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, minno ws Phoxinus phoxinus used shallows in a selective manner, preferring s hallows in which water temperature was higher than the ambient river t emperature. During the morning and evening, the minnows occasionally e ntered the shallows but did not linger. They spent considerably longer periods in the shallows when water temperature was greater than in th e main channel of the river. During these periods the minnows were ina ctive. Gut fullness of wild minnows varied temporally and on one occas ion spatially, with gut fullness rising rapidly after dawn and remaini ng high throughout the day. Minnows held in enclosures in the river ha d significantly higher gut fullness than those held in the shallows. T he minnows use shallows that are warmer than the adjacent river prefer entially 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 tem perature and a low prey density, warmer habitat. (C) 1998 The Fisherie s Society of the British Isles.