Temporal variations in the fecundity of Arcto-Norwegian cod (Gadus morhua)in response to natural changes in food and temperature

Sexually mature Arcto-Norwegian female cod, Gadus morhua, were sampled off northern Norway either during spawning migration (Vesteralen) or at spawnin g sites (Lofoten) from 1986 to 1996. This period comprised a dramatic, near ly cyclical change in the Barents Sea ecosystem. The stock of the main food item, viz. the Barents Sea capelin Mallotus villosus villosus, changed fro m a low (1986), to a high (1991) and again to a low (1994) level of abundan ce while the climate changed from a cold (less than or equal to 1989) to a warm regime. The relative annual potential fecundity (i.e. number of vitell ogenic oocytes per g prespawning fish) increased by approximately 40% from 1987 to 1991. However, information from a back-calculation technique calibr ated in the laboratory using spawning fish indicated that this change might have been as high as 80 to 90%. Ovaries were analysed by the gravimetric, the automated particle counting and the stereometric method (modified to us e with ovaries too large to section whole). All three methods gave similar fecundity estimates. The latter method was applied to quantify atresia of d eveloping oocytes in the good-condition year of 1991. Atresia was rare, occ urring in only 30% of the ovaries and where it was present in only 1 to 4% of the vitellogenic oocytes. Spawning females sampled from 1991 to 1996 gra dually produced fewer eggs and demonstrated clear interannual variations in vitellogenic oocyte mean size and distribution thought to reflect a delica te reproductive tactic to minimise negative nutritional effects on egg size and egg quality. Estimates of annual potential fecundity for the duration of the study were significantly positively correlated with environmental te mperature and the availability of capelin during vitellogenesis. (C) 1998 E lsevier Science B.V. All rights reserved.