Reference Document Home | Publications Home
CONTENTS
Introduction
The Fishery
Stock Abundance Indices and Life History Parameters
Estimates of Stock Size and Fishing Mortality
Georges Bank Haddock Stock Status
Conclusions
References
Appendix. Evaluation of accuracy and precision of US haddock aging during 20042005 (PDF)

Northeast Fisheries Science Center Reference Document 06-11

Stock Assessment of Georges Bank Haddock, 1931-2004

Jon Brodziak, Michele Traver, Laurel Col and Sandy Sutherland
National Marine Fisheries Service, 166 Water Street, Woods Hole MA 02543-1026

Web version posted July 5, 2006

Citation: Brodziak J, Traver M, Col L, Sutherland S. 2006. Stock assessment of Georges Bank haddock, 1931-2004. Northeast Fisheries Science Center Ref. Doc. 06-11; 114 p.

Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, the Northeast Fisheries Science Center completed both technical and policy reviews for this report. These predissemination reviews are on file at the NEFSC Editorial Office.

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ABSTRACT

The Georges Bank haddock stock assessment is updated through 2004. Assessment results show that fishing mortality has remained below the overfishing threshold since 1995 and that spawning biomass has increased substantially in recent years, but still remains below 0.5 of BMSY. As such, the stock is currently overfished but is not experiencing overfishing. Recruitment has increased markedly in recent years, with two strong year classes (1998 and 2000) currently providing much of the fishery yield. The 2003 year class is one of the largest ever recorded and has the potential to rebuild the stock in a few years if fishing mortality remains low. Decreases in mean weight at age during 2001-2004 may be due to density-dependent growth or to changes in trophic interactions. Overall, the Georges Bank haddock stock is rapidly recovering from the severely depleted condition observed in the mid-1990s.


1. INTRODUCTION

The Georges Bank haddock stock has been commercially exploited since the 1880s (German 1987).  Fishing intensity on the stock increased during the 20th century as harvest technology improved and fishing effort increased (Hennemuth and Rockwell 1987, Murawski et al. 2002).  Prior to the mid-1990s, Georges Bank haddock had been overfished by modern standards for several decades (Brodziak and Link 2002).  The stock experienced significant long-term declines in spawning biomass and recruitment since the 1960s (Brodziak et al. 2001) and collapsed in the early 1990s (NEFSC 1994).  Fishery management measures implemented since 1994 have included large year-round closed areas, restrictions on fishing effort, and increases in trawl mesh size (Fogarty and Murawski 1998) and have decreased fishing mortality and led to marked increases in stock size (Brodziak et al. 2002).  Further information on the Georges Bank haddock fishery, stock assessment, and biology is available in Brodziak et al. (2002), Brodziak (2005), Brodziak and Legault (2005), Brown and Munroe (2000), Clark et al. (1982), German (1987), Hennemuth and Rockwell (1987), Van Eeckhaute and Brodziak (2004), and Van Eeckhaute and Brodziak (2005).

The Georges Bank haddock stock is a transboundary resource shared by the US and Canada since implementation of the Hague Line in October 1984 (Christie 1987).  The eastern Georges Bank haddock management unit is jointly managed by the two countries (Figure 1) while the US manages the western Georges Bank unit.  In May 2004, a formal quota sharing agreement between Canada and the US was implemented for the eastern Georges Bank haddock management unit.  This agreement includes total allowable catch allocations for each country as well as in-season monitoring of the catch of haddock.

The Georges Bank haddock stock was last assessed at the 2002 Groundfish Assessment Review Meeting (Brodziak et al. 2002).  Based on that assessment of the combined western and eastern Georges Bank management units (Figure 1) through 2001, the stock was overfished but not experiencing overfishing.  Spawning biomass in 2001 was 74,400 mt, roughly 30% of BMSY but over 6-fold greater than the near-record low of 11,400 mt in 1993.  Fishing mortality in 2001 was F=0.22, roughly 85% of FMSY (FMSY =0.26).  In this paper, we update the Georges Bank haddock stock assessment using revised fishery data for 1972-2001 along with fishery data for 2002-2004 and research survey data for 2002-2005.  Current estimates of spawning biomass and fishing mortality are used to determine stock status.  Sensitivity analyses of 2002 assessment results to changes in tuned virtual population analysis (VPA) software and revised fishery and biological data are also conducted.


2. THE FISHERY

The Georges Bank haddock stock has been commercially exploited since the 19th century with reliable catch statistics available beginning in 1904 (Table 1.1, Figure 2).  Since then, the fishery for Georges Bank haddock has gone through seven periods: (1) the initial expansion from 1904-1923 when annual catch averaged 17,400 mt; (2) the rapid expansion and decline during 1924-1930 when catches averaged 73,200 mt; (3) the thirty-year period of fishery stability during 1931-1960 when annual catch averaged 46,300 mt; (4) the rapid expansion and decline during 1961-1968 when catches averaged 73,000 mt and foreign distant water fleets began to harvest the resource (Table 1.2); (5) the pre-Hague line fishery during 1969-1984 when catches averaged about 13,500 mt; (6) the fishery nadir during 1985-2000 when catches averaged only 5,600 mt; and (7) the nascent recovery from 2001-2004 when annual catches have increased to average 13,700 mt per year.  Catches have generally increased each year since 1995 as the stock has been rebuilding under restrictive management measures.  In 2004, the total commercial catch was 17,600 mt, over 7-fold larger than the lowest recorded catch in 1995.

