Food of introduced mosquitofish: ontogenetic diet shift and prey selection

The mosquitofish Gambusia holbrooki of Lake Banyoles, Catalonia, Spain, wer e strictly littoral. Their population was dominated by females throughout t he year, except in spring when males were more common. Their diet was based on littoral cladocerans, particularly Chydorus sphaericus, Scapholeberis r amneri, Ceriodaphnia reticulata, and Pleuroxus laevis, and nematoceran (bas ically chironomid) adults. There was a large variety of prey of terrestrial (collembolans, ants) or aquatic neustonic origin (S. ramneri, emerging nem atoceran adults), showing the microhabitat of mosquitofish closely linked t o the water surface. In contrast to other studies, terrestrial insects were not especially important in summer. In addition to seasonal and between-si te variation, there was an ontogenetic diet shift from microcrustaceans, pa rticularly cladocerans (smallest fish also using diatoms and copepod naupli i) to larger prey, namely nematoceran adults. However, there was an unusual , inverse relationship between fish length and the size of two prey (S. ram neri and the collembolan Sminthurides sp.), parallelling the overall increa se in mean prey size. (C) 1999 The Fisheries Society of the British Isles.