Impacts of population and fishery spatial structures on fishery stock assessment

Fish populations and fishing efforts in most fisheries exhibit spatial heterogeneity. However, spatial considerations are generally ignored in fishery stock assessment and management because of a lack of spatially explicit data and poor understanding of the spatial dynamics of most fisheries. This study uses a simulation approach to evaluate the consequences of misspecifying spatial structure and migration during the assessment process. We developed an operating model to simulate a fishery using US Atlantic herring (Clupea harengus) as our model species. This population consists of two well-defined spawning substocks distributed and mixed in four management areas. Simulations were done for three alternative "true" populations, each having a different spatial structure both biologically and with regard to the geographic distribution of fishing effort. Stock assessments were then performed for the three simulated "true" populations using standard methodologies and assumptions currently used. Management-area-based assessments lead to overestimation of spawning stock biomass and underestimation of fishing mortality because of the interaction within the management area between the spatial structure of the population and that of the spatially heterogeneous fishery removals. In contrast, when fishing is spatially homogeneous, movement across management boundaries may not be relevant to modeling population dynamics. Such an idealized situation does not typically hold, however.