Accounting for spatial population structure at scales relevant to life history improves stock assessment: The case for Lake Erie walleye Sander vitreus

Abstract

Stock assessments commonly allow parameters to vary across fishery or jurisdictional boundaries, often by treating each region as a unit stock. However, animals generally disperse in response to spatial habitat features to satisfy particular life history requirements, and these features are often not congruent with fishery or jurisdictional boundaries. Thus, populations are often spatially structured at scales distinct from those acknowledged in assessments. Furthermore, when the spatial structure arises from dispersal of a common pool of recruits, redefining unit stock boundaries may not adequately capture these dynamics. Here we test the utility of spatially referencing parameters (vulnerability and catchability) in a statistical catch-at-age stock assessment model as a simple approach to account for life history variation of walleye ( Sander vitreus) when information on explicit movement rates is unavailable. We apply several alternative assessment models to Lake Erie walleye – a population identified as displaying age-specific differences in the extent of dispersal from spawning grounds – to investigate the importance of accounting for spatial heterogeneity at ecologically important scales in stock assessments. Comparisons of the most parsimonious assessment models (based on a deviance information criterion) with and without spatially referenced parameters (by basin) highlighted the importance of estimating regional vulnerability and catchability. There was strong statistical evidence that incorporating spatially referenced parameters at a scale relevant to walleye dispersal patterns improved model fit, and the change altered estimates of stock size and fishing mortality. For example, estimates of total age-2 and older walleye abundance in the most recent year decreased by 16% (34% for ages 7 and older) and fully selected fishing mortality increased by 70% after incorporating walleye spatial population structure. These results emphasize the importance of considering spatial aspects in stock assessments at scales relevant to the life history of the species or group of species under consideration.