Habitat selection by kokanee salmon and smallmouth bass in thermally heterogenous environments: the importance of growth maimization to diel habitat shift

Diel habitat shifts consisting of vertical or horizontal movements are common in both marine and freshwater fishes. Hypotheses proposed to explain this behavior include maximizing food consumption, minimizing risk of predation, and maximizing bioenergetic efficiency. The bioenergetic hypothesis is often considered under conditions of thermal heterogeneity within the environment. The general objective of this study was to evaluate the importance of bioenergetic efficiency (i.e., maximization of net energy assimilation) as an influence on habitat selection and diel habitat shifts of two fishes. The vertical distribution and movement patterns of the planktivorous kokanee salmon (Oncorhynchus nerka) were monitored in Nantahala Lake, North Carolina, with vertical gill nets and hydroacoustics surveys. The hydroacoustics surveys indicated only limited movement (about 8 m) by adults on one occasion, but a substantial vertical migration (up to 45 m) for juveniles at another time. A bioenergetics model was developed to simulate diel vertical migrations by kokanee to examine the energetic advantages and consequences of different migratory strategies. The model demonstrated that vertical migrations can be energetically advantageous when a predator and its prey are thermally segregated. The model also predicts ontogenetic and seasonal differences in the optimal migration strategy. The daily movement patterns of adult smallmouth bass (Micropterus dolomieu) were monitored in Melton Hill Reservoir, Tennessee, with temperature-sensitive ultrasonic transmitters so that habitat selection and daily movements could be determined with regard to temperature, depth, and nearness to shore and physical structure. The expected offshore migrations were not evident in the tracked fish, and a high affinity for nearshore cover was observed even when temperatures exceeded preferred levels. Based on the field observations, laboratory experiments with a thermal gradient tank were designed to evaluate the relative importance of temperature, food availability, and cover on habitat section by smallmouth bass. Both food and cover affected the habitat selection of the fish within the thermal gradient, and cover was often selected with apparent disregard for bioenergetic efficiency. These results demonstrate the importance of bioenergetic efficiency to daily movements under certain conditions, but also demonstrate that several factors can have a simultaneous effect on diel habitat shifts.