Ontogenetic habitat and diet selection in estuarine-dependent fishes: Comparisons of observed patterns with model predictions

Unexplained patterns of ontogenetic habitat shift in estuarine-dependent fishes were investigated by applying predator models to natural estuarine prey gradients. The predator models simulated ontogenetic changes in feeding that occur between the larval and juvenile stages (5-70 mm), and were applied to distributions of 32 prey types (equivalent diameters of 0.09-6.11 mm) along an estuarine gradient extending from tidal fresh water in the upper Little Manatee River estuary to open saline waters of Tampa Bay, Florida. Prey distributions along this gradient were monitored at two-week intervals for two years. Sixteen predator models were developed, each consisting of a unique combination of one of eight search modes and one of two prey selection modes. Four models were restricted to water-column feeding, and the remaining twelve incorporated varying degrees of epibenthic feeding. Regardless of search mode, selection mode, or time of day, the models indicated that maximum prey availability for larvae (5 mm) was at an open bay location, while that for juveniles ({dollar}ge{dollar}10 mm) was within the tidal river. The shift in prey availability occurred as the model predators switched from feeding on calanoid copepods to epibenthic crustaceans (primarily mysids and amphipods) and then to fish prey primarily Anchoa mitchilli juveniles).; Predicted habitat and diet shifts were compared with habitat and diet shifts observed in silver perch (Bairdiella chrysoura), sand seatrout (Cynoscion arenarius), and spotted seatrout (C. nebulosus). All three species moved landward during the late larval and/or early juvenile stage, exhibiting habitat and diet shifts that were in good agreement with model predictions. For these species, the results support feeding advantages as the explanation for the ontogenetic habitat shift pattern known as estuarine dependence.; Search and selection modes appeared to change during ontogeny. Pelagic search modes were joined by demersal and semidemersal search modes during juvenile development. Silver perch maintained small relative prey sizes (2-4% of predator length) throughout the length range examined, and achieved stomach filling by ingesting large numbers of small, easily captured prey. All species exhibited some combination of nonselective and selective feeding upon encountering prey. Conformity of seatrout diets to predictions of optimal foraging theory increased during development. Larger juvenile seatrout increased mean prey volume and relative size (2-20%) while decreasing prey number. These ontogenetic trends were smoother in sand seatrout than spotted seatrout, indicating better control over diet optimality in the sand seatrout.; The biological structure of the estuarine prey gradient is discussed in relation to theories associated with the river continuum concept. The role of submerged aquatic vegetation (SAV) as nursery habitat was evaluated through diet comparisons and evaluation of prey abundance in SAV vs. non-SAV areas. Habitat-specific diet comparisons of silver perch and spotted seatrout, which are generally associated with SAV as juveniles, indicated no consistent advantage to feeding in SAV. The sand seatrout which is not associated with SAV, fit model predictions better than the SAV-associated species, and also exhibited a narrower dietary breadth at juvenile sizes, indicating a greater degree of diet optimization.