Management Implications of Spatially Varying Catchability in the Gulf Menhaden Fishery in the Gulf of Mexico

The catchability of a harvested species to its fishery often varies in time or space, but is considered a constant value in many stock assessment models, including the stock assessment model for Gulf menhaden in the Gulf of Mexico. The spatial distribution of catches in the Gulf menhaden fishery changes in response to declines in dissolved oxygen concentrations. Catchability of Gulf menhaden is likely to vary due to changes in the distribution of the fishery, but whether such variations influence management advice for the fishery remains unknown. We fit the original stock assessment model to simulated catch data of Gulf menhaden based on a two-area population model with known differences in catchability to determine whether management recommendations would be sensitive to assumptions about catchability. We compared scenarios where catchability was varied for one or both areas in the population model, and where catchability was either directly proportional to the area of hypoxia in the Gulf of Mexico, or followed alternative functional forms. We found that despite large changes in catchability of Gulf menhaden to its fishery, stock status indicators changed very little from the original assessment model that assumed constant catchability.