Diet Variability in Relation to Season and Habitat Utilisation in Brown Trout, Salmo trutta L., in a Southern Irish Stream

Habitat utilisation by salmonids and the identification of the important factors in habitat selection received considerable attention in the past, but little is known of the impact of such utilisation on other aspects of salmonid ecology, particularly diet. Present field study was undertaken to investigate diet variability in different aged brown trout in relation to habitat, site and season.
The study was carried out in five sites in a single stream, sampling taking place seasonally over 2 years in riffle and pool habitats. Habitat utilisation and diet (using non-destructive stomach flushing techniques) were monitored from fish populations in each site and on each sampling occasion. 0+ and 1+ trout showed differential habitat utilisation.
Diet variation with habitat, fish age, season and site was analysed using the TWINSPAN hierarchical classification technique. This analysis produced clearly defined clusters of samples related to season and within summer and autumn related to habitat. There was little differences in diet during winter and spring. Age and sampling year were also influential but site had no significant effect on diet.
Based on knowledge of prey types and availability in the different habitats, the results suggest the adoption of specific foraging tactics involving pool dwelling fish moving out to forage in riffles at certain times of year. The data support the contention that trout have activity-specific habitats, at least for part of the year, with resting refuges in pool habitats and foraging in riffles.