Premigration Patterns of Growth, Condition, and Resource Use by the Spring‐Spawned and Summer‐Spawned Cohorts of Juvenile Bluefish in the Hudson River Estuary, New York

In fish populations with multiple age‐classes of similar size and close proximity, interactions between age‐classes may be an important source of density‐dependent population structuring if there is a disparity in competitive ability, creating a competitive bottleneck. However, intercohort competitive interactions within a single year‐class have rarely been investigated and may have implications for recruitment success. Bluefish Pomatomus saltatrix produce at least two cohorts (spring‐ and summer‐spawned) of offspring each year, and juveniles of this species are abundant in the lower Hudson River estuary, New York, during summer and early autumn. Our objectives were to evaluate the potential for a competitive bottleneck between these spring and summer cohorts by assessing the cohort‐specific growth, condition, and patterns of resource use prior to the autumn migration in 2008 and 2009. The size advantage gained by the spring cohort resulting from earlier hatching dates was maintained throughout both summers. However, the summer juveniles achieved higher lipid content by the end of each summer. When the two cohorts inhabited the estuary at the same time, they exhibited high diet overlap and low spatial overlap. Although Bay Anchovy Anchoa mitchilli predominated the prey base, the two Bluefish cohorts exhibited cohort‐specific size‐selective feeding, such that the spring cohort preyed upon larger Bay Anchovy than the summer cohort, probably corresponding to different Bay Anchovy year‐classes. In this large estuary, the two juvenile Bluefish cohorts exhibited unique strategies with respect to growth and condition, which widened the competitive bottleneck by separating their niches spatially and by foraging on different age‐classes of Bay Anchovy.