Invertebrate accessibility and vulnerability in the analysis of brown trout (Salmo trutta L.) summer habitat suitability

Water discharge regulation can affect food availability, accessibility and vulnerability and thus, the trophic habitat suitability for lotic salmonids. To analyse brown trout habitat suitability, we therefore combined the relative importance of the food availability (overall abundance of benthic and drifting invertebrates), with the potential food vulnerability (accessibility, conspicuousness and ease of handling), the latter depending on both physical habitat characteristics (flow patterns and structural complexity of habitat) and invertebrate traits (size and other biological traits). We considered the trophic patterns of trout at two spatial scales: the reach scale (unregulated reach versus regulated reach) and the macrohabitat scale (e.g. riffles and pools). Discharge regulation reduced trout abundance, biomass, and temperature-independent growth rates. In the regulated reach, trout had a lower total prey consumption, a higher consumption of terrestrial There were no macrohabitat-specific differences in the total prey consumption and in the potential food availability within each reach. However, brown trout diets differed between the macrohabitats of each reach, in relation to differences in potential invertebrate vulnerability, invertebrates and a higher diet diversity than in the unregulated reach, indicating that trout were food-limited. However, the potential availability of food supplies per individual trout was similar for the two reaches. Thus, trout prey consumption in the regulated reach should have been predominantly affected by the decrease in both the availability of large invertebrates in the drift and their vulnerability in the total food supplies. There were no macrohabitat-specific differences in the total prey consumption and in the potential food availability within each reach. However, brown trout diets differed between the macrohabitats of each reach, in relation to differences in potential invertebrate vulnerability. Therefore, the potential vulnerability of invertebrates to predation was more relevant in the ecological evaluation of salmonid habitat suitability than the total food availability. The analysis at the macrohabitat scale provided a better understanding of the switches in brown trout diet and enabled a finer and more realistic analysis of trout feeding patterns. Copyright (C) 2002 John Wiley Sons, Ltd.