Relative contribution of spring- and summer-spawned bluefish cohorts to the adult population: effects of size-selective winter mortality, overwinter growth, and sampling bias

Abstract

Length distributions of juvenile bluefish (Pomatomus saltatrix) are bimodal, consisting of spring- and summer-spawned fish. Research during the 1990s from the northeastern United States suggested that the summer cohort contributes little to the adult population and that overwinter mortality may limit their survival. We examined length distributions of juvenile bluefish from before and after winter and found that the mean length of the summer cohort increased during winter. Based on a winter-growth experiment and temperatures from the two winters examined, changes in mean length were due to size-selective mortality for one year class and growth for the other. Despite evidence for winter mortality, summer-spawned bluefish were commonly caught at age 1. We reexamined the relative contribution of each cohort to the adult population using archived scales from North Carolina fisheries. Cohort origin of adults was determined by back-calculating length at age 1. One-third of adults consisted of summer-spawned fish, contrasting with previous research from the northeast. The differences in relative cohort abundance between the northeastern and southeastern United States arise from sized-based migration in age-1 and age-2 bluefish (the dominant age groups sampled in these studies) and the size-selective gear and sampling season in which they were collected.