The relationship between individual habitat use and diet in brown trout

Abstract

SummarySalmonids exhibit considerable variation within and between individuals in growth, diet, foraging strategy and habitat use, but little is known about how these characteristics covary. Previous work has shown that habitat use strongly influences growth rates in brown trout (Salmo trutta).We examined individual variation in diet of PIT‐taggedSalmo truttain three stream enclosures in relation to individual habitat use, size, sex and growth. Each enclosure consisted of a fine substratum pool and a coarse substratum riffle. By placing antennae between these habitats, we continuously monitored habitat use under field conditions. Fish were recaptured four times over the 2‐month study period, and diet, which was examined through stomach flushing, was related to habitat use over the previous 48 h. Individual fish growth was also measured.Based on habitat use, trout were classified as either movers or stayers, with stayers being of two types, those using only pools and those using only riffles and movers using and swimming between pools and riffles. The stayers in pools took more terrestrial prey than the stayers in riffles, whereas the latter fed more on aquatic invertebrates such as the crustaceanGammarus pulex,the plecopteranLeuctraand cased caddis larvae. Movers had diets intermediate between the stayers in pools and the stayers in riffles.Results of canonical correspondence analysis showed that variation in diet amongst individual fish over the study period was significantly influenced by enclosure, growth and % time spent in the pool during the day. Graphical models of diet analysis showed that population and mean individual prey diversity tended to differ amongst enclosures and suggested that stayers in pools consumed a greater prey diversity, whereas stayers in riffles consumed more prey. Discriminant analysis of diets revealed significant discrimination by habitat and sex on two of the four sampling occasions, based on daytime habitat use, but only on one date based on night‐time habitat use. Stoneflies and terrestrial prey contributed most to the separation.