EFFECTS ON ECOSYSTEM STRUCTURE AND FUNCTION OF FISH FARMING AS SIMULATED IN LITTORAL BRACKISH-WATER MESOCOSMS

The effects of fish cage farming were simulated in littoral brackish w ater mesocosms. Two different dry fish feeds were used in the experime nt, one commercially available feed and a purpose made feed containing 30% feral Baltic herring from the Archipelago Sea, SW Finland. Functi onal as well as structural effects of the fish farming simulation were obtained. The production limiting factor in the mesocosm appeared to be ammonium excreted by the fish. Excretion of nutrients was studied s eparately. Fish farming simulation initiated a successional developmen t in the mesocosms in the form of higher sedimentation, higher sedimen t carbon content, a peak of BOD7 outflow, no or limited total organic carbon efflux and higher respiration as compared to the control over t he experimental period. rue functional effects coincided with higher s ediment invertebrate biomass and abundance: the Baltic mussel, Macoma baltica and Chironomus plumosus were all favored by nutrient enrichmen t. Increased availability of ammonium also favored periphytic growth a fter an initial production peak of phytoplankton. Production of other than farmed fish (Gasterosteus aculeatus) was doubled by biomass. Base d on the separate fish nutrient excretion test, an eco-management mode l was put together, The results from this calculation imply that a 30% addition of feral herring in dry feed would result in a 1000 t yr(-1) net removal of ammonium with a total fish production of 12000 t. (C) 1998 Elsevier Science B.V. All rights reserved.