INTERNATIONAL COUNCIL FOR ICES CM 2008/I:02 THE EXPLORATION OF THE SEA Session I: Fishing capacity, effort, and fishing mortality The understanding of fishery dynamics and their links to management

Projecting Equilibrium, Mixed-species Yield of New England Groundfish

Nikki Jacobson and Steve Cadrin

NOAA/UMass Cooperative Marine Education and Research Program School for Marine Science and Technology

New England groundfish are harvested in multispecies fisheries and are managed under a single Fishery Management Plan. Current practice is to determine fishing limits through single-species projection models that allow for stock rebuilding or sustainable yield. Effectiveness of alternative harvest scenarios for achieving management targets is then evaluated using multispecies patterns of catch per effort, by season and location. However, the current mixed-species analyses are static, in that they do not project population dynamics. Our approach incorporates the dynamic aspects of single-species projections with technological interactions for multispecies evaluations. The computational approach extends a traditional (but seldom applied) mixed-species yield-per-recruit model by incorporating stock-recruitment relationships. This equilibrium multispecies projection approach provides the first step towards a non-equilibrium, short-term evaluation of alternative management scenarios to achieve stock rebuilding and sustainable yield targets. Keywords: mixed-species, groundfish Contact author: Nikki Jacobson, NOAA/UMass CMER Program, School for Marine Science and Technology, 200 Mill Road, Suite 325, Fairhaven, MA 02719 U.S.A. [tel: +1 508 910 6394 email: jacobson.n@gmail.com]

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Introduction The purpose of fishery models is to answer questions and inform decision makers of the consequences of alternative harvest scenarios (Hilborn and Walters, 1992; Hollowed et al., 2000). The most common models consider one species or stock, rather than multiple stocks. This is mainly due to two factors: most fisheries target one particular species, and estimating maximum sustainable yield for more than one species is a difficult task (Boreman et al., 1997). However, with an increased interest in ecosystem management and recognition that many fisheries catch a wide range of fishes, multispecies models are gaining more attention. Although single species models dominate current management, they may not be the most appropriate when multiple stocks interact. There are two types of interactions to consider when developing a multispecies model: biological and technological. The choice of which to use depends on such things as the stocks under consideration, the data available, and the information desired. Biological models apply to interactions between stocks through natural processes such as predation, environmental disturbance, and competition (Rogers and Pikitch, 1992). Technological interactions (i.e., mixed-species harvesting; Murawski and Finn, 1988), apply to stocks caught together by non-selective gear. Biological interactions are often expensive to monitor and requiring extensive data collection over many years. Technological interactions are often simpler, cheaper, and quicker to define and study (Hilborn & Walters, 1992). Biological models may be broad in scope, possibly addressing the ecosystem as a whole, while technological models address only select stocks (Hollowed et al., 2000). Technological interactions may also be more applicable to management because they can be partially controlled. While not currently used for management in the Northeast, there are population dynamics models for multi-species assemblages involving technological interactions (e.g., Ricker et al., 1958; Paulik et al., 1967; Murawski, 1984; Pikitch, 1987; Murawski et al., 1991; Spencer et al., 2002; Holland, 2003; Vinther et al., 2004). Common conclusions of these analyses, the vulnerability of less-productive stocks to depletion by mixed-stock fisheries and greatest sustained yield is achieved by harvesting each stock separately, illustrate the difficulty in multispecies’ optimization. However, it is now widely accepted that many fisheries harvest a range of stocks, requiring new approaches for assessment and management (Walters and Bonfill 1999). In recent decades the limitations of single species models have been recognized and multispecies models have become more prevalent at the research and exploratory phases. New England Groundfish The New England groundfish fishery suffered from a collapse of several major stocks in the early 1990’s due to overfishing (Boreman et al., 1997; Hennessey and Healey, 2000). Once one of the most productive fisheries in the world, by 1990 the spawning stock biomass of the principal species had declined to the lowest levels ever recorded (Hennessey and Healey, 2000).

