Seasonal shifts in lacustrine trophic ecology: differences in the diet of three Salvelinus species between summer and winter

Understanding aquatic trophic interactions is fundamental for assessing fish population dynamics, yet current knowledge is derived almost exclusively from warm seasons, with winter remaining poorly studied. In this study, we compared stomach contents between summer and winter for three Salvelinus species— S. alpinus (Arctic charr), S. namaycush (lake trout), and S. fontinalis (brook trout)—in five subalpine lakes with contrasting fish assemblages (i.e., in allopatry or sympatry). Diet variations between seasons and fish assemblages were observed, with generalist feeding tactics increasing in winter, particularly under high interspecific competition, confirming that Salvelinus species continue feeding in winter. These shifts were also reflected in corresponding changes in physiological characteristics, with greater liver and ventricle masses in summer, while lower gut weights were observed in summer compared to winter. Cannibalism was also documented in brook trout, a rarely observed behavior. Dietary shifts were driven not only by season but also by life-history traits, with piscivory emerging only in larger individuals, regardless of season. This study highlights the role of seasonal changes and intra- and interspecific competition in shaping trophic niches, with varying wintering strategies among fish assemblages illustrating the behavioral plasticity of Salvelinus species in coping with different environmental pressures.