NONADDITIVE EFFECTS OF TERRESTRIAL AND AQUATIC PREDATORS ON JUVENILE ESTUARINE FISH

Small fish frequently avoid large fish predators by moving into shallo w refuges, but this can increase encounters with terrestrial piscivore s. In previous experiments, we documented that juvenile spot (Leiostom us xanthurus) respond to southern flounder (Paralichthys lethostigma) by moving into shallow water. This might increase their risk of predat ion by birds. We conducted a pond-scale field experiment to test the h ypothesis that predation by birds and flounder will have nonadditive e ffects on spot survival and size. We expected spot to have lower survi val in the presence of both predators than expected based on their add itive effects (i.e., facilitation among predators). The experiment was a 2 X 2 factorial with and without flounders and bird access. Flounde r significantly reduced spot survival while birds did not, but there w as a significant interaction effect. Spot survived better with both pr edators than expected based on adding separate predator effects. Mean spot growth rate was unaffected by treatment. Spot moved to shallow wa ter in the presence of flounder and aggregated more tightly in the pre sence of birds. The observed nonadditive effects could have been due t o interference between flounder and birds. However, because all the fl ounder in our experiments survived and their growth was unaffected by birds, we favor the idea that behavioral changes (increased aggregatio n) by spot to avoid bird predation also further reduced their vulnerab ility to flounder. Our results suggest that modeling the effects of mu ltiple predators will be more complicated than pimply adding up the ef fects observed in experiments with single predators.