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New England groundfish stocks are managed by the New England Fishery Management Council under the multispecies fishery management plan and are harvested in Georges Bank, in the Gulf of Maine and off southern New England. The plan was adopted in 1986 to manage key groundfish stocks from Maine to Cape Hatteras, and currently there are 12 species caught in large-mesh fisheries consisting of 19 stocks under this plan (Mayo and Terceiro, 2005). The stocks are:

1. Georges Bank Cod 2. Georges Bank Haddock 3. Georges Bank Yellowtail Flounder 4. Southern New England/Mid-Atlantic Yellowtail Flounder 5. Gulf of Maine/Cape Cod Yellowtail Flounder 6. Gulf of Maine Cod 7. Witch Flounder 8. American Plaice 9. Gulf of Maine Winter Flounder 10. So. New England/Mid-Atlantic Winter Flounder 11. Georges Bank Winter Flounder 12. White Hake 13. Pollock 14. Acadian Redfish 15. Ocean Pout 16. Gulf of Maine/Georges Bank Windowpane 17. So. New England/Mid-Atlantic Windowpane 18. Gulf of Maine Haddock 19. Atlantic Halibut.

These nineteen stocks are managed under one fishery management plan because they are linked through technological interactions. Landings in the Northeast are mainly by otter trawl which is a non-selective fishing gear (Murawski et al., 1983). Mayo et al. (1981) determined that commercial otter trawl landings in the Northeast had overall species compositions quite similar to natural species distributions when gear selectivity and discards were taken into account. Gillnet and long-line fisheries also account for a significant, yet smaller, portion of groundfish catches (Holland, 2003). Less is known about the species selectivity of these passive gears. In reaction to the fishery collapse in the early 1990s, a variety of regulatory measures were implemented to promote recovery of depleted groundfish stocks off New England. Reductions in number of days at sea, year-round and seasonal area closures, mesh size increases, and a moratorium on entry to the fishery reduced fishing mortality significantly on all stocks, and some began to rebuild (NEFMC, 1985; Holland and Maguire, 2003). In 1996, the Magnuson Stevens Fishery Conservation and Management Act required fishery management plans to identify overfished stocks, or stocks nearing an overfished condition, and to rebuild them in at least ten years (DOC, 1996). In addition, these plans were required to minimize bycatch and bycatch mortality. Amendment 13 was added to the multispecies groundfish plan to address the requirements; rebuild groundfish stocks to

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Bmsy (biomass that produces maximum sustainable yield) and end overfishing (i.e., reduce fishing mortality to less than Fmsy; the rate that produces maximum sustainable yield). To meet this goal, more extensive seasonal and year-round area closures, gear restrictions, size limits, trip limits, participant limits and days-at-sea were implemented (Amendment 13). However, as of 2004, twelve of the groundfish stocks were still either overfished or rebuilding from overfished conditions (Mayo and Terceiro, 2005). One of the major goals of all fishery management plans in the U.S. is to “…achieve and maintain, on a continuing basis, the optimum yield from each fishery” (DOC, 2006). Optimum yield means the amount of fish that will provide the greatest overall benefit to society and is equal to maximum sustainable yield, or less if reduced for economic, social or ecological factors. Maximum sustainable yield is defined as the largest average yield that can continuously be taken from a stock under existing environmental conditions. At their current state, many of the groundfish stocks in the Northeast are yielding less than their potential maximum sustainable yield (FAO, 2006). Projecting Yield of New England Groundfish With the requirement to rebuild all groundfish stocks, updates on the progress and likelihood of recovery are performed for each stock by the Northeast Fisheries Science Center (NEFSC) (Mayo and Terceiro 2005). A single species projection model, called AGEPRO (Brodziak et al., 1998), is used to perform these evaluations and determine the appropriate fishing mortality rate to produce MSY or achieve rebuilding to Bmsy. AGEPRO is an age-structured stochastic projection of abundance which incorporates initial population size, recruitment, mortality, growth and maturity (NEFSC, 2002; Brodziak et al., 1998). Two scenarios of fishing mortality have been evaluated for New England groundfish using AGEPRO. First, fishing at Fmsy is simulated. If there is < 50% chance that the stock will recover to Bmsy, the maximum F that allows a 50% chance of recovery is calculated by iteratively changing the fishing mortality rate. Simulations using AGEPRO are used to evaluate alternative harvest strategies in terms of expected landings, discards, spawning biomass and recruitment. Therefore, AGEPRO provides a method to consider uncertainties in both initial population abundance and future recruitment for management advice (NEFSC, 2002). Once appropriate fishing mortalities have been determined for each stock through AGEPRO, the closed area model (CAM) is used to analyze biological and economic impacts of the management measures proposed to achieve the defined fishing levels. CAM projects changes in mortality from various area closures, trip limits and days at sea. Inputs to the model are prices by species and month, effort by vessel and month, and average catch per unit effort by species, gear type, area and month (NEFMC, 2006). The deadline for recovery of New England groundfish stocks approaches, with a significant portion of groundfish stocks still rebuilding or being overfished (NEFSC, 2005). These stocks are managed under one FMP because it is acknowledged that they are linked through technological interactions, while a single species projection model is

