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Hypotheses for the Decline of Cod in the North Atlantic*

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Hypotheses for the Decline of Cod in the North Atlantic* MARINE ECOLOGY PROGRESS SERIES Vol. 138: 293-308, 1996 Published July 25 Mar Ecol Prog Ser REVIEW Hypotheses for the decline of cod in the North Atlantic* Ransom A. ~yers'#**,Jeffrey A. ~utchings~,N. J. Barrowman' 'Northwest Atlantic Fisheries Centre, Science Branch, PO Box 5667, St. John's, Newfoundland, Canada AlC 5x1 'Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 451 ABSTRACT We examine alternative hypotheses for the decllne of 20 cod Gadus morhua stocks in the North Atlantlc The year of the lowest observed biomass of spawners did not correspond to low juve- nile survival for the cohorts that should have contnbuted to the stock in that year However, fishing mortdl~tywas very high for the years preceding the collapse The collapse of the cod stocks was not caused by a lack of resilience at low population abundance because all spawners were able to produce many potential replacements at low population size We show that as populations collapsed, flshlng mortality increased untll the populations were reduced to very low levels We conclude that increased fishing mortality caused the population decl~nes,and often the collapses, of the cod stocks KEY WORDS Gadus morhua Cod North Atlantic Decline Stocks Tlme serles Spawners Recru~tment Catch Mortality Collapse Density-dependent mortality Fishlng INTRODUCTION a fishery. We have reformulated these questions as follows: During the last few years many of the world's cod (1)What was the extent and timing of the population Gadus morhua stocks have rapidly declined to the decline? point where fishing has been effectively eliminated. (2) To what extent were increased juvenile and fish- This has caused a major economic and social disaster ing mortality responsible for the population decline? for some regions. By 1993,6Canadian stocks of cod had (3) How resistant was the population to increased collapsed to the point where a total moratorium was de- fishing through compensation in recruitment? clared on fishing (Hutchings & Myei-S 1994). In 1994, (4) What is the resilience of the population, i.e. Iceland, which depended on cod for 70 % of its export its abllity to reproduce and grow at low population earning, reduced its cod catches by in 1 yr. Similar sizes? drastic reductions occurred in Norway In 1989 (Jakab- (5) Do catchability, spatial distribution, and fishing sen 1994).The cod populations off the eastern USA are mortality change at low population sizes? at unprecedented low levels (Murawski et al. 1996).Re- We address these questions using methods similar to ductions in catch have occurred or are contemplated in those employed by Beverton (1990). We differ from other regions (ICES 1994a).The purpose of this paper is Beverton in that we believe that it is important to sep- to investigate the causes of these collapses. arate juvenile mortality into natural mortality and that In an analysis of small pelagic fish, Beverton (1990) indirectly caused by fishing, e.g. discarding of young posed 5 fundamental questions about the collapse of fish. In addition, we ask: (6) How did the perception of fishing mortality change over time? 'Dedicated to the memory of the late R J. H. Beverton, whose work inspired this paper (7) How does the collapse of cod compare with the "E-mail myers@mi-spocknwafc nf ca collapse of small pelagic fish examined by Beverton? O Inter-Research 1996 Resale of full art~clenot permitted 294 Mar Ecol Prog Ser 138: 293-308, 1996 TESTING HYPOTHESES that year to be low if this hypothesis is true (Fig 1).The last 5 year classes that contributed to the SSB in the We first consider the timing and cause of the decline year of SSB minimum are in the cod stocks. We use the Spawning Stock Biomass (SSB) as our metric of the stock status, which is de- noted in year y as S,. and is defined as where A,,,, is the average age at which 50 % of females A are mature. The average of the juvenile survival over s, = C N,,,w,,a p,,, (1) these 5 yr represents the survival of those recruits that a=l contributed to the minimum SSB. The juvenile survival where A is the maximum age observed, W,,,is the that contributed to the maximum SSB was computed in weight at age a at the time of spawning in year y, N,., a similar way (Table 1). The choice of 5 yr makes our is the numbers of fish alive at the time of spawning in analysis comparable to that of Beverton (1990). year y at age a, and Py,,is the proportion mature at age Juvenile survival rate is the ratio of the number of a in year y. recruits to the SSB of the parent stock. The juvenile If environmental change in early life is the primary survival rate (termed reproductive rate by Beverton cause of population declines, we would expect to see a 1990) is estimated from egg production to recruitment decline in the survival to recruitment. If y,,,, is the year to the fishery. This index of 