VARIABILITY IN MEDIAN SIZE AND AGE AT SEXUAL MATURITY OF 

 ATLANTIC COD, GADUS MORHUA, ON THE SCOTIAN SHELF IN 



THE NORTHWEST ATLANTIC OCEAN 



Terry D, Beacham 1 



ABSTRACT 



Median length and age at sexual maturity of Atlantic cod on the Scotian Shelf declined about 50% in most 

 stocks between 1959 and 1979. Atlantic cod in more northerly stocks matured at older ages than did those in 

 more southerly stocks. Males generally matured at younger ages and smaller sizes than did females. Large, 

 immature Atlantic cod disappeared from the stocks between 1959 and 1979. During 1975-79, virtually all 

 Atlantic cod age 5 and older were mature. The decline in median length and age at sexual maturity may be due 

 to the commercial fishery removing larger, older immature fish, a general decline in stock biomasses between 

 1960 and 1975 due to heavy exploitation, or both. 



Vertebrate population dynamics are determined by 

 the composite effects of reproduction, growth, dis- 

 persal, and mortality. The median age at which in- 

 dividuals attain sexual maturity profoundly impacts 

 potential population growth (Cole 1954; Stearns 

 1976). Size and age at sexual maturity are the direct 

 linkage between individual growth and reproductive 

 potential of a population, and therefore they are 

 parameters of prime concern in population dyna- 

 mics. 



Atlantic cod, Gadus morhua, is economically the 

 most important finfish species landed in the Atlantic 

 region of Canada. Heavy exploitation in the 1960's 

 and early 1 970's resulted in declines in stock biomass 

 of many Atlantic cod stocks in the Maritimes 

 (Halliday 1976; Beacham 1980). Canadian landings 

 of Atlantic cod on the Scotian Shelf [Northwest 

 Atlantic Fisheries Organization (NAFO) Divisions 

 4VWX] (Fig. 1) increased in the late 1970's while 

 landings by other countries, chiefly Spain and the 

 Soviet Union (Halliday 1976), declined. The south- 

 ern Gulf of St. Lawrence cod stock (Division 4T) also 

 supports an extensive fishery (Beacham 1980). 



Spawning biomass of a stock is of direct concern in 

 the management of a fishery and can be evaluated, 

 when the abundance and proportion mature at each 

 age are known. The presence of Atlantic cod in 

 several areas of NAFO Subarea 4 (McKenzie 1956; 

 McCracken 1959; Templeman 1962; Martin and 

 Jean 1964) presented an opportunity to investigate 



'Department of Fisheries and Oceans, Fisheries Research Branch, 

 Pacific Biological Station, Nanaimo, B.C. V9R 5K6, Canada. 



variation in median size and age at sexual maturity of 

 Atlantic cod among areas. The major purpose of this 

 paper is to present historical changes in median size 

 and age at maturity for Atlantic cod in NAFO Sub- 

 divisions 4Vn and 4Vs and Divisions 4 W and 4X and 

 to attempt to account for some of this variability 

 among cod in these areas. 



MATERIALS AND METHODS 



Data analyzed in this paper were collected during 

 1959-79 from groundfish surveys of the Canadian 

 research vessels MV Harengus, E. E. Prince, and A. 

 T. Cameron. In 1970, annual stratified random design 

 groundfish surveys were initiated on the Scotian 

 Shelf with the July surveys of the A. T. Cameron. Pre- 

 vious to 1970, surveys were not always conducted an- 

 nually in the areas examined for the present analysis. 

 Annual values for median (50% mature point) length 

 and age at sexual maturity of Atlantic cod on the Sco- 

 tian Shelf before 1970 were calculated from any sur- 

 vey during the summer, not only July. Maturity 

 ogives based on length and age were calculated for 

 four periods in the study, each corresponding to 

 about a 5-yr interval. All surveys conducted in each 

 interval were included in the calculation of the 

 maturity ogives. All Atlantic cod in the surveys for 

 which maturity stage was recorded and either length 

 or age were known were included in the determina- 

 tion of median size and age at maturity. Details of the 

 surveys, including vessels and gear used and areas 

 surveyed, were outlined by Halliday and Koeller 

 (1981). Fork length (centimeters) and maturity stage 



Manuscript accepted September 1982. 

 FISHERY BULLETIN: VOL. 81, NO. 2, 1983. 



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