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used to determine appropriate fishing levels and give management advice. Previous multispecies models which incorporate technologically linked species or stocks (e.g., Murawski, 1984, Muraswki et al., 1991) have been equilibrium analyses which are important for long term calculations of optimum effort and yield. In contrast, non-equilibrium projections are useful for evaluating short term alternative management scenarios for rebuilding and sustainable yield targets through incorporation of time-varying population dynamics (e.g., recruitment). A non-equilibrium, multispecies projection model may be more appropriate for the New England groundfish complex than the current single species model or previously developed equilibrium multispecies models. A dynamic, equilibrium model is explored in this paper as the first step in analysis of a non-equilibrium, dynamic model.

Data and Methods Inputs for the analysis come from multiple sources. Fishery dependent data provide yield (Y) and effort (f) for each species by area, season, gear type and target species. Estimates of abundance, natural mortality, partial recruitment, weight at age and fishing mortality are obtained from 2005 stock assessments (Mayo and Terceiro, 2005). Finally, survey data supplies the seasonal distributions of each stock, and stock-recruit relationships are provided by NEFSC (2002). Our model is based on Murawski’s (1984) multispecies yield per recruit model which is a modified form of the Thompson and Bell (1934) single species yield per recruit model. The term ‘yield per recruit’ is the expected lifetime yield of a fish, which varies among species due to growth rate, natural mortality, and exploitation pattern. The model was applied to a single fishery and multiple fisheries for the groundfish species Atlantic cod, haddock, yellowtail flounder, and winter flounder. Gear selectivity and fishing effort could be evaluated on both combined and single species yield. Total yields were maximized at 6 to 10 thousand days of effort. Murawski’s model (Equation 1) calculates the total yield of species i through summation of the yields at each age t, from first age of recruitment to the maximum age. These yields at age are functions of total deaths (D), fishing mortality (F), partial recruitment (PR), natural mortality (M) and weight (W).

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Equation 1

( )∑ ⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛+

=λt

tit

ititit

itititi

r

WPRFM

PRFDY*

Yi – yield of species i tr – age at recruitment to fishery tλ – maximum fishable age Dit – total deaths of species i at age t Fit – instantaneous fishing mortality rate of species i at age t Mit – instantaneous natural mortality rate of species i at age t PRit – partial recruitment factor of species i at age t Wit – mean fish weight of species i at age t

The single species form was extended for multispecies application by including catchability coefficients (q) which are proportions of fishing effort (f) to fishing mortality. Fishing mortality is a function of catchability and fishing effort (Equation 2); for a given level of fishing effort on a mixed species fishery, fishing mortality will differ for each species due to availability and vulnerability to the gear. Catchability coefficients allow for evaluation of the impacts various levels of effort may have on the fishing mortality of each species.

Equation 2

fqF ii =

While recruitment is initially assumed to be constant in Murawski’s multispecies model, levels of recruitment are acknowledged to differ among species. Therefore, each species’ total yield function (Equation 1) is weighted by its average recruitment with a relative recruitment multiplier (RRM) to account for these differences (Equation 3).

Equation 3

i

ki

iims RRMYY ∑

=

=

Yms – multispecies yield Yi – yield of species i RRM – relative recruitment multiplier

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Model Extension We extend Murawski’s model by replacing relative recruitment multipliers with 2004 recruitment estimates. The Murawski model assumes constant, recruitment which is expressed as a weight (RRM) on the total yield of each species. While this assumption is common, annual variation is more realistic and is an important component of the system’s short-term dynamics (Hollowed et al., 2000). Stock-recruit relationships are used to estimate recruitment from stock biomass. With this modification a non-equilibrium multispecies yield per recruit model is formulated (Equations 4 and 5).

