Predatory behaviour and selectivity of a primary piscivore: comparison of fish and non-fish prey

Young-of-the-year (YOY) bluefish Pomatomus saltatrix become piscivorous at about 40 min total length, coinciding with a habitat shift from offshore wa ters to estuarine nursery areas, Although YOY bluefish are primarily pisciv orous in estuarine waters, shrimp prey can often be an important component of diets in mid-summer. In this study, we first compare the feeding behavio ur and prey selection patterns of bluefish feeding on fish Menidia menidia and sand shrimp Crangon septemspinosa prey in a series of laboratory experi ments, and then assess prey-type selectivity in the field. Compared to cons uming fish prey, ingesting shrimp prey involved added manipulation and clos e inspection by the predator, Feeding on shrimp prey resulted in lower atta ck success rates, longer handling times, and lower relative profitabilities than feeding on fish prey. When given a choice, bluefish selectively inges ted fish prey over similarly-sized shrimp prey. The results of these experi ments suggest that species-specific differences in prey response lead to th e observed differences in predator behaviour and selectivity. Examination o f field-collected spring-spawned bluefish showed that their diet was domina ted by fishes; however, sand shrimp were important prey in early summer. Se lectivity index values showed that bluefish exhibited positive selection fo r fishes and avoidance of shrimp. The occurrence of shrimp in bluefish diet s probably results from high relative abundance of shrimp and the small siz e 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 h ave a strong effect on bluefish size at the end of the growing season and, therefore size-dependent survival.