AN ENERGETIC MODEL OF MICROHABITAT USE FOR RAINBOW-TROUT AND ROSYSIDE DACE

We constructed an energetic model to determine the optimal focal point current velocity (i.e., microhabitat) for rainbow trout (Oncorhynchus mykiss) and rosyside dace (Clinostomus funduloides) in Coweeta Creek, a fifth-order stream in North Carolina, USA. Energetic costs were evaluated by quantifying the metabolic expenditure associated with swimming at a given velocity. We estimated benefits by measuring potential energetic gains of feeding at a given velocity. This included estimates of the ability of the fish to capture prey at different current velocities as well as estimates of the frequency and the energy content of drifting prey at various velocities. We derived separate models for small (53-70 mm SL [standard length]) and medium (71-125 mm SL) trout, and medium (47-52 mm SL) and large (53-70 mm SL) dace for all seasons, deriving net energy gain as a function of current velocity. We predicted fishes would occupy velocities at which net energy gain was maximized. Predicted velocities were compared with those utilized by fishes inhabiting Coweeta Creek. Optimal velocities predicted by energetic models ranged from 7.7 to 22.1 cm/s, and closely matched actual velocity use (average deviation = 2.6 cm/s). Prey capture success appeared to be the most important component in the models. Consequently, we constructed models based solely upon aspects of capture success; the average deviation from velocity use with these models was only 1.8 cm/s. Thus, the ability of dace and trout to capture prey at varying velocities appears to be the dominant factor affecting microhabitat selection in these species.