2.1. Commercial Landings

US haddock landings during 2001-2004 were derived from mandatory dealer reports and fishing vessel logbooks as in previous assessments conducted since 1994.  Landings were prorated into stock areas (Figure 1) using vessel trip reports (VTR) and dealer data available at the time of the assessment preparation (through July 2005).  The stratification design for the Georges Bank haddock stock was based on market category (large, scrod), port group, gear group and calendar quarter as in previous assessments.  These standard prorations are considered preliminary since VTR data processing and auditing procedures have not been finalized.  As a result, minor changes in landings by stock area may occur through time.  For example, the revised prorated US Georges Bank haddock landings totaled 4,631 mt in 2001, a 0.1% decrease from the prorated value in the last assessment (Brodziak et al. 2002).  As in previous years, most US landings during 2001-2004 were taken with otter trawl gear (Table 2).  US landings in 2004 totaled 7,179 mt, a 55% increase over 2001 (Table 2).

Canadian landings during 2001-2004 were taken from the most recent assessment of eastern Georges Bank haddock (Van Eeckhaute and Brodziak 2005).  Canadian landings data are obtained through dockside monitoring and at-sea observers.  As in previous years, most Canadian landings during 2001-2004 were taken by otter trawl and longline gear (Table 2).  Canadian landings in 2004 totaled 9,745 mt, a 44% increase over 2001 landings.

Total US and Canadian landings of Georges Bank haddock during 1964-2004 averaged 14,876 mt per year (Table 2).  Total annual landings were relatively high during the 1960s, declined during the 1970s, and briefly increased in the late-1970s and early-1980s due to the recruitment of the strong 1975 and 1978 year classes.  Total landings declined to less than 9,000 mt during 1985-2000, but since 2001 have exceeded 11,000 mt.  The US percentage of total landings decreased from over 70% in the 1960s to a low of 8% in 1994 and 1996.  Since 1999, the US percentage has generally been above 40%.

2.2. Recreational Landings

Recreational landings of Georges Bank haddock are usually low (<100 mt/yr) although party charter vessels may target haddock on some trips.  Estimates of the annual recreational catch using data collected by the Marine Recreational Fishery Statistics Survey during 2001-2004 ranged from 0% to <0.1% of the commercial catch.  As such, recreational landings of Georges Bank haddock were considered to be negligible and were not included in the total landings estimates.

2.3. Commercial Discards

Discards of Georges Bank haddock in the US fishery are believed to have remained at a relatively low and constant level since the mid-1950s (Brown and Munroe 2000).  Estimated discards have been included in years when resource conditions or management regulations have led to haddock discards substantially higher than the low background level.  In particular, discards were included in the total catch during 1974, 1977, 1978, and 1980 when the strong 1975 and 1978 year classes were heavily discarded.  Discards were also included during 1994-1998 when low trip limits generated substantial regulatory discards of Georges Bank haddock.  Since then, the Georges Bank haddock stock has produced three strong year classes (1998, 2000, and 2003) that were subject to discarding during 2001-2004.  To account for potential discards of these large year classes and also to provide a total accounting of total fishery removals in recent years, US commercial discards were incorporated into the total catch estimates during 2001-2004.

US discards of Georges Bank haddock during 2001-2004 were estimated fishery observer sampling data for western and eastern Georges Bank.  Fishing trips with observers accounted for 2% of the US western Georges Bank catch in 2001 and 14% in 2004 (Table 3.1).  On eastern Georges Bank, observed catches increased from 1% of the US catch in 2001 to 9% in 2004 (Table 3.2).  Recent increases in US observer coverage of haddock landings reflect increases in the total number of observed groundfish trips.

Quarterly US discards of haddock for the western and eastern Georges Bank management units (Figure 1) during 2001-2004 were estimated for otter trawl, longline (hook), gillnet, and other (all other gears that captured haddock) fishing gears using reported landings and observed discard to kept ratios as in previous assessments (Brown and Munroe 2000).  Discard to kept ratios for quarters with no observer data were imputed using an average ratio for that gear type. US discards of western Georges Bank haddock increased from about 100 mt during 2001-2003 to over 400 mt in 2004 (Table 4.1).  US discards of eastern Georges Bank haddock increased from about 50 mt during 2001-2003 to over 150 mt in 2004 (Table 4.2).

Canadian fishing vessels are required to land their entire catch of Georges Bank haddock (i.e., full retention is mandated).  Discarding of Georges Bank haddock may occur, however, in the Canadian sea scallop fishery which has been prohibited from retaining haddock since 1995.  Estimates of discards of eastern Georges Bank haddock in the Canadian sea scallop fishery during 1972-2004 (Van Eeckhaute and Brodziak 2005) were also included in the updated fishery catch data.  Canadian scallop fishery discards of haddock ranged from a high of 186 mt in 1985 to a low of 29 mt in 2000 and have remained below 100 mt since 1998.  The Canadian scallop fishery discards of haddock are included in the updated total catch time series of Georges Bank haddock (Tables 1.1-1.2).

