A curvilinear regression model was used to determine an expected stomach capacity for any size Bear Lake sculpin with a standard length between 4-9.5 centimeters. Stomach contents were then expressed as a percent fullness and the change in percent fullness with time intervals was used to compute an instantaneous digestion rate. The instantaneous consumption rate was computed by the formula C=log P1-log P0 +d, where C=instantaneous consumption rate. The amount of food consumed during a time interval was computed from the formula It=CtSt, where It=amount of food ingested during a time interval expressed in percent fullness of stomach. The 24 hour consumption rate was computed by summing the consumption rates for 4 consecutive time intervals of 6 hours each. Studies in September, December, and May give instantaneous digestion rates of -.424, -.214, and -.235 and total food consumption rates of 22.2, 10.2, and 6.1 percent of stomach capacity for each individual fish in the population.

Bear Lake sculpin (Cottus extensus) exhibited an ontogenetic habitat shift during their first year of life. Soon after hatching in the littoral zone, the fry swam to the surface where they dispersed throughout the lake. Most juveniles that initially settled in the profundal zone returned to the littoral zone during their first summer. Patterns in the daily growth increments of otoliths confirmed the history of habitat residence and the individual size at the habitat switch. We used this habitat shift to test a μ/g model incorporating a growth rate-mortality risk trade-off. A trade-off occurred in the littoral zone because both growth rate and mortality risk were greater in this habitat. Given initial profundal residence, the directed movement of juveniles to the more profitable littoral zone was consistent with model predictions. Contrary to model predictions, juvenile sculpin initially occupied both habitats and switched habitats at a wide range of sizes. lack of a discrete switch size may have resulted, in part, from the lack of a strong trade-off or from an inability of fish to respond facultatively to environmental variables at the large spatial scale of this system.

Underyearling Bear Lake sculpin exhibit a diel pattern of vertical migration throughout the pelagic region of Bear Lake (Utah-Idaho) until they are approximately 22 mm standard length. Individuals move from the bottom of the lake (5° C) during the day into the water column (13-16° C) at night. The migration, however, is not related to feeding. Although the dominant copepod in the water column, Epischura nevadensis, do undergo a similar diel vertical migration, stomach analysis of juvenile sculpin captured by trawling shows that they do not feed in the water column. Instead, from July through October, their diet is dominated (70-93%) by benthic copepods and ostracods. Also, gut fullness of sculpin increases through the daylight period and decreases through the night, reaching minimum levels just before the dawn descent. Furthermore, feeding trials conducted in the laboratory show that juvenile sculpin feed most efficiently at light intensities found on the bottom (30-60m) of Bear Lake during the day. Feeding rate coefficients increase markedly from 1013 photons m-2S-1 until peaking at intermediate intensities of 1016 photons m-2S-1 and then decline at higher light levels. Although they do not migrate to feed, the movement into the warmer water appears to increase the sculpin's digestion rate, thereby allowing continued feeding during the day. This supports the hypothesis that diel vertical migration in Bear Lake sculpin is a thermoregulatory strategy that increases growth rate.