Predator-prey interactions between cod (Gadus morhua) and perch (Perca fluvitatilis) and three crustaceans (Idotea baltica, Gammarus oceanicus and Palaemon adspersus) in the Baltic Fucus communities

Three different laboratory studies have been carried out in order to elucidate the interactions between two predators and their prey in the Fucus vesiculosus community. The first study investigated the behavioural response of Gammarus oceanicus, Idotea baltica and Palaemon adspersus in the chemicai presence of their predators Gadus morhua and Perca fluviatilis. A lowering of night activity among the crustaceans was expected. The second study was carried out in 1 m3 mesocosm-tanks were the predators had the opportunity to actively prey on both I. baltica and G. oceanicus among live F. vesiculosus plants. This experiment lasted före one week and the decrease of the crustaceans during this period was noted as result. In the third study was the predators ability to forage in complete darkness investigated. This was carried out in the same mesocosm tanks but without Fucus and for 5 hours. None of the crustacean species decreased their night activity as response to the smell of the predators. This may indicate that chemically mediateda signals are of less importance in the Fucus community due to high turbulence and swift waterexchange in this splashzone habitat. Cod consumer more of both G. oceanicus and I. baltica than perch in the two different foraging-studies. In these conditions the cod was the most efficient predator. The decreased stocks of cod was the most efficient predator. The decreased socks of cod in the Baltic Sea may have reduced the top-down control in the foodweb of the fucus community.