2.4. Commercial Port Sampling

US commercial fishery length sampling intensity for western Georges Bank haddock averaged over 200 lengths/100 mt during 2001, 2003 and 2004, but was only 124 lengths/100 mt in 2002 (Table 5.1).  Age sampling averaged about 50 ages/100 mt during 2001-2004.  Sampling intensity for eastern Georges Bank haddock during 2001-2004 was similar to that for western Georges Bank (Table 5.2), although there were some quarters where no market category length samples were available due to low landings.  US vessels fishing for haddock use similar otter trawl gear in both management areas and observed fishery length composition data are also similar across areas.  US commercial length frequency data for eastern Georges Bank haddock were augmented with length composition data from US statistical areas 521, 522 and 525 during 2001-2002 and areas 522, and 525 during 2003-2004 to provide quarterly fishery length composition data.  US discard length compositions for western and eastern Georges Bank were derived from domestic fishery observer data.  Discard length compositions for quarters with no observer data were imputed using an average discard length frequency by gear type.

The annual size composition of the Canadian eastern Georges Bank haddock catch during 2001-2004 was characterized using 67905, 46802, 69398, and 78113 length samples, respectively.  These samples were collected from all principal fishing gears (otter trawl, gillnet, and longline) by month.  The annual sampling intensities for the Canadian eastern Georges Bank haddock fishery during 2001-2004 were 1003 lengths per 100 mt, 721 lengths per 100 mt, 1024 lengths per 100 mt, and 801 lengths per 100 mt, respectively.  Details of the Canadian commercial fishery port and at-sea observer sampling intensity for eastern Georges Bank haddock during 2001-2004 are reported in Gavaris and Van Eeckhaute (2002),Van Eeckhaute, Gavaris, and Brodziak (2003), Van Eeckhaute and Brodziak (2004), and Van Eeckhaute and Brodziak (2005).

2.5. Commercial Catch at Length Composition

US commercial fishery landings and discarded numbers at length were calculated from the available length composition data for western and eastern Georges Bank during 2001-2004.  In 2001, the US fishery caught about 1.9 million haddock on western Georges Bank and roughly 320 thousand haddock on eastern Georges Bank (Tables 6.1-6.2).  The average length of the US haddock caught in 2001 was about 2% larger on western Georges Bank in comparison to eastern Georges Bank (Figures 3.1-3.2).  In 2002, the U.S fishery caught 2.5 million haddock on western Georges Bank and 504 thousand fish on western and eastern Georges Bank (Tables 6.1-6.2).  The average length of US haddock caught on western Georges Bank haddock was 7% greater than on eastern Georges Bank haddock (Figures 3.3-3.4).  Total US haddock catches on western and eastern Georges Bank were 1.9 million and 822 thousand fish, respectively, in 2003 (Figure 3.5), with the average length of haddock caught on western Georges Bank about 3% greater than on eastern Georges Bank (Figure 3.6).  In 2004, the US fishery caught 3.4 and 1.3 million haddock, respectively, on western and eastern Georges Bank (Tables 6.1-6.2).  The average length of haddock caught on western Georges Bank haddock was about 3% greater than on eastern Georges Bank (Figures 3.7-3.8).

Canadian commercial fishery landings and discarded numbers at length were calculated from the available length composition data for eastern Georges Bank during 2001-2004.  The annual size composition of the Canadian otter trawl catch of eastern Georges Bank haddock averaged 51 cm during 2001-2004.  In contrast, the Canadian longline catch averaged 58 cm during 2001-2002, was 57 cm in 2003, and declined to 54 cm in 2004.  Details of the Canadian commercial fishery catch at length of eastern Georges Bank haddock during 2001-2004 are reported in Gavaris and Van Eeckhaute (2002), Van Eeckhaute, Gavaris, and Brodziak (2003), Van Eeckhaute and Brodziak (2004), and Van Eeckhaute and Brodziak (2005).

2.6. Commercial Fishery Catch at Age

Total catch numbers at age of Georges Bank haddock were estimated using available fishery length and age composition data during 2001-2004.  The accuracy and precision of age composition data were evaluated and found to be satisfactory for age-structured assessment analyses (Appendix [PDF file]).  The Canadian catch at age of eastern Georges Bank haddock during 1972-2004 was taken from the most recent assessment of this management unit (Van Eeckhaute and Brodziak 2005).  The US catch at age of western and eastern Georges Bank haddock during 2001-2004 was estimated using fishery length and age-length composition data collected by port samplers and fishery observers, along with research survey age-length composition data to characterize sublegal discards.