Equation 4

( )[ ] [ ]( ) ( )∑ ⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛+

−= +−−−λt

tit

itiit

itiMfPRqfPRqttiti

r

ititiitir WfPRqM

fPRqeeRY 1

Yi – Yield of stock i tr – Age at recruitment to fishery tλ – Maximum fishable age t – Current age Rit – Recruits of stock i of age t qi – Catchability of stock i f – Mixed species fishing effort Mit – Instantaneous natural mortality rate of stock i of age t PRit – Partial recruitment factor of stock i of age t Wit – Mean fish weight of stock i of age t

Equation 5

∑=

=ki

iims YY

Yms – Multispecies yield Yi – Yield of stock i

Seven stocks with age-based stock assessment data were included in this analysis: Georges Bank cod, Georges Bank haddock, Georges Bank yellowtail, Cape Cod/Gulf of Maine yellowtail, Gulf of Maine cod, American plaice and winter flounder. For the initial analysis, median recruitment values based on available historical recruitment information were used. For the second analysis, equilibrium recruitment (R*) values were used when possible. Stock recruit relationships were used for four stocks; the Beverton-Holt relationship for Georges Bank cod and Gulf of Maine cod, and a stepwise relationship for Georges Bank

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haddock based on average recruitment and Georges Bank yellowtail based on median recruitment (Figures 1-4). Median recruitment was assumed for American plaice, winter flounder and Cape Cod yellowtail. Spawner-per-recruit is the spawning stock biomass that will come from a certain number of recruits at a certain F in equilibrium (Equation 6). These values were incorporated into the four stock recruit equations. Equation 7 calculates R* for the Beverton-Holt stock recruit relationship, Equation 8 calculates R* for stepwise stock recruit relationships. Equation 6

R

SSBSPRF =

Equation 7

F

F

SPRbSPRaR −

=**

a – alpha b – beta Equation 8 if SPRF * Rlow< x lowRR =* R*=Rhigh if SPRF * Rhigh ≥ x

Results

Our initial analysis updates Murawski’s original multispecies yield per recruit model using median recruitment and 2004 stock conditions. Calculations of mixed-species yield (Table 1, Figure 5) show the greatest yield is achieved by harvesting each stock separately. Calculations of mixed-species spawning stock biomass (Table 2, Figure 6) show 40% of the maximum SSB, a proxy for Bmsy for several New England groundfish, for each stock and the assemblage is attained at a more conservative effort level than maximum yield. The output suggests that maximum yield is produced by about 90,000 days at sea (DAS, Emax) while 40% of maximum spawning potential is maintained at around 20,000 DAS. Our next analysis incorporates equilibrium recruitment estimates in place of median recruitment values where appropriate. As stated earlier, four stocks have defined stock recruit relationships: Georges Bank cod, Gulf of Maine cod, Georges Bank haddock and Georges Bank yellowtail flounder. Equilibrium recruitment estimates link fishing mortality with stock recruitment relationships, creating a more realistic/dynamic model. Figure 7 shows this relationship for the four stocks mentioned above; with more fishing effort, fewer recruits grow to become spawners. The three stocks where median recruitment was used in lieu of a stock recruit relationship (Cape Cod yellowtail, American Plaice and witch flounder) show no affect of fishing mortality on recruitment.

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Calculations of mixed-species yield from the second analysis (Table 3, Figure 8) show similar relationships between maximizing individual stocks and the assemblage. In addition, 40% of the maximum SSB for the assemblage and individual stocks is attained at a more moderate effort level than MSY. However, there are some interesting differences which can be attributed to the equilibrium recruitment values. Increasing effort past 50,000 DAS causes a sharp decrease in assemblage yield due to the equilibrium recruitment relationship of Georges Bank haddock. This affect can also be seen in the spawning stock biomass of the assemblage at the same effort level (Table 4, Figure 9). The output suggests maximum yield is produced by about 50,000 DAS (Emsy) while 40% of maximum spawning potential is maintained at around 30,000 DAS. Both analyses provide estimates which are consistent with recent evaluations of the management plan, requiring a reduction from the 35,000 days fished in 2005. While we assumed constant fishing and natural mortality with respect to seasonality, spatial patterns, fishing gear and target species in both analyses, the latter analysis incorporates equilibrium recruitment values as opposed to median values. In both cases, the potential to overfish sensitive stocks when maximizing the assemblage yield is a major issue.

