Prey selectivity in planktivorous juvenile fishes associated with floating objects in the western Mediterranean

Abstract

Aspects of the feeding ecology of planktivorous juvenile fishes were assessed by comparing stomach content samples from purse seine catches with food environment samples from vertical hauls of a WP-2 mesozooplankton net. Sampling was performed fortnightly over 2 years, around moored experimental floating objects and at open water control stations in oceanic Mediterranean waters. Species composition, abundance and seasonal occurrence of both potential prey (zooplankton) and predators (fish) were studied. Dietary composition was examined for six fish species, and prey selectivity was assessed by means of several selectivity indices: Ivlev's selectivity index, linear food selection index and forage ratio. A comparison of the results showed significant variations among indices, related to both the degree of selection and the relative abundance of the food type in the environment. Most fish caught were pelagic juveniles, and had fed on plankton; however, there was variability in diet composition and prey selectivity among species allowing their separation into generalists and specialists. The pilot fish Naucrates ductor (Linnaeus) was a specialist, preying upon neustonic hyperiid amphipods, whereas the three congeneric species of Trachurus: T. picturatus (Bowdich), T. trachurus (Linnaeus) and T. mediterraneus (Steinachner) fed on copepods and other planktonic crustaceans. The species studied appeared to partition food resources to minimize the overlap in use of a very specific habitat.