Annual US catch numbers at age of landed and discarded haddock during 2001-2004 for western (Table 6.1) and eastern Georges Bank haddock (Table 6.2) were computed using quarterly age-length keys.  The US catch at age of western Georges Bank haddock was dominated by the strong 1998 year class which contributed 27%, 39%, 50%, and 37% of total catch numbers in 2001 to 2004.  In 2004, the strong 2000 year class contributed 29% of total western Georges Bank catch while the very strong 2003 year class contributed 11% at age-1.  For eastern Georges Bank haddock, there were limited age-length data in some quarters (Table 5.2).  As a result, Canadian commercial fishery age-length keys from eastern Georges Bank were used to augment US age-length composition data for quarters 2, 3, and 4, while the Canadian spring survey age-length key was used for quarter 1.  The 1998 year class was also prominent in US catches of eastern Georges Bank haddock and contributed 25%, 37%, 37%, and 20% respectively of total catch numbers in 2001 to 2004 (Table 6.2).  In 2004, the strong 2000 year class was dominant and contributed 31% of the total US catch on eastern Georges Bank.  The 2003 year class contributed about 12% of the total US eastern Georges Bank catch in 2004.

Canadian catch numbers at age on eastern Georges Bank during 2001-2004 (Table 6.3) were derived using quarterly age-length composition and fishery length composition data from the most recent assessment (Van Eeckhaute and Brodziak 2005).  Similar to the US catch, the 1998 year class dominated Canadian catches and contributed 44%, 48%, 29%, and 20% respectively of the total Canadian haddock catch during 2001-2004.  In 2004, the strong 2000 year class became dominant and contributed 54% of the total Canadian catch while the 2003 year class only contributed 3%.

The total catch at age of Georges Bank haddock during 1963-2004 was assembled by combining the western Georges Bank and eastern Georges Bank US and Canadian catch-at-age matrices (Table 7).  This was the catch at age input to the tuned virtual population analysis.

2.7. Commercial Fishery Mean Weights at Age

Commercial fishery mean weights at age of US western and US and Canadian eastern Georges Bank haddock catches were computed for landings and discards (Table 8).  Mean weight-at-age data for western and for US and Canadian eastern Georges Bank haddock were averaged to derive the mean weights at age of Georges Bank haddock during 2001-2004 (Table 9).  Commercial fishery mean weights at age of Georges Bank haddock during 2001-2004 were below their long-term average for all age classes, with declines ranging from -7% to -44%. The consistent pattern of declining weight at age suggests that density-dependent growth or possibly environmental changes have reduced commercial haddock size at age in recent years.


3. Stock Abundance Indices and Life History Parameters

3.1. US Research Survey Abundance Indices

The annual NEFSC autumn bottom trawl survey was initiated in 1963 and the annual NEFSC spring bottom trawl survey in 1968. NEFSC spring survey and autumn survey stratified mean number and weight per tow indices were computed using survey catch data from strata 13-25 and 29-30 during 1963 to 2005 (Table 10, Figure 4, Figure 5).  Haddock survey indices were adjusted for estimated differences in the fishing power of the R/V Albatross IV and the R/V Delaware II, where the R/V Albatross IV was the standard (Table 11).  In addition, haddock survey indices were adjusted for estimated differences in catchability of BMV trawl doors (used prior to 1985) versus polyvalent trawl doors (1985 onwards).  During 1972-1981, the NEFSC spring survey deployed a different net (Yankee 41) than the standard net (Yankee 36).  No survey adjustment coefficients were estimated for this gear change (Sissenwine and Bowman 1978).  As a result, the nine years of spring survey data collected during 1972-1981 were treated as a separate survey abundance index series in tuning the VPA analysis. NEFSC spring and autumn survey catch at length data (Figures 6.1-6.5) and survey-specific age-length keys were used to compute stratified mean number per tow at age through time (Table 12, Table 13, Figures 7.1-7.2) and by age class (Figures 8.1-8.2 and Figures 9.1-9.2).  Note that the 2005 autumn survey data were not available at the time of assessment preparation and were not used as inputs to the tuned VPA analysis.

3.2. Canadian Research Survey Abundance Indices

The Canadian DFO initiated a winter bottom trawl survey on Georges Bank in 1986.  Canadian winter survey stratified mean number per tow indices (Figure 5) were computed using standardized research survey data collected by the R/V Needler on Georges Bank during 1986 to 2004.  Canadian winter survey catch per tow at age indices during 1986-2004 (Table 14, Figure 9.3) were computed using survey-specific age-length keys (L. Van Eeckhaute, DFO, pers. comm.).  In 2005, the R/V Needler was not available to complete the entire Georges Bank winter survey and the R/V Templeman was used.  Because no inter-vessel calibration data exists for these two vessels, the 2005 Canadian winter survey data were not used as inputs to the tuned VPA analysis.

3.3. Trends in Survey Abundance Indices

The NEFSC spring and autumn abundance indices exhibit similar trends through time (Figure 5).  Autumn indices declined from record highs in the 1960s to low levels in the early-1970s.  The spring and autumn indices both increased in the mid-1970s due to the strong 1975 and 1978 year classes and then declined in the early 1980s.  Both US and Canadian abundance indices were low from the mid-1980s to the mid-1990s and have since increased.

3.4. Natural Mortality

Natural mortality was assumed to be 0.2 in this assessment, as in previous assessments of the Georges Bank haddock stock (Brown and Munroe 2000, Brodziak et al. 2002) and eastern Georges Bank haddock management unit (Van Eeckhaute and Brodziak 2005).  Catches of haddock in excess of 15 years of age in both US and Canadian survey time series are consistent with this assumption for natural mortality.