Conclusion

A multispecies management plan has been implemented for the Northeast groundfish complex due to the acknowledgement of technological interactions (NEFMC, 1985). However, sustainable or rebuilding yield levels continue to be determined through single species models, and a multi-species model for addressing maximum yield and management actions is the next step. When yearly information is of interest, non-equilibrium modeling is applicable because it incorporates the inter-annual variability in mortality and recruitment. It is important to update management actions and goals to changing stock status. As some stocks (like Georges Bank haddock and yellowtail flounder) recover, new data becomes available and management targets and thresholds should change with these statuses (NEFSC, 2002). Our analyses demonstrate the affects of incorporating some population dynamics into management evaluations. Our findings are consistent with previous analyses regarding the difficulties associated with multispecies management. When maximizing total yield of the assemblage, more sensitive stocks will be overfished. This is a serious issue for the New England groundfish stocks with current laws preventing overfishing from occurring and many of them are on rebuilding plans. Regardless of the difficulties, a multispecies fishery requires a multispecies model for evaluation of management actions on the assemblage and the associated stocks. We provide initial analyses towards a short term, non-equilibrium model which would be a useful addition to current management by evaluating the immediate effects of fishing effort and gear selectivity on the groundfish stocks in the Northeast. The next step in this research is to develop non-equilibrium projections from current stock conditions using

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the same mixed-species process equations. Possible extensions to the model include the incorporation of economic data (e.g. costs, revenues, profits) and stochasticity in the form of recruitment variability and estimate uncertainty.

Acknowledgements We thank Steve Murawski and Brian Rothschild for their guidance and enthusiasm in this project, Massachusetts Marine Fisheries Institute for funding, SMAST Study-Fleet Program and Northeast Fisheries Science Center for sharing their data, and Fred Serchuck for his constructive review.

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References Boreman, J., Nakashima, B.S., Wilson., J.A., and Kendall, R.L. editors. 1997. Northwest Atlantic groundfish: perspectives on a fishery collapse. American Fisheries Society, Bethesda, Maryland. Brodziak, J., Rago, P., and Conser, R. 1998. A general approach for making short-term stochastic projections from an age-structured fisheries assessment model. In F. Funk, T. Quinn II, J. Heifetz, J. Ianelli, J. Powers, J. Schweigert, P. Sullivan, and C.-I. Zhang (Eds.), Proceedings of the International Symposium on Fishery Stock Assessment Models for the 21st Century. Alaska Sea Grant College Program, Univ. of Alaska, Fairbanks. Department of Commerce (DOC). 1996. Magnuson Stevens Fishery Conservation and Management Act. Department of Commerce (DOC). 2006. Magnuson Stevens Fishery Conservation and Management Act. FAO. 2006. The state of world fisheries and aquaculture. Rome, Italy. Hennessey, T. and Healy, M. 2000. Ludwig’s ratchet and the collapse of New England groundfish stocks. Coastal Management. 28: 187-213. Hilborn, R., and Walters, C. 1992. Quantitative fisheries stock assessment: Choice, dynamics and uncertainty. Chapman & Hall, New York. Holland, D. S. 2003. Integrating spatial management measures into traditional fishery management systems: the case of the Georges Bank multispecies groundfish fishery. ICES J. Mar. Sci. 60: 915–929. Holland, D. S. and Maguire, J.J. 2003. Optimal effort controls for the multispecies groundfish complex in New England: what might have been. Can. J. Fish. Aquat. Sci. 60: 159-170. Hollowed, A. B., Bax, N., Beamish, R., Collie, J., Fogarty, M., Livingston, P., Pope, J., and Rice, J. C. 2000. Are multispecies models an improvement on single-species models for measuring fishing impacts on marine ecosystems? ICES J. Mar. Sci. 57: 707–719. Mayo, R. K., Lange, A. M. T., Murawski, S. A., Sissenwine, M. P., and Brown, B. E. 1981. Estimation of discards in mixed trawl fisheries off the northeast coast of the United States, based on bottom trawl survey catches. ICES C. M. 1981/G:62. Mayo, R.K.; Terceiro, M., editors. 2005. Assessment of 19 Northeast groundfish stocks through 2004. 2005 Groundfish Assessment Review Meeting (2005 GARM), Northeast

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Fisheries Science Center, Woods Hole, Massachusetts, 15-19 August 2005. U.S. Dep. Commer., Northeast Fish. Sci. Cent. Ref. Doc. 05-13; 499 p. Murawski, S. A. 1984. Mixed-species yield-per-recruit analyses accounting for technological interactions. Can. J. Fish. Aquat. Sci. 41: 897- 916. Murawski, S. A, and Finn, J. T. 1988. Biological bases for mixed-species fisheries: species co-distribution in relation to environmental and biotic variables. Can. J. Fish Aquat. Sci. 45: 1720-1735. Murawski, S. A., Lange, A. M., and Idoine, J. S. 1991. An analysis of technological interactions among Gulf of Maine mixed-species fisheries. ICES Mar. Sci. Symp. 193: 237-252. Murawski, S. A., Lange, A. M., Sissenwine, M. P., and Mayo, R. K. 1983. Definition and analysis of multispecies otter-trawl fisheries off the northeast coast of the United States. J. Cons. Int. Explor. Mer, 41: 13-27. New England Fishery Management Council. 1985. Fishery management plan for the Northeast multi-species fishery. New England Fishery Management Council. Saugus, Massachusetts.

