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Estimating synchronous changes in condition and density in eastern Bering Sea fishes

Arnaud GrĂ¼ss*, Jin Gao, James T. Thorson, Christopher N. Rooper, Grant Thompson, Jennifer L. Boldt, Robert Lauth

*Corresponding author:

ABSTRACT: Estimating fish condition, the relative weight of an individual fish given its body length, is a convenient way to relate the physiological health and energetic status of fishes to their productivity. Despite evidence of density-dependence effects on condition in some species, previous research has not jointly estimated synchronous changes in condition and density operating at fine spatial scales (a few kilometers). Therefore, we developed a spatio-temporal modeling approach that simultaneously estimates correlated variation in density (measured as numbers per area) and condition. We applied our approach to six eastern Bering Sea (EBS) groundfish species (four flatfishes and two gadoids) for the period 1992–2016, and estimated correlations in spatial variation (unmeasured variation that is stable over time) and spatio-temporal variation (unmeasured variation that changes between years). Spatial variation in density had a strong negative and significant association with spatial variation in condition for three flatfishes and a positive association for one gadoid. Spatio-temporal variation in density had a significant association with spatio-temporal variation in condition for one flatfish (negative) and one gadoid (positive). Moreover, for the six study species, bottom temperature was identified as an important predictor of both density and condition. The increasing trend in bottom temperatures between 1992 and 2016 was accompanied by an overall increase in the abundance-weighted condition of five species. We conclude that forecasts of changes in weight-at-age within some EBS groundfish assessments will require an understanding of both density-dependence and bottom temperature effects on fish condition to better prepare for future climate and exploitation changes.