3.5. Research Survey Mean Weights at Age

Time series of mean weights at age of Georges Bank haddock captured during NEFSC spring and autumn surveys and the Canadian winter survey were evaluated to determine whether trends in survey mean weights at age were consistent with those in the commercial fishery during 2001-2005.  NEFSC spring survey mean weights at age of Georges Bank haddock were substantially below their long-term averages (Table 15.1).  Declines in spring mean weight at age ranged from -23% at age-7 to -37% at age-8.  NEFSC autumn survey mean weights at age showed similar declines (Table 15.2), with weights at age during 2001-2004 ranging from -22% (age-1 and age-2) to -55% (age-8) of their long-term averages.  Canadian winter survey mean weights at age also showed recent declines (Table 15.3), with weights at age during 2001-2004 ranging from -7% at age-3 to -25% at age-1 of their long-term averages.  Overall, there was a consistent pattern in declining mean weights at age across age groups and research surveys.

3.6. Research Survey Mean Lengths at Age

Mean lengths at age of Georges Bank haddock in the NEFSC spring and autumn surveys were examined to evaluate changes during 2001-2005 relative to the long-term (1963-2005) average.  Age groups with fewer than 3 samples or older than age-9 were excluded from the analysis due to low sample size.

For age-0 haddock, recent mean length was 9% below the long-term autumn average (Figure 10.1). Similarly, for age-1 and age-2 haddock, recent mean lengths at age were 11% and 9% respectively, below the long-term average during spring and autumn (Figures 10.2 and 10.3).  Mean lengths of age-3 haddock were 10% and 11% below their long-term average (Figure 10.4), while recent mean lengths at age for age-4 and age-5 haddock were 9% and 10% below the long-term average during spring and autumn (Figures 10.5 and 10.6).  Recent mean lengths at age for age-6 through age-9 haddock during spring were 5%, 4%, 9%, and 3% respectively, below their long-term averages (Figures 10.7 to 10.10).  Similarly, in the autumn surveys, recent mean lengths for age-6 through age-8 haddock were 12%, 9%, and 8% respectively, below their long-term average (Figures 10.7-10.9).  Overall, in recent years there was a consistent decline of about 10% in mean length at age across age groups.

3.7. Research Survey Weight at Length Relationships

Individual length and weight measurements of Georges Bank haddock obtained during spring and fall bottom trawl surveys conducted since 1992 were used to evaluate whether any changes in length-weight relationships have occurred.  Linear regression was applied to log-transformed length-weight data to estimate the scaling parameter “A” and the exponent parameter “B” of the allometric equation (Table 16) for predicting weight (W) for a given length (L), W = A • LB.

For each survey, the point estimate of the scale coefficient “A” was adjusted for transformation bias using the low-bias estimator given in Hayes et al. (1995).  The nonparametric bootstrap (Efron and Tibshirani 1993) was applied to estimate standard errors of the length-weight parameters.

Annual spring survey length-weight relationships during 1992-2000 (Figure 11.1) and 2001-2005 (Figure 11.2) were generally similar to the overall average spring length-weight relationship for 1992-2005, although spring weight at length was below average in 1994 and 2003.  Annual autumn length-weight relationships during 1992-2000 (Figure 11.3) and 2001-2004 (Figure 11.4) were also similar to the average autumn relationship, although autumn weight at length was slightly below average during 2001-2004.

Haddock condition was evaluated using the predicted mean weights at lengths at 25, 50, and 75 cm using the spring and autumn survey length-weight relationships (Figures 12.1-12.2).  Haddock condition was relatively constant during 1992-2005 with some suggestion of modest declines in recent years. Average predicted spring weights during 2001-2005 for lengths 25, 50, and 75 cm were 6%, 5%, and 5% below the 1992-2005 average.  Similarly, average predicted autumn weights for lengths 25, 50, and 75 cm were 5%, 3%, and 2% below the 1992-2004 average.  Hence, haddock condition factor appears to have had a minor impact on recent declines in mean weights at age relative to the 2001-2005 declines in mean length at age.

3.8. Proportion Mature at Age

Maturity data for individual Georges Bank haddock have been collected during spring and autumn bottom trawl surveys since 1992.  Logistic regression was applied to derive the recent female proportion mature at age-k (Pk), where , (Table 17) using the same approach as in previous assessments (Brodziak et al. 2002, Brown and Munroe 2000).  Parameter estimates were a=-4.7330 and b=2.5685.  Predicted proportion mature at age was approximately equal to the observed proportion mature at age for all ages except age-1, for which the predicted percent mature was 10% and the observed percent mature was only 1%.  Given the discrepancy between model prediction and observed percent mature at age-1, the observed female percent maturity at age-1 of 1% was used in subsequent analyses(Table 17) .  The logistic regression results indicate that female median age at maturity of Georges Bank haddock during 2001-2004 was 1.8 years with a standard error of 0.2 years.


