Prey selectivity in planktivorous juvenile fishes associated with floatingobjects in the western Mediterranean

Aspects of the feeding ecology of planktivorous juvenile fishes were assess ed by comparing stomach content samples from purse seine catches with food environment samples from vertical hauls of a WP-2 mesozooplankton net. Samp ling was performed fortnightly over 2 years, around moored experimental flo ating objects and at open water control stations in oceanic Mediterranean w aters. Species composition, abundance and seasonal occurrence of both poten tial prey (zooplankton) and predators (fish) were studied. Dietary composit ion was examined for six fish species, and prey selectivity was assessed by means of several selectivity indices: Ivlev's selectivity index, linear fo od selection index and forage ratio. A comparison of the results showed sig nificant variations among indices, related to both the degree of selection and the relative abundance of the food type in the environment. Most fish c aught were pelagic juveniles, and had fed on plankton; however, there was v ariability 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 amphip ods, whereas the three congeneric species of Trachurus: T. picturatus (Bowd ich), T. trachurus (Linnaeus) and T. mediterraneus (Steinachner) fed on cop epods and other planktonic crustaceans. The species studied appeared to par tition food resources to minimize the overlap in use of a very specific hab itat.