Condition and size composition effects on the simulated relationship between spawner biomass and total egg production for Northeast Arctic cod (Gadus morhua L.)
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Spawner biomass (SB) is often assumed to be proportional to total egg production (TEP) by fish stocks. Recent studies suggest that this assumption may not be valid, particularly when the physiological condition of fish varies. This study used Monte Carlo techniques to simulate the relationship between SB and TEP for Northcast Arctic cod (Gadus morhua). In the first simulation exercise, reproductive traits reflecting condition (length-specific maturity, weight and fecundity) were varied according to food abundance (capelin biomass) while the size composition (numbers at length) was held constant. In the second exercise, the size composition, as described by the modal length of spawners, was varied and condition was held constant. In the final exercise, condition and size composition were varied simultaneously. The first exercise indicated that variation in TEP was not constant but increased as the SB increased. The second exercise showed that, for a given SB, TEP increased with increasing modallength of spawners. Comparing the results of the first and second exercise showed that food-dependent variation in condition introduced greater variability into the relationship between SB and TEP than did shifting the size composition. When condition and size composition were varied simultaneously, as would be expected in wild stocks, TEP exhibited approximately 4-fold level of variation at any given SB.