How Marine Reserves Could Affect the Performance of Spatial and Conventional Stock Assessments

Fish populations occur heterogeneously in space naturally, and as a result of anthropogenic influences such as spatial management. Few stock assessments account for spatial heterogeneity, assuming instead that populations (or fishing mortality) are distributed evenly across space. A special case of spatial management is the use of no-take marine reserves. In this study we use simulation modeling to analyze the ability of several stock assessment methods to estimate current biomass, unfished biomass, and depletion after the implementation of a single large no-take marine reserve. We use age- and sex-structured two-dimensional spatial operating models with five patterns of ontogenetic movement to represent the “true” underlying population dynamics. Results show that assessing populations as a single stock without accounting for the no-take marine reserve results in severe underestimation of biomass. Performing separate assessments for fished and protected areas leads to improved estimation performance in the absence of movement between assessment areas, but can severely overestimate biomass otherwise.