New England Fishery Management Council. 2002. Re-evaluation of biological reference points for New England Groundfish. Northeast Fish. Sci. Cent. Ref. Doc. 02-04; 395.

New England Fishery Management Council. 2006. Framework Adjustment 42 to the Northeast Multispecies Fishery Management Plan. Paulik, G. J., Hourston, A. S., and Larkin, P. A. 1967. Exploitation of Multiple Stocks by a Common Fishery. J. Fish. Res. Bd. Canada. 24(12): 2527-2537. Pikitch, E.K. 1987. Use of a mixed-species yield-per-recruit model to explore the consequences of various management strategies for the Oregon flatfish fishery. Canadian Journal of Fisheries and Aquatic Sciences 44(Suppl. 2):349-359. Ricker, W. E. 1958. Maximum sustainable yields from fluctuating environments and mixed stocks. J. Fish. Res. Bd. Can. 15: 991-1006. Rogers, J. B., and Pikitch, E. K. 1992. Numerical definition of groundfish assemblages caught off the coasts of Oregon and Washington using commercial fishing strategies. Can. J. Fish. Aquat. Sci. 49: 2648-2656. Spencer, P.D., Wilderbuer, T.K., and Zhang, C.I. 2002. A mixed-species yield model for eastern Bering Sea shelf flatfish fisheries. Can. J. Fish. Aquat. Sci. 59: 291-302.

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Thompson, W.F., and Bell, F.H. 1934. Effect of changes in intensity upon total yield and yield per unit of gear. Rep. Int. Fish. Comm. 8: 7–49. Vinther, M., Reeves, S.A., and Patterson, K.R. 2004. From single-species advice to mixed-species management: taking the next step. ICES J. Mar. Sci. 61: 1398-1409. Walters, C.J., and Bonfil, R. 1999. Multispecies spatial assessment models for the British Columbia groundfish trawl fishery. Can. J. Fish. Aquat. Sci. 56: 601-628.

Appendix Table 1. Equilibrium yield (mt) of seven New England groundfish stocks and the assemblage at various effort levels based on average recruitment. Red values depict maximum yield. (DAS – days at sea; GBC – Georges Bank cod; GBH – Georges Bank haddock; GBYT – Georges Bank yellowtail; CCYT – Cape Cod yellowtail; GMC – Gulf of Maine cod; PLAICE – American plaice; WITCH – witch flounder)

DAS GBC GBH GBYT CCYT GMC PLAICE WITCH ASSEMBLAGE0 0 0 0 0 0 0 0 0

10,000 9,043 3,299 6,372 1,917 9,599 1,886 866 32,983 20,000 13,088 5,110 6,718 2,326 11,151 3,092 1,329 42,815 30,000 14,948 6,180 6,620 2,443 11,286 3,877 1,585 46,939 40,000 15,806 6,852 6,500 2,476 11,144 4,396 1,732 48,908 50,000 16,188 7,297 6,403 2,484 10,976 4,743 1,817 49,907 60,000 16,335 7,602 6,326 2,483 10,834 4,976 1,867 50,423 70,000 16,367 7,819 6,264 2,481 10,721 5,133 1,896 50,681 80,000 16,342 7,978 6,213 2,479 10,633 5,239 1,912 50,794 90,000 16,290 8,096 6,170 2,478 10,561 5,309 1,919 50,822 100,000 16,228 8,187 6,133 2,477 10,501 5,354 1,921 50,800

Table 2. Spawning stock biomass (mt) of seven New England groundfish stocks and the assemblage at various effort levels, based on average recruitment. Red values depict 40% of maximum SSB. (DAS – days at sea; GBC – Georges Bank cod; GBH – Georges Bank haddock; GBYT – Georges Bank yellowtail; CCYT – Cape Cod yellowtail; GMC – Gulf of Maine cod; PLAICE – American plaice; WITCH – witch flounder)

