Effects on ecosystem structure and function of fish farming as simulated in littoral brackish water mesocosms

Abstract The effects of fish cage farming were simulated in littoral brackish water mesocosms. Two different dry fish feeds were used in the experiment, one commercially available feed and a purpose made feed containing 30% feral Baltic herring from the Archipelago Sea, SW Finland. Functional 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 separately. Fish farming simulation initiated a successional development in the mesocosms in the form of higher sedimentation, higher sediment carbon content, a peak of BOD 7 outflow, no or limited total organic carbon efflux and higher respiration as compared to the control over the experimental period. The functional effects coincided with higher sediment invertebrate biomass and abundance: the Baltic mussel, Macoma baltica and Chironomus plumosus were all favored by nutrient enrichment. Increased availability of ammonium also favored periphytic growth after an initial production peak of phytoplankton. Production of other than farmed fish ( Gasterosteus aculeatus ) was doubled by biomass. Based on the separate fish nutrient excretion test, an eco-management model 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 12 000 t.