Small fish frequently avoid large fish predators by moving into shallow refuges, but this can increase encounters with terrestrial piscivores. In previous experiments, we documented that juvenile spot (Leiostomus xanthurus) respond to southern flounder (Paralichthys lethostigma) by moving into shallow water. This might increase their risk of predation by birds. We conducted a pond-scale field experiment to test the hypothesis that predation by birds and flounder will have nonadditive effects on spot survival and size. We expected spot to have lower survival in the presence of both predators than expected based on their additive effects (i.e., facilitation among predators). The experiment was a 2 ϫ 2 factorial with and without flounders and bird access. Flounder significantly reduced spot survival while birds did not, but there was a significant interaction effect. Spot survived better with both predators than expected based on adding separate predator effects. Mean spot growth rate was unaffected by treatment. Spot moved to shallow water in the presence of flounder and aggregated more tightly in the presence of birds. The observed nonadditive effects could have been due to interference between flounder and birds. However, because all the flounder in our experiments survived and their growth was unaffected by birds, we favor the idea that behavioral changes (increased aggregation) by spot to avoid bird pre-dation also further reduced their vulnerability to flounder. Our results suggest that modeling the effects of multiple predators will be more complicated than simply adding up the effects observed in experiments with single predators.
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