Density-dependent overwinter survival in young-of-year bluefish (Pomatomus saltatrix)? A new approach for assessing stage-structured survival

Identifying the life history stages where density-dependent mortality occurs is essential for understanding fish population dynamics. Here we present an approach to evaluate density-dependent survivorship using catch-at-length data on bluefish (Pomatomus saltatrix) from fishery-independent data in the northwest Atlantic, focusing on the stage transition from age-0 to age-1. Abundance indices for each stage were generated by partitioning annual length frequency distributions into catch estimates for individual stages and by using a generalized additive model to standardize survey effort across years. We tested for the existence of density dependence by fitting the data to alternative stage-transition models and using model selection procedures to identify the best-fitting model. We found that that the transition from fall age-0 to age-1 is density dependent and that that significant dampening occurred throughout much of the time series we examined. Estimates of process error were small, suggesting relatively modest variance around the stage-transition model and little interannual variation in survivorship other than that generated by density-dependent effects. Further, we found our conclusions are robust to any potential bias imposed by observation error. We conclude that an improved understanding of bluefish population dynamics might follow from a fuller exploration of the processes dictating overwinter survivorship.