4. Estimates of Stock Size and Fishing Mortality

4.1. Virtual Population Analysis Formulation

Virtual population analysis (VPA) was used to estimate Georges Bank haddock stock size and fishing mortality at age during 1963-2004.  Because this assessment is an update and not a benchmark assessment, alternative models, such as statistical catch at age analysis, were not applied.  The VPA formulation (Table 18) was identical to that used in the 2002 GARM assessment (Brodziak et al. 2002).  The updated VPA included US research survey indices from 2001-2005 and Canadian research survey indices from 2001-2004 as well as updated catch-at-age data for 1972-2004.  The precision of VPA estimates was evaluated using nonparametric bootstrap analysis of tuning indices.  VPA estimates of spawning stock size and fishing mortality during 1931-1962 were taken from Brown and Munroe (2000).  The historic VPA estimates are for a period when no research survey tuning indices are available and are not affected by updated catch data in this assessment.

4.2. VPA Diagnostics

VPA diagnostics indicated a good overall fit to the survey data, the current assessment exhibited the lowest mean squared residual observed in the last seven assessments (Table 18). Coefficients of variation of numbers at age estimates for ages 1-8 in the terminal year plus one ranged from 58% at age-1 to 23% at age-6. Maximum coefficients of variation of catchability in the surveys ranged from 0.15 (NEFSC fall survey) to 0.51 (NEFSC spring survey Yankee 41 net, 1973-81. VPA residual patterns were generally without trend (Figure 13) although year effects were apparent across ages in some cases (e.g., 2002 NEFSC spring survey residuals were negative for all ages).

4.3. VPA Results

Georges Bank haddock stock size changed markedly during 1963-2005 (Table 19, Figure 14.1). VPA results indicate that stock size averaged 352 million fish during 1963-1968, but was reduced by more than 10-fold to an average of about 26 million fish during 1969-1974. Stock size increased to average 76 million fish during 1975-1984 due to the recruitment of the strong 1975 and 1978 year classes, but subsequently declined and averaged only 27 million fish during 1985-1994. With the implementation of restrictive management measures in the mid-1990s, stock size increased and averaged 67 million fish during 1995-2000. Georges Bank haddock stock size increased 4-fold from 164 million in 2001 to 712 million in 2005 due to moderate fishing mortality and the recruitment of the strong 2000 and 2003 year classes.

Georges Bank haddock spawning biomass also changed substantially over the past five decades. Spawning biomass peaked in the 1960s averaging 132 kt during 1963-1968 (Table 20, Figure 14.2), declined to a near-record low of 15 kt in 1973, and averaged only 27 kt during 1969-1974. Spawning biomass increased moderately to average 48 kt during 1975-1984 and then declined again, averaging only 23 kt during 1985-1994, with a record low of 14.6 kt in 1993. During the mid-1990s, spawning biomass increased. Since 2001 spawning biomass has approached levels observed in the 1960s and averaged 115 kt during 2001-2004. Spawning biomass increased by 22% from 96 kt in 2001 to 117 kt in 2004.

Fishing mortality (unweighted average of age-4 through age-7 Fs) on Georges Bank haddock ranged from less than 0.1 to over 0.6 during 1963-2004 (Tables 21 and Table 22, Figure 14.3). F averaged 0.52 during 1963-1968 and declined to a low of 0.06 in 1974. F increased over 5-fold in the mid-1980s and averaged 0.35 during 1985-1994. Fishing mortality declined in the mid-1990s and since 1995 has remained below the overfishing threshold of FMSY=0.26, although fishing mortality increased 33% from F=0.18 in 2001 to F=0.24 in 2004.

Georges Bank haddock recruitment (stock size at age-1) fluctuated substantially during 1963-2005 ranging from a low of 0.3 million age-1 fish in 1971 to a high of 788.9 million in 2003 (Table 19, Figure 14.4). Recruitment during 1963-1968 averaged 117 million fish and was dominated by the exceptional 1963 year class (462 million age-1 fish). Recruitment declined to average only 8 million fish during 1969-1974, but subsequently increased and averaged 26 million fish during 1975-1984, primarily due to the strong 1975 (106 million) and 1978 (84 million) year classes (YCs). Recruitment averaged only 8 million fish during 1985-1994, but increased to average 22 million fish during 1995-2000. Since 2001, recruitment has averaged 179 million fish. In particular, the 1998 (47 million) and 2000 (91 million) year classes are strong, while the 2003 year class (789 million) currently appears to be the largest observed (Figure 14.4).

4.4. Stock-Recruitment Relationship

Analyses stock-recruitment data indicate that spawning stock size affects recruitment of Georges Bank haddock (Brodziak and Legault 2005; Brodziak et al. 2001, NEFSC 2002). For example, Brodziak et al. (2001) found that when spawning stock biomass is above 82 kt, the odds of recruitment being above average is 20 times greater than when biomass is below 82 kt and the average YC size is 5-fold higher. Thus, recruitment is higher, on average, when spawning biomass exceeds a threshold value.

The Working Group on the Re-evaluation of Reference Points for New England Groundfish determined that an appropriate productivity threshold for Georges Bank haddock was 75 kt of spawning biomass (NEFSC 2002). Above this threshold, average recruitment is 96 million age-1 fish (Figure 15) while below this threshold average recruitment is about 21 million fish. When spawning biomass is above 75 kt, the odds of achieving recruitment above the 1931-2005 median value are 23 times greater than when spawning biomass is below 75 kt. This difference is highly significant (P < 0.001) based on Fisher’s exact test applied to the 2x2 contingency table of stock-recruitment data.

