Cod Spawning Behavior and Observed Sex Ratios: A Liability and an Asset

Catches of Atlantic cod (Gadus morhua) from known spawning grounds are often heavily skewed towards males.  Recent acoustic telemetry work in the Gulf of Maine has identified sexually dimorphic behavior as the principal cause of this phenomenon.  During the spawning season, females are more densely aggregated and less mobile than males, making them less susceptible to capture. While there is some evidence of spatial segregation between sexes at small scales, there is little reason to expect that the operational sex ratio is unequal at larger scales.  If ignored, this behavioral influence on the observed sex ratio can introduce a negative bias on female-based measures of stock reproductive capacity.  On the other hand, this phenomenon can also be utilized to identify specific spawning arenas, aiding in the design of effective conservation measures.  We describe a ground-breaking study on the spawning behavior of wild Atlantic cod in relation to the role of sex ratio data in stock assessments and fishery management plans.  Our results demonstrate how an understanding of fine-scale reproductive ecology can improve inferences at the population level.