Spawner quality effects on two measures of reproductive potential: have we been counting our eggs before they've hatched?

Abstract

Recent studies have found positive associations between recruitment and indices of spawner quality (condition) for stocks exhibiting an indeterminate relationship between recruitment and spawner biomass. This suggests that spawner biomass is insensitive to interannual variability in condition affecting total egg production. This hypothesis was tested for the Northeast Arctic (NA) cod stock using a simulation approach. Because abundance of the Barents Sea capelin stock is an important determinant of condition of NA cod, the simulations varied length-specific proportion mature, weight, and fecundity of NA cod according to capelin biomass while holding numbers at length constant. Total egg production and spawner biomass and their associated errors were estimated for levels of capelin biomass which encompassed the range observed in the Barents Sea. Increasing condition significantly increased both total egg production and spawner biomass, however, the magnitude of increase in total egg production was approximately double that in spawner biomass. Incorporating hypothesized effects of condition on atresia and skipped spawning increased the difference between total egg production and spawner biomass. Comparing sim~~lation results for different numerical abundances and size compositions indicated that spawning stocks dominated by smaller length classes had larger differences between total egg production at the minimum and maximum capelin biomasses.