'juvenile' survival will of minimum SSR, then we would expect survival of the include the egg and larval stage as well as the early year classes that should have contributed to the SSB in demersal stage up to the age that fishing begins. For northern regions, the age of recruitment to the fishery is 3 yr, e.g. the Barents Sea stock; while in warmer waters, the age of recruitment is 1 yr , e.g. the Irish Sea stock. Using a scaling factor, the survival rate can be converted into a standardized form which allows sur- vival rates to be compared across stocks. The scaling factor we use is SPR,,,, the spawning stock biomass per recruit when fishing mortality is zero (Goodyear 1977, Gabriel et al. 1989, Mace & Sissenwine 1993, Mace 1994),which is defined as B 5yeas %I i Survival where a, is the age of recruitment to the fishery and M, Rate is natural mortality at age c. Thus, the standardized juvenile survival rate to recruitment is C S years i Estimates of SPRF,o were extracted from the database > compiled by Mace & Sissenwine (1993) and are based on the same age-at-recruitment, growth, and matura- Fishing Mortality tion schedules used in the Myers et al. (1995~)database of spawner, recruitment, catch and fishing mortality es- timates. Note that when the standardized juvenile sur- 1 IIIIJIIIIIIItIIIII vival rate is less than 1, the population would not be ex- pected to replace itself. We use the term 'survival rate' Year instead of the term 'reproductive rate' used by Beverton Fig. 1. Expected pattern in the change of (A) Spawnlng Stock (1990) because Eqs. (3) and (4) measure survival to the Biomass (SSB). (B) standardized juvenile sunrival rate, and (C)fishing mortality under alternative hypotheses. If the min- commercial fishery, i.e. some of the mortality may be imum SSB was caused by decreased juven~lesurvival for the caused by discarding and not be related to reproduc- 5 yr that shouid have most contributed to the SSB (i.e. the 5 yr tive success in the ordinary sense. previous to the number of years required for maturity, A,,,), In several cases, the time series for recruitment and then we should expect juvenile survival as in (B).If the mini- SSB extend back to the beginning of the large-scale mum SSB was caused by an increased fishlng mortality in the 5 yr before the minimum, then we should expect the pattern industrial fishery (late 1950s/early 1960s), e.g. West of fishing mortality in (C) Greenland, Labrador/NE Newfoundland, St. Pierre Table 1. Statistics on the maximum (Max) and mnlmum (Min.) observed Spawning Stock B~omass(SSB, thousands of tonnes) for 20 cod populations. The j~~venllesurvival rates (standardized ratio of the number of recruits to t.he SSB of the parent stock; Eq. 4) are averages for the 5 most recent year classes that contributed to the maximum or m~n~mumSSB (except near the beginning of a data series, in which case less than 5 yr of data was used). For stocks for which it is not possible to estimate the surv~valrate of the cohorts that contributed to the maximum or mlmmum observed SSB, we used the f~rst5 observed survival rates; these survival rates are denoted by asterisks. The instantaneous fishing mortality rates given are averages for the 4 yr preceding the maximum or minimum SSB as well as the year itself (again, except near the beginning of a data series, in which case less than the total of 5 yr of data was used). Also given are the ages (in yr) used in the calculation of fishing mortality Stock NAFO/ICES MBX Min Min SSB/ Survival rate Fishing mortality Year SSB Year SSB Max SSB for Max for Min for Max for Min Ages West Greenland 1 1956 1496 1977 27 1/55 3.02' 2.4 0.23 0.72 6 .8 Labrador/NE Nfld 2J3KL 1962 1599 1992 22 1/73 7.75' 8.23 0.33 1.48 7-9 S Grand Bank 3N0 1967 193 1993 17 1/11 17.9 1.84 0.72 0.39 7-9 N Gulf of St. Lawrence 3Pn4RS 1984 188 1993 10 1/19 13.47 3.01 0.47 1.03 7-9 St. Pierre Bank 3Ps 1960 139 1993 32 1/4 5.48' 7.58 0.4 0.84 7-9 S Gulf of St. Lawrence 4TVn 1956 429 1993 49 1/9 2.44 2.6 0.31 0.82 7-14 E Scotian Shelf 4VsW 1963 103 1993 16 1/6 14.65 3.87 0.51 0.85 7-9 SW Scotian Shelf 4X 1966 90 1993 36 1/2 6.09 4.22 0.58 0.79 6-12 Georges Bank 5 2 1980 93 1992 48 1/2 7.94 7.11 0.46 072 4-10 SE Baltic 22 & 24 1980 50 1992 7 1/7 37 81 21.04 0.94 1.25 3-7 Central Baltic 25-32 1980 818 1992 79 1/10 15.29 4.04 0.74 1.08 4-7 Celt~cSea VIIg,f 1989 13 1976 3 1/4 28.14 5.83 0.83 0.58 2-5 Faroe Plateau Vb 1984 115 1992 17 1/7 7.69 3.7' 0.49 0.64 3-7 Iceland Va 1955 1380 1988 188 1/7 4.49' 8.94 0.3 0.78 8-12 Irish Sea VlIb,c,h-k 1973 11 1992 3 1/4 31.86 40.93 0.66 1.07 2-5 Kattegat South llIa 1973 52 1991 4 1/13 8.48 11.08 0.43 1.22 2-6 Barents Sea I & 11 1946 4078 1987 149 1/27 2.32' 12.94 0.11 0.88 5-10 North Sea 1V 1970 271 1991 56 1/5 20.21 26.57 0.55 0.9 2-8 Skagerrak North llla 1982 3 3 1987 14 1/2 35.45 15.77 0.9 1.09 3-6 West of Scotland IVa 1968 56 1992 15 1/4 3.22 12.63 0.49 0.83 2-5 Fishing Mortality SS6 and Catch Su~ivalRate Fishing Mortality SSB and Catch Survival Rate 140 1.2 - 10 - 0.8 - 80 St.
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