DIETARY VARIATION IN A FRESH-WATER FISH SPECIES - RELATIVE CONTRIBUTIONS OF BIOTIC INTERACTIONS, ABIOTIC FACTORS, AND SPATIAL STRUCTURE

We used multivariate analyses to examine which Variables among the env ironmental and spatial components can best account for dietary variati on in a freshwater fish, brook trout, Salvelinus fontinalis. The diet composition of brook trout was quantified in 37 lakes of the Laurentia n Shield, Quebec, Canada. Among the 25 measured environmental variable s, fish species composition, sampling date, macrophyte abundance, and trout body length were the best predictors of diet composition. The to tal variation in diet composition was partitioned into four components : pure environmental 21.6%, pure spatial 23.2%, shared 19.9%, and unex plained 35.3%. A significant spatial trend in diet composition existed even after accounting for the main effects measured by the environmen tal variables. The two sets of spatial variables, when combined with t he environmental descriptors, extracted different components of the di etary variation. The study allowed us to (1) highlight the role of spa tial structure in diet variation of brook trout, (2) determine the rel ative contribution of both environmental and spatial components, and ( 3) generate testable hypotheses concerning mechanisms underlying the o bserved structure. Dependent variables other than diet composition, su ch as the density of different species at different sampling sites, ca n be used within the same statistical framework in studies of communit y ecology.