DAS GBC GBH GBYT CCYT GMC PLAICE WITCH ASSEMBLAGE0 264,613 97,942 65,015 26,516 186,681 80,293 30,679 751,739

10,000 182,973 77,456 24,903 11,201 74,029 64,697 22,839 458,098 20,000 134,927 64,410 15,962 6,580 40,949 53,298 17,740 333,865 30,000 104,880 55,580 12,230 4,575 27,823 44,782 14,278 264,148 40,000 85,043 49,283 10,186 3,517 21,427 38,287 11,834 219,577 50,000 71,314 44,592 8,881 2,882 17,820 33,240 10,046 188,775 60,000 61,420 40,967 7,962 2,465 15,552 29,250 8,698 166,314 70,000 54,039 38,081 7,273 2,172 14,002 26,046 7,653 149,267 80,000 48,366 35,726 6,733 1,954 12,875 23,436 6,824 135,915 90,000 43,894 33,763 6,295 1,785 12,013 21,284 6,154 125,188 100,000 40,291 32,098 5,932 1,650 11,329 19,488 5,601 116,388 40% SSB 105,845 39,177 26,006 10,606 74,672 32,117 12,272 300,696

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Table 3. Equilibrium yield (mt) of seven New England groundfish stocks and the assemblage at various effort levels based on equilibrium recruitment. Red values depict maximum yield. (DAS – days at sea; GBC – Georges Bank cod; GBH – Georges Bank haddock; GBYT – Georges Bank yellowtail; CCYT – Cape Cod yellowtail; GMC – Gulf of Maine cod; PLAICE – American plaice; WITCH – witch flounder)

DAS GBC GBH GBYT CCYT GMC PLAICE WITCH ASSEMBLAGE0 0 0 0 0 0 0 0 0

10,000 51,156 45,561 6,893 1,917 14,989 1,886 866 123,267 20,000 69,418 70,571 7,267 2,326 16,480 3,092 1,329 170,484 30,000 73,536 85,344 7,161 2,443 15,683 3,877 1,585 189,628 40,000 71,393 94,627 7,031 2,476 14,570 4,396 1,732 196,225 50,000 66,479 100,760 6,926 2,484 13,555 4,743 1,817 196,763 60,000 60,402 13,208 3,701 2,483 12,700 4,976 1,867 99,336 70,000 53,927 13,585 3,664 2,481 11,983 5,133 1,896 92,669 80,000 47,420 13,861 3,635 2,479 11,374 5,239 1,912 85,919 90,000 41,056 14,066 3,610 2,478 10,846 5,309 1,919 79,283 100,000 34,910 14,223 3,588 2,477 10,380 5,354 1,921 72,853

Table 4. Spawning stock biomass (mt) of seven New England groundfish stocks and the assemblage at various effort levels based on equilibrium recruitment. Red values depict 40% of maximum SSB. (DAS – days at sea; GBC – Georges Bank cod; GBH – Georges Bank haddock; GBYT – Georges Bank yellowtail; CCYT – Cape Cod yellowtail; GMC – Gulf of Maine cod; PLAICE – American plaice; WITCH – witch flounder)

DAS GBC GBH GBYT CCYT GMC PLAICE WITCH ASSEMBLAGE0 1,577,631 1,352,501 70,326 26,516 303,171 80,293 30,679 3,441,117

10,000 1,035,009 1,069,608 26,938 11,201 115,598 64,697 22,839 2,345,890 20,000 715,676 889,450 17,266 6,580 60,517 53,298 17,740 1,760,526 30,000 515,966 767,518 13,229 4,575 38,662 44,782 14,278 1,399,010 40,000 384,117 680,564 11,018 3,517 28,013 38,287 11,834 1,157,350 50,000 292,870 615,776 9,606 2,882 22,007 33,240 10,046 986,427 60,000 227,112 71,174 4,658 2,465 18,230 29,250 8,698 361,587 70,000 178,052 66,161 4,255 2,172 15,650 26,046 7,653 299,988 80,000 140,348 62,069 3,939 1,954 13,773 23,436 6,824 252,343 90,000 110,625 58,659 3,683 1,785 12,338 21,284 6,154 214,527 100,000 86,676 55,766 3,470 1,650 11,198 19,488 5,601 183,849 40% SSB 631,053 541,000 28,131 10,606 121,268 32,117 12,272 1,376,447

Figure 1. Stepwise stock recruit relationship for Georges Bank haddock based on average recruitment.

