The effects of a spatial gradient of fishing pressure in a developing fishery on stock assessments

Fisheries stock assessments make spatial assumptions about the behavior of the fish populations and the fishery.  One common assumption is that all fish of a given age or size are equally vulnerable to fishing.  This assumption is often violated, especially in developing fisheries when effort expands spatially as a fishery grows.  We conducted simulation experiments to examine the effects of differential fishing pressures on two partially mixed regions of a single population on estimates from statistical catch-at-age models.  Data sets were simulated from a two-region age-structured model, where one region of the population was fished at a gradually increasing level, and the other region was fished only towards the end of the simulation.  At the end of the simulation model the entire population was fully exploited.  Models that assumed a well-mixed stock produced biased estimates of fishing mortality and biomass when the stock was not well mixed.  The direction of the bias depended on the spatial configuration of fishing and its evolution over time.  Accounting for spatial differences in fishing pressure and how they have changed over time is important to consider in stock assessments that guide fisheries management.