ONTOGENIC SHIFT IN THE DIET OF YOUNG-OF-YEAR BLUEFISH POMATOMUS-SALTATRIX DURING THE OCEANIC PHASE OF THE EARLY LIFE-HISTORY

Along the U.S. east coast, the bluefish Pomatomus saltatrix spawns in offshore continental shelf waters during at least two distinct periods : spring and summer. Juveniles migrate to inshore nurseries where they complete the first growing season. Previous studies have shown that d iet during the oceanic larval stage consists of copepods, while older juveniles captured inshore feed largely on teleost prey. To determine timing of the ontogenetic shift in diet to piscivory, we examined the feeding habits of 189 early-juvenile bluefish (18-74 mmTL). Samples we re collected from continental shelf waters of the Middle Atlantic Bigh t (MAB) during spring and summer of 1988 and 1989. Spring- and summer- spawned P. saltatrix differed in body size, prey size, and in the prop ortions of prey types consumed. Copepods were the most common prey typ e in fish <60 mm. Teleost prey appeared initially in the diet of 30 mm individuals and became the major dietary item in spring-spawned fish >40 mmTL. Gut fullness and incidence of piscivory peaked in late after noon and were positively correlated with daylight hours. There was no evidence of an abrupt increase in mouth width associated with this ont ogenetic shift in diet. Because juvenile bluefish migrate inshore soon after becoming piscivores, their impact as predators on the abundance of other young fishes is probably focused on inshore/estuarine, rathe r than offshore species.