Effects of turbidity on feeding of southern flounder on estuarine prey

Southern flounder Paralichthys lethostigma are predators on young brown shrimp Farfantepenaeus aztecus in turbid estuarine nurseries, and this predator-related mortality can affect shrimp recruitment to the fishery. Water clarity appears important in affecting feeding rates and prey selection of this ambush predator. Earlier experiments with southern flounder showed increased feeding on brown shrimp in turbid water. We propose a simple model which predicts that increased predation rates occur at intermediate turbidity levels due to increased prey activity. The effect should vary with prey size, and predation rates should decline when the reactive zone is smaller than the strike zone. We conducted an experiment using 2 sizes of shrimp prey and 4 levels of turbidity and showed a significant interaction between prey size and turbidity. These results appeared similar to those predicted by the hypothetical model, and predation rates on small shrimp were not reduced until turbidities reached 50 Formazin Turbidity Units (FTU). In a second experiment, we found no significant effect of turbid water (up to 25 FTU) on strike distance of southern flounder. Additional experiments were conducted on prey selection. In clear water, southern flounder fed mainly (93 to 94%) on shrimp, but at 50 FTU, these predators selected long-nose killifish (64 to 77% of prey eaten). Our experimental results support the conclusion that environmental factors such as water clarity can have substantial effects on feeding rates, prey selection, and perhaps mortality of prey, and that such interactions should be considered when developing trophic models.