0 50000 100000 150000

0e+0

02e

+05

4e+0

56e

+05

8e+0

5

SSB (tons)

age

1 ab

unda

nce

(thou

sand

s)

Figure 2. Stepwise stock recruit relationship for Georges Bank yellowtail flounder based on median recruitment.

0 5000 10000 15000 20000 25000

020

000

4000

060

000

8000

0

SSB (tons)

age

1 ab

unda

nce

(thou

sand

s)

16

Figure 3. Beverton-Holt stock recruit relationship for Georges Bank cod.

0e+00 2e+04 4e+04 6e+04 8e+04 1e+05

010

000

2000

030

000

4000

0

SSB (tons)

age

1 ab

unda

nce

(thou

sand

s)

Figure 4. Beverton-Holt stock recruit relationship for Gulf of Maine cod.

0 5000 10000 15000 20000 25000 30000

050

0010

000

1500

020

000

2500

030

000

SSB (tons)

age

1 ab

unda

nce

(thou

sand

s)

17

Figure 5. Total yield (mt) of the assemblage, GBC – Georges Bank cod, GBH – Georges Bank haddock, GBYT – Georges Bank yellowtail, CCYT – Cape Cod yellowtail, GMC – Gulf of Maine cod, PLAICE – American plaice, and WITCH – witch flounder based on median recruitment.

Fishing Effort (DAS)

Yie

ld (t

ons)

0

10000

20000

30000

40000

50000ASSEMBLAGE

0 20000 40000 60000 80000 100000

CCYT GBC

GBH GBYT

0

10000

20000

30000

40000

50000GMC

0

10000

20000

30000

40000

50000

0 20000 40000 60000 80000 100000

PLAICE WITCH

18

Figure 6. Spawning stock biomass (mt) of the assemblage, CCYT – Cape Cod yellowtail, GBC – Georges Bank cod, GBH – Georges Bank haddock, GBYT – Georges Bank yellowtail, GMC – Gulf of Maine cod, PLAICE – American plaice, and WITCH – witch flounder based on median recruitment.

Fishing Effort (DAS)

SS

B (t

ons)

0e+00

2e+05

4e+05

6e+05

ASSEMBLAGE

0 20000 40000 60000 80000 100000

CCYT GBC

GBH GBYT

0e+00

2e+05

4e+05

6e+05

GMC

0e+00

2e+05

4e+05

6e+05

0 20000 40000 60000 80000 100000

PLAICE WITCH

19

Fishing Mortality

Rec

ruits

(tho

usan

ds)

50000

100000

150000CCYT

0 1 2 3

GBC GBH

GBYT GMC

50000

100000

150000PLAICE

50000

100000

150000

0 1 2 3

WITCH

Figure 7. Recruitment as a function of fishing mortality. (CCYT – Cape Cod yellowtail; GBC – Georges Bank cod; GBH – Georges Bank haddock; GBYT – Georges Bank yellowtail; GMC – Gulf of Maine cod; PLAICE – American plaice; WITCH – witch flounder)

20

Figure 8. Total yield (mt) of the assemblage, – Cape Cod yellowtail, GBC – Georges Bank cod, GBH – Georges Bank haddock, GBYT – Georges Bank yellowtail, CCYT GMC – Gulf of Maine cod, PLAICE – American plaice, WITCH – witch flounder based on equilibrium recruitment.

Fishing Effort (DAS)

Yie

ld (t

ons)

0

50000

100000

150000

200000ASSEMBLAGE

0 20000 40000 60000 80000 100000

CCYT GBC

GBH GBYT

0

50000

100000

150000

200000GMC

0

50000

100000

150000

200000

0 20000 40000 60000 80000 100000

PLAICE WITCH

21

Fishing Effort (DAS)

SS

B (t

ons)

0e+00

1e+06

2e+06

3e+06

ASSEMBLAGE

0 20000 40000 60000 80000 100000

CCYT GBC

GBH GBYT

0e+00

1e+06

2e+06

3e+06

GMC

0e+00

1e+06

2e+06

3e+06

0 20000 40000 60000 80000 100000

PLAICE WITCH

22

Figure 9. Spawning stock biomass (mt) of the assemblage, CCYT – Cape Cod yellowtail, GBC – Georges Bank cod, GBH – Georges Bank haddock, GBYT – Georges Bank yellowtail, GMC – Gulf of Maine cod, PLAICE – American plaice, and WITCH – witch flounder based on equilibrium recruitment.