points:
FMSY = 0.175,
MSY = 35,200 mt and
SSBMSY = 217,000 mt.
The MSY includes commercial landings only and does not include recreational
landings or discards.
Projections
Short term projections will not be presented in this assessment. These
analyses will be conducted by Plan Development Team (PDT) of the New
England Fisheries Management Council. A comparison of the projections
conducted for Amendment 13 and the current bootstrap estimates of F and
SSB are presented in Figure 22.
CONCLUSIONS
Georges Bank Atlantic cod are overfished, and overfishing is occurring.
Biomass indices derived from research surveys indicate that the stock
remains below the long-term average of the 43-year time series. Fishing
mortality has been steadily declining since 1997, except for a slight
increase in 2001, and is currently at the lowest exploitation in the
time series. Spawning stock biomass reached a record low in 1995
and slowly increased, due to the growth of the 1996 and 1998 year classes;
however, since 2001 SSB has been declining. The 2002-2004 F trajectory
is less than that projected for A13 and the SSB is slightly higher than
the A13 projection. Catch during 2002-2004 was also less than the A13
projection. Retrospective analysis indicates a pattern in which estimates
of F in the last year of the VPA are less than the converged estimates
of F.
The 1999 and 1998 year classes account for the majority of the USA catch
and the 1998 year class accounts for the majority of the Canadian catch
in 2004. The 1998 (12.8 million age 1 fish) year class, while below
the long term average (14.7 million age 1 fish), represents the strongest
year class since the last above-average year class that occurred in 1990
(17.8 million age 1 fish). The 1994, 2000, 2001, and 2002 year classes
are the lowest in the time series. The 2003 (21.2 million age 1
fish) year class is the first above average year class since 1990, and
recruited to the fishery during 2005.
The NEFSC and DFO survey biomass and abundance indices fluctuated during
2002 to 2005; however, all the indices continue to remain below the long
term average. The most recent NEFSC surveys indicate that the 2003
year class may be similar in size to the 1998 year class, and the DFO
spring survey indicates that the year class is above average.
The lack of strong recruitment in the last decade suggests that recovery
of this stock will be largely dependent on maintaining reduced fishing
mortality in the near term and husbanding the strong 2003 year class
-- and potentially the 2004 year class -- to increase SSB.
ACKNOWLEDGMENTS
We appreciate the constructive review of all the members and participants
of the GARM. We thank J.J. Hunt and B. Hatt for providing data for the
Canadian fishery, and Tom Nies for the most recent NEFMC management measures.
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