Trophic Flexibility of Stream-Dwelling Salmonids: Disentangling Common Ontogenetic and Seasonal Patterns

This review synthesises the literature on ontogenetic and seasonal changes in feeding patterns of nine genera of the family Salmonidae (Coregonus, Prosopium, Stenodus, Hucho, Oncorhynchus, Salmo, Salvelinus, Brachymystax and Thymallus), disentangling the general mechanisms driving those changes and the common dietary patterns. Stream-dwelling salmonids are characterised by their trophic flexibility, exhibiting consistent ontogenetic and seasonal patterns. One of the most outstanding ontogenetic dietary changes is the switch to piscivory along ontogeny. However, there is no solid evidence supporting piscivorous behaviour in riverine grayling (Thymallus spp.) and whitefish (Prosopium williamsoni and Coregonus lavaretus) despite been frequently shown in lacustrine populations. Other general ontogenetic patterns include allometry in prey size and a higher consumption of terrestrial invertebrates (i.e. switch to surface-foraging). This consumption of allochthonous resources is especially important during the summer in temperate territories when insect phenology maximises and thus, becomes accessible to salmonids. In addition, availability of imagoes of aquatic invertebrates (Ephemeroptera, Trichoptera, Diptera and Plecoptera) is highly related to emergence (emergence peaking around spring and early summer), but also highly dependent on climate-related geographical variability. Thus, climate influence on seasonal life-history traits of prey taxa (hatching, emergence, diapause, etc.) can strongly determine seasonal changes in feeding of salmonids through bottom-up mechanisms. Understanding global dietary patterns can be complex because the environmental conditions change broadly across salmonid’s distribution as well as seasons, but still common patterns can be extended to geographic regions with similar climatic conditions.