Prey selection of the shallow water fish Pomatoschistus minutus (Gobiidae, Teleostei) in the SW Baltic

Abstract

The aim of this investigation is to demonstrate the reasons of an opportunist feeder to select some components of the available food supply and to avoid others. Object of this test is the eurytopic and euryhaline sand goby, Pomatoschistus minutus (Pallas), from the Luebeck Bight. It inhabits both sand habitats as well as bottoms mixed with pebbles or continuous hard substrates. Therefore, it is often in close contact with Mytilus-belts which present a very rich food supply of small crustaceans. The selectivity behaviour of sand gobies from a sand/clay ecotone was compared with that of populations from a stony pier and a sand bottom during the reproduction period. Regarding the numbers of prey organisms, crustaceans of the periphyton were generally preferred at the pier but avoided in the ecotone. The same is valid for prey organisms of the psammal which were preferred by gobies of the sand bottom but mostly avoided by populations of mixed bottoms. Analyses of size selections revealed that the preferred gammarids or Jaera isopods were between 2 and 5 mm length. A balanced relation of goby biomass and utilizable food supply (predatory impact index) seems to effect positive selectivity of gammarids in the Mytilus-belts or of harpacticoids in the sand bottom. © 1990 Biologische Anstalt Helgoland.