Use of shallow marginal habitat by Phoxinus phoxinus: a trade‐off between temperature and food?

Fish may alter their habitat use in accordance with the profitability of differing habitat patches. During summer, in the River Frome, minnows Phoxinus phoxinus used shallows in a selective manner, preferring shallows in which water temperature was higher than the ambient river temperature. During the morning and evening, the minnows occasionally entered the shallows but did not linger. They spent considerably longer periods in the shallows when water temperature was greater than in the main channel of the river. During these periods the minnows were inactive. Gut fullness of wild minnows varied temporally and on one occasion spatially, with gut fullness rising rapidly after dawn and remaining high throughout the day. Minnows held in enclosures in the river had significantly higher gut fullness than those held in the shallows. The minnows use shallows that are warmer than the adjacent river preferentially but must return to the river to feed. Thus, there is evidence for a trade‐off between a habitat with high prey density, but low temperature and a low prey density, warmer habitat.