Fish in space: local variations of home range and habitat use of a stream‐dwelling fish in relation to predator density

A key response of animals to local environmental variation is altered use of space, but studies simultaneously examining local variation in habitat use and space use are uncommon. We predicted that elevated abundance of avian predators would result in grayling Thymallus thymallus, a stream-dwelling fish, using mesohabitats containing more cover, superimposed on seasonal changes in use of key resources (and hence space use) for functions such as reproduction. Using radio-telemetry, the pattern of space and habitat use by 40 wild grayling was determined in neighbouring stream sections in relation to season and predator density. Grayling used different habitats between seasons, but displayed similar patterns of habitat use in adjacent sections. Although patterns of habitat use were stable between stream sections, space use was not. In two winter periods, grayling ranged significantly more widely where there were significantly greater densities of avian predators, especially cormorant, Phalacrocorax carbo. No such differences were apparent in summer when cormorants were absent, but experimental manipulation of predator densities was not possible, so results are correlative. Support for a predator effect is provided from significantly greater rates of injury, associated with avian beak scar marks, present on grayling from the section with highest avian predator densities, compared with adjacent sections with lower levels of avian predators. Unlike many studies of fish behaviour to elevated predation risk, in which fish make greater use of refuge' habitat, grayling exhibited wide-ranging behaviour and high activity, possibly reflecting avoidance behaviour.