Cohort splitting in bluefish, Pomatomus saltatrix, in the US mid-Atlantic Bight

Atlantic bluefish exhibit cohort splitting, whereby two modes of juvenile recruits originate from spatially distinct springand summer-spawning regions in US Atlantic shelf waters. We evaluate the pattern of cohort splitting in a transition area (US Maryland coastal region and Chesapeake Bay) between the two major spawning regions. Spring and summer cohorts were differentially represented in Maryland estuarine (Chesapeake Bay) and coastal waters. The spring cohort was dominant in Chesapeake Bay, but was not well represented in the ocean environment, and the converse true for the summer cohort. We hypothesized that ocean temperatures control the bimodal spawning behavior and extent of cohort splitting. As evidence, we observed an intervening early summer cohort produced in years when shelf temperatures during early summer were suitably warm for spawning. In most years however, two dominant cohorts were evident. We propose that vernal warming dynamics in the midAtlantic Bight influence spawning behavior and the resultant bimodal pattern of seasonal juvenile cohort production commonly observed along the US east coast.