In the present study, energy densities of 80 adult brown trout (Salmo trutta), seasonally sampled in an alpine stream in the eastern Alps and energy densities of their main preys items, were determined. The energy density (J/g wet mass) and dry weight content (%) of fish were highly correlated (p<0.001) and averaged 5,611.6 ± 859.9 J/g wet mass and 25.3 ± 2.1% dry weight, respectively. Energy density values were significantly higher in fish sampled in spring then in other seasons. no major changes in the energy content were observed due to age or sex. Macroinvertebrates, particularly Ephemeroptera and Diptera, were the major food source of brown trout in the sampled area. Their gross energy content varied within a wide range of values (1,654-5,110 J/g wet weight), depending on the taxa and family or genus within a given taxon.