Survival ratios of Georges Bank haddock, as indexed by recruitment per spawning biomass, fluctuated about an average of 0.72 recruits per kg (R/S) during 1931-2004 (Figure 16). Survival ratios averaged 0.77 R/S with a standard error of 0.07 during 1931-1960 when spawning biomass averaged 102 kt. Since then survival ratios have declined 12% on average to 0.68 R/S with a standard error of 0.18 during 1961-2004 when spawning biomass averaged 64 kt. Since 1961, three strong year classes (1963, 1975, and 2003) have had survival ratios of roughly 3 or higher (Figure 16). Each of these year classes experienced very high survival during early life history stages, suggesting the influence of favorable environmental conditions in inducing large recruitment events. Spawning biomass also affects survival ratios as survival ratios average 0.82 when spawning biomass is above 75 kt in comparison to only 0.59 (-28%) when spawning biomass is below 75 kt.

4.5. Precision of Stock Size and Fishing Mortality Estimates

Bootstrap analyses of estimates of Georges Bank haddock stock size (Figure 17.1), spawning biomass (Figure 17.2), and fishing mortality (Figure 17.3) show some variability beginning in the late-1990s. Prior to that, the VPA estimates are unaffected by the precision of estimates of recent cohorts. The relatively high variability in stock size estimates in recent years is primarily due to uncertainty in the estimated size of the 2003 year class at age-1 in 2004 (Figure 18.1) and at age-2 in 2005 (Figure 18.2, CV=34%). Bootstrap analyses also indicate that estimates of spawning biomass and average F in 2004 are relatively precise, having coefficients of variation between 13% to 18% (Table 23, Figure 19).

4.6. Retrospective Analysis

Retrospective analysis was used to evaluate whether there were any patterns in recent estimates of spawning biomass, fishing mortality, and recruitment.  In this evaluation, the tuned VPA model was rerun after deleting the most recent year of data, and corresponding estimates of the current year spawning biomass and fishing mortality were then compared. The average of the one-year retrospective changes calculated for five pairs of years (2004/2003, 2003/2002, 2002/2001, 2001/2000 and 2000/1999) was used to judge whether a retrospective pattern existed. Average changes of less than ±10% were considered to show no pattern, changes of ±10% to 20% were considered to indicate a moderate retrospective pattern, and changes over ±20% were considered to have a strong retrospective pattern. A positive retrospective pattern indicated that, on average, the estimated value of spawning biomass, fishing mortality, or recruitment increased when another year of data was added to the assessment.

Estimates of spawning biomass exhibited no retrospective pattern (Figure 20.1). On average, spawning biomass estimates for the last year of the VPA changed by +3% when an additional year of data was added. Fishing mortality estimates also exhibited no retrospective pattern (Figure 20.2) with an average change of -3% during 1999-2004. Estimates of Georges Bank haddock recruitment also did not have a retrospective pattern (Figure 20.3) with an average change of only +5% during 1999-2004. Overall, the retrospective analysis showed that estimates of Georges Bank haddock spawning biomass, fishing mortality, and recruitment had no retrospective pattern.

4.7. Sensitivity Analysis on Effect of Updated VPA Software
and Updated Fishery and Biological Data on 2002 VPA Results

Two sensitivity analyses were conducted to evaluate how changes in VPA software and Georges Bank haddock catch data affected VPA results. In 2004, the NEFSC VPA software was enhanced to include more options for estimation of stock size and F. The old software (FACT) was compared to the new software (GARM) using the VPA input data for Georges Bank haddock from the 2002 assessment (Brodziak et al. 2002). Results of this comparison showed that there were only minor differences in 2001 estimates of numbers-at-age in the terminal year plus one, fishing mortality at age in the terminal year, average F, and spawning biomass (Table 24). Overall, the software change had no significant impact on VPA results.

The input data for the Georges Bank haddock VPA was also revised in this assessment to include Canadian scallop fishery discards in the catch at age for 1972-2004, US discard-at-age estimates for 2001-2004, updated proration of 2001 US haddock landings to stock area, revised mean weight-at-age data in 2001, and revised female percent mature at age in 2001. The effect of using the revised data with the GARM VPA software was compared to the effect of using the old data with the FACT VPA software (Table 24). The use of the new data increased estimates of stock size at age and spawning biomass by roughly 10% (Table 24) but had no discernable effect on estimates of fishing mortality. The primary effect of using the revised catch data was to increase stock size to account for increases in catch at age due to the inclusion of additional estimates of fishery discards.


5. Georges Bank Haddock Stock Status

The US Sustainable Fisheries Act of 1996 requires that fishery conservation and management measures prevent overfishing and rebuild depleted stocks to biomasses consistent with producing maximum sustainable yield (MSY). Overfishing occurs whenever fishing mortality exceeds a threshold that jeopardizes the reproductive capacity of a stock to produce maximum sustainable yield. Guidelines to the Act also specify that a depleted resource is one that has been reduced below a minimum stock size threshold. For Georges Bank and Gulf of Maine haddock, the minimum stock size threshold is one-half the biomass needed to produce MSY (BMSY). It is possible for a stock to be classified as overfished (due to previous overharvesting) even though the annual harvest rate is below the overfishing threshold. This has been the case for haddock, which has been rebuilding in recent years.

