EUTROPHICATION-RELATED CHANGES IN MACROVEGETATION AND FORAGING OF YOUNG COD (GADUS-MORHUA L) - A MESOCOSM EXPERIMENT

Predation by 1-year-old cod (Gadus morhua) on three decapod crustacean prey (Crangon crangon, Carcinus maenas and Palaemon adspersus) was examined in outdoor flow-through mesocosms. Experimental treatments included varying percent cover (70-80; 30-40 and 0 or 10%, respectively) of filamentous algae (Enteromorpha prolifera, E. intestinalis or Cladophora spp.) on bare sand and on patches of Zostera marina. Foraging efficiency of cod on Crangon crangon and Carcinus maenas was greatest in unvegetated treatments. Survival of Crangon crangon and Carcinus maenas was significantly enhanced by the addition of moderate (30-40%) levels of filamentous algal cover to sand substrates, while further increased habitat complexity (70-80% cover) didn't significantly increase the survival of these two species. This suggests a threshold effect in the influence of habitat structure where only small changes in macrovegetation are needed to dramatically decrease foraging efficiency of cod on Crangon crangon and Carcinus maenas. In contrast, no significant difference in predation-induced mortality of P. adspersus was observed between experimental treatments, indicating that P. adspersus is equally susceptible to predation in all habitat types tested. Our studies emphasize the potential effects of habitat alteration from barren sandy embayments providing optimal foraging areas for cod, to progressively more algal-covered habitats in which cod are less successful foragers. During the past decade, shallow coastal areas along the Swedish west coast have been subjected to increasing eutrophication and a general proliferation of filamentous algae. By affecting predator-prey relationships, eutrophication-related structural changes in macrovegetation might cause considerable alterations in trophic relationships in shallow coastal waters.