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 ex amined in outdoor flow-through mesocosms. Experimental treatments incl uded varying percent cover (70-80; 30-40 and 0 or 10%, respectively) o f filamentous algae (Enteromorpha prolifera, E. intestinalis or Cladop hora spp.) on bare sand and on patches of Zostera marina. Foraging eff iciency of cod on Crangon crangon and Carcinus maenas was greatest in unvegetated treatments. Survival of Crangon crangon and Carcinus maena s was significantly enhanced by the addition of moderate (30-40%) leve ls of filamentous algal cover to sand substrates, while further increa sed habitat complexity (70-80% cover) didn't significantly increase th e survival of these two species. This suggests a threshold effect in t he influence of habitat structure where only small changes in macroveg etation are needed to dramatically decrease foraging efficiency of cod on Crangon crangon and Carcinus maenas. In contrast, no significant d ifference in predation-induced mortality of P. adspersus was observed between experimental treatments, indicating that P. adspersus is equal ly susceptible to predation in all habitat types tested. Our studies e mphasize 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 relatio nships, eutrophication-related structural changes in macrovegetation m ight cause considerable alterations in trophic relationships in shallo w coastal waters.