How variable is the fecundity within and between cod stocks?

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Stock reproductive potential is determined by the number of fish at age, sex ratio, proportion of mature female at age and fecundity. Fecundity data, which are fundamental in estimating egg production are not collected routinely for most fish stocks. Thus, variability in fecundity is largely ignored in the estimations of reproductive potential. The objective of this study was to identify patterns of spatial and temporal variation in the fecundity within and between different stocks of Atlantic cod (Gadus morhua) in the North Atlantic and Baltic Sea. A review of fecundity data for cod of different stocks and years was made. The examination of fecundity at size indicated a very large variability in the fecundity of cod between stocks and years. The fecundity of a 60 cm cod varied from 210 000 eggs to 2 250 000 eggs. Baltic and North Sea cod stocks were very distinct from the others, highest fecundity-at-size notwithstanding years being observed for these stocks. A different pattern of variation was observed for the other stocks. Temporal variation in fecundity appeared as important as or even more important than differences associated with stocks. Without excluding genetic differences, variations in the environmental conditions and fishing pressure could possibly explain much of the variation in the fecundity of these stocks. At a smaller scale, it also appears that the increase in the reproductive investment with age/size can differ between stocks and years. Thus, routine measurement of fecundity should be essential in estimating reproductive potential of cod.