5.1. Biological Reference Points

For Georges Bank haddock, spawning biomass (BMSY) and the fishing mortality to produce MSY (FMSY) are BMSY = 250,300 mt and FMSY = 0.26 (NEFSC 2002). The overfished threshold (BTHRESHOLD) for Georges Bank haddock is BTHRESHOLD =  ½ BMSY = 125,200 mt. The overfishing threshold (FTHRESHOLD) for Georges Bank haddock is FTHRESHOLD = FMSY = 0.26.

5.2. Stock Status in 2004

In 2004, spawning biomass was 116,800 mt (93% of BTHRESHOLD and 47% of BMSY). Therefore, the Georges Bank haddock stock was overfished in 2004 (Figure 21). In 2004, the fishing mortality was 0.24 (92% of  FTHRESHOLD). Therefore, overfishing was not occurring on the Georges Bank haddock stock in 2004 (Figure 22).

5.3. Comparison with Projected Amendment 13 Rebuilding Trajectory

The formal rebuilding plan for Georges Bank haddock adopted in Amendment 13 calls for fishing at the overfishing threshold FMSY=0.26 during 2004-2008 (NEFMC 2003). In 2009, the fishing mortality would be reduced marginally to FREBUILD=0.245, a value projected to produce at least a 50% chance that spawning biomass will meet or exceed BMSY=250,300 mt in 2014. This rebuilding strategy is subject to change in 2008 if observed progress towards rebuilding spawning biomass or reducing fishing mortality is not consistent with the projected rebuilding trajectory.

The projected Amendment 13 rebuilding trajectory for Georges Bank haddock was compared to VPA estimates of spawning biomass and fishing mortality in 2004. For this stock, an adaptive rebuilding plan was adopted in which FREBUILD=FMSY=0.26 during 2004-2008. Median spawning biomass on the rebuilding trajectory was projected to be 129.8 kt in 2004 (Figure 23.1). For comparison, the projected 80% confidence interval for SB2004 was (97.9, 138.8) kt and the SSBREBUILD in 2004 falls within the probable range of the VPA estimate of SSB2004. Similarly, the 80% confidence interval based on bootstrapping was (0.21, 0.31) and the FREBUILD value for 2004 falls within the probable range of the VPA estimate of F2004 (Figure 23.2). Overall, current estimates of SB and F are consistent with projected values on the Amendment 13 rebuilding trajectory.


6. CONCLUSIONS

Fishery management measures implemented since 1994 have decreased fishing mortality on Georges Bank haddock. Fishing mortality on the stock averaged F=0.35 per year during 1980-1993, or about 36% higher than the current overfishing limit (FMSY =0.26) for this resource. Since 1994, annual fishing mortality for Georges Bank haddock has averaged about F=0.17, about 50% lower than during 1980-93 and 30% below FMSY.

The response of the Georges Bank haddock to reductions in fishing mortality during the 1990s was dramatic. Under persistent overfishing in the 1980s, spawning biomass declined from 67,400 mt in 1980 to only 14,600 mt in 1993. Since 1994, spawning biomass has increased substantially as fishing mortality has declined. By 2003, spawning biomass had increased to 131,900 mt, the highest level since 1966 and over a 9-fold increase since 1993. However, even though stock size has increased markedly in recent years, the Georges Bank haddock stock is still overfished as spawning biomass is still less than half of the rebuilding target.

Recruitment of Georges Bank haddock displayed a similar positive response to reduced fishing mortality. Recruitment averaged only 8 million age-1 recruits per year during 1980-1993. Since 1994, average recruitment has increased over 10-fold to about 87 million fish. Prospects remain positive for continued high recruitment as spawning biomass is currently above the 75 kt threshold. Recent US and Canadian assessments and research survey data suggest that the 2003 year class is exceptionally abundant. This year class has the potential to rebuild the stock to BMSY in a few years if fishing mortality remains below FMSY.

Recruits per spawner data shows that survival ratios for Georges Bank haddock were relatively low from the late-1960s to early-1990s in comparison to ratios during the 1930s-1960s. The impact of large-scale area closures to fishing, reductions in fishing effort, and trawl mesh size increases during the 1990s have all had a positive effect on recruits per spawning biomass (R/SB). During 1980-1993, R/SB averaged about 0.33 recruits per kg. Since 1994, average R/SB, excluding the exceptional 2003 year class, has increased to 0.46 recruits per kg. Further increases in R/SB may still occur since, at least historically, the expected value of R/SB was higher. Overall, the recent increases in R/SB indicate that survival ratios are approaching the historical average of about 0.76 recruits per kg observed during 1931-1960, despite recent decreases in fish size at age. If the recent increases in recruitment and survival can be sustained, it is possible that historic yields on the order of 50,000 mt per year may be achievable.


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