THE EFFECTS OF PREDATION ON THE SURVIVAL AND SIZE-DISTRIBUTION OF ESTUARINE FISHES - AN EXPERIMENTAL APPROACH

The effect of southern flounder, Paralichthys lethostigma, predation on the survival and size-distribution of spot, Leiostomus xanthurus, was investigated using feeding experiments in outdoor tanks and estuarine ponds. In the outdoor tank experiments, southern flounder, 143 mm average standard length (SL), fed for 48 h on spot (25-30 mm SL), over a range of densities (4-20 M-2). In these trials, flounder showed a Type II functional response to spot density. In the estuarine pond experiment, flounder density (2 or 4 flounders 70 M-2) contributed significantly to spot mortality relative to predator-free controls. The effect of flounder density on spot mortality was non-linear; in the 4-flounder treatment the daily instantaneous mortality due to flounder (0.0177) was 3.5 times that in the 2-flounder treatment (0.0051). The total spot mortality rate in the 4-flounder treatment (0.028) was similar to that observed from published field observations. Flounder also altered the size distribution of spot relative to predator-free controls. Predation by flounder resulted in fewer intermediate-sized spot at the end of the experiment when compared to predator-free controls. Flounder also significantly reduced survival and mean length of two killifishes, Cyprinodon variegatus and Fundulus heteroclitus, that invaded the pond. Flounder predation was a significant size-structuring force on the prey fish assemblage in the pond. Results from both the outdoor tank experiment and the pond experiment indicate that flounder may represent a significant source of mortality for juvenile estuarine fishes.