FISHERY BULLETIN: VOL. 81, NO. 2 



All the Atlantic cod stocks examined in the present 

 study showed a downward trend both in median size 

 and median age at sexual maturity with time. Several 

 hypotheses would explain these events. However, to 

 the extent that size and age at maturity are genetical- 

 ly determined (Aim 1959), fish which mature at 

 smaller sizes or younger ages had a selective advan- 

 tage during the intensive fishery that has occurred on 

 many of the Atlantic cod stocks since 1960 in Sub- 

 area 4. These genotypes reproduce before being fully 

 recruited to the fishery, whereas genotypes that ma- 

 ture at larger lengths or older ages tend to be 

 removed before reproduction. This process could 

 contribute to the decreasing abundance of larger, im- 

 mature fish with time and account for the shifting of 

 the maturity ogives towards smaller sizes and 

 younger ages. Size-selective fishing has been pos- 

 tulated to account for declines in size of individuals 

 through the disproportionate removal of fast- grow- 

 ing individuals (Ricker et al. 1978;Favroetal. 1979). 

 It may be, therefore, that late-maturing genotypes 

 were removed from the Atlantic cod stocks in a 

 period of heavy exploitation. 



Environmental and genetic effects are difficult to 

 separate. There may be a lag effect if size and age at 

 sexual maturity have not increased at the same rate 

 as stock biomass. Because fish attaining maturity at 

 low stock biomass remain mature with increasing 

 biomass, then the lag in increase in size and age at 

 maturity results from year classes spawned under 

 high stock biomass conditions requiring a longer 

 period to attain maturity. There was an increase in 

 median size and age at maturity in 1979 for Atlantic 

 cod stocks on the Scotian Shelf, but further data are 

 necessary to determine if this will be continuing. If 

 median size and age at sexual maturity increase in the 

 next 5-10 yr to levels similar to those between 1959 

 and 1964, this would suggest that these parameters 

 were responding to stock biomass with a lag effect, 

 although this interpretation is confounded by a 

 decline in fishing intensity and thus selection. 

 However, if median size and age at sexual maturity 

 remain at the 1975-79 levels or decrease for the next 

 5-10 yr, this would suggest that there has been a 

 genetic change within the stocks, with Atlantic cod 

 maturing at larger sizes and older ages being selected 

 against. 



The rapid recoveries of many groundfish stocks 

 during periods of restricted exploitation after heavy 

 fishing, with no apparent genetic change, suggest 

 that biomass has greater influence on variability in 

 population parameters than do genetic changes. 

 Heavily exploited stocks of North Sea cod and had- 

 dock recovered rapidly during World War II, when 



fishing mortality was reduced (Gulland 1971). Pacific 

 halibut, Hippoglossoides stenolepis, stocks recovered 

 from low population levels with no apparent genetic 

 change (Miller 1957). 



In summary, the present study has indicated that 

 there has been a decline in median length and age at 

 sexual maturity for Atlantic cod in several areas in 

 NAFO Subarea 4 in the 1960's and 1970's. However, 

 whether this decline can be ascribed to genetic or to 

 environmental changes cannot be determined be- 

 cause application of the selective force (fishing inten- 

 sity) concurrently changes the environment (stock 

 biomass). Controlled selection experiments may pro- 

 vide some indication of potential for genetic change 

 in Atlantic cod. 



ACKNOWLEDGMENTS 



I am indebted to those people who have crewed the 

 research vessels and aged the Atlantic cod. Charles J. 

 Krebs of the University of British Columbia wrote 

 the computer program for probit analysis. Albert 

 Vromans ably assisted in much of the analysis. M. C. 

 Healey (Pacific Biological Station), R. G. Halliday 

 (Marine Fish Division), and two referees made many 

 helpful comments on a draft of this manuscript. The 

 manuscript was prepared with the assistance of the 

 staff at the Pacific Biological Station. 



LITERATURE CITED 



Alm, G. 



1959. Connection between maturity, size and age in fishes. 

 Experiments carried out at the Kalarne fishery research 

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BEACHAM, T. D. 



1980. 1980 assessment of cod in Division 4T and 4Vn (Jan- 

 Apr). Can. Atl. Fish. Sci. Adv. Ctte. Res. Doc. 80/22. 

 61 p. 

 1982. Some aspects of growth, Canadian exploitation, and 

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 Beacham, T. D., B. A. Fowler, and A. H. Vromans. 



1980. 1980 analyses of inshore cod stock in Subdivision 4Vn 

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 16, 24 p. 

 Cole, L. C. 



1954. The population consequences of life history phenom- 

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1979. Population-genetic study of the effects of selective 

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 Can. 36:552-561. 

 Fleming, A. M. 



1960. Age, growth and sexual maturity of cod (Gadus morhua 

 L.) in the Newfoundland area, 1947-1950. J. Fish. Res. 

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