MEPS 217:157-165 (2001)  -  doi:10.3354/meps217157

ABSTRACT: Young-of-the-year (YOY) bluefish Pomatomus saltatrix become piscivorous at about 40 mm total length, coinciding with a habitat shift from offshore waters to estuarine nursery areas. Although YOY bluefish are primarily piscivorous in estuarine waters, shrimp prey can often be an important component of diets in mid-summer. In this study, we first compare the feeding behaviour and prey selection patterns of bluefish feeding on fish Menidia menidia and sand shrimp Crangon septemspinosa prey in a series of laboratory experiments, and then assess prey-type selectivity in the field. Compared to consuming fish prey, ingesting shrimp prey involved added manipulation and close inspection by the predator. Feeding on shrimp prey resulted in lower attack success rates, longer handling times, and lower relative profitabilities than feeding on fish prey. When given a choice, bluefish selectively ingested fish prey over similarly-sized shrimp prey. The results of these experiments suggest that species-specific differences in prey response lead to the observed differences in predator behaviour and selectivity. Examination of field-collected spring-spawned bluefish showed that their diet was dominated by fishes; however, sand shrimp were important prey in early summer. Selectivity index values showed that bluefish exhibited positive selection for fishes and avoidance of shrimp. The occurrence of shrimp in bluefish diets probably results from high relative abundance of shrimp and the small size of shrimp relative to fish prey. We conclude that the timing of bluefish entry into estuaries relative to peaks in abundance of various prey could have a strong effect on bluefish size at the end of the growing season and, therefore size-dependent survival.

KEY WORDS: Bluefish · Crangon septemspinosa · Menidia menidia · Piscivory · Pomatomus saltatrix · Predation · Prey selectivity · Shrimp prey