A Spatial Model for Population-Selectivity

Different age-classes of fish typically do not experience the same rates of fishing mortality because fish size and behavior change with age.  Differential fishing mortality-at-age is usually described as selection or selectivity.  Processes causing selectivity operate at the level of the fishing gear but also more broadly at the level of the population.  In this paper a relatively simple model of population-level selectivity is developed from a set of survival equations that are coupled to allow movement of fish between spatially distinct sub-populations.  The model has a particularly surprising emergent property.  Even though the same asymptotic gear-selectivity applies across all subpopulations the overall population-selectivity will be dome-shaped unless fishing mortality is uniform across all subpopulations.  Hence the sometimes controversial feature of “cryptic biomass”, which follows from having domed-selection, may be a natural consequence of the non-uniform application of fishing.