Abstract
1. Stream dwelling fishes acquire most of their dietary energy by consumption of aquatic and terrestrial macroinvertebrates. These prey subsidies supply similar amounts of energy and macronutrients, but the fact that terrestrial macroinvertebrates have lower contents of omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA), such as eicosapentaenoic acid (EPA), than aquatic macroinvertebrates is largely neglected.
2. EPA is a precursor for synthesis of docosahexaenoic acid (DHA), which is an important structural lipid representing about a third of all fatty acids in the brain of stream dwelling fishes. Laboratory studies have shown a positive impact of DHA on brain development and cognition in fishes. However, little is known how diet quality influences the DHA content of brain tissues and its effect on fish brain development in the wild.
3. Here we use previously demonstrated shift towards higher reliance on n-3 LC-PUFA poor terrestrial prey of native brown trout Salmo trutta living in sympatry with an invasive brook trout Salvelinus fontinalis to explored how dietary deprivation of n-3 LC-PUFA affects biochemical composition of tissues and brain volume and morphology of wild fish.
4. We found that higher reliance on terrestrial prey of sympatric brown trout resulted in lower EPA content in muscle tissues. The DHA content in muscles did not differ between allopatric and sympatric brown trout, but it decreased in sympatric brown trout with increasing reliance on terrestrial prey. The brain volume, but not brain morphology, was positively related to content of DHA in muscle tissues in sympatric and allopatric brown trout.
5. Our study demonstrates that reliance on subsidies from terrestrial food-web can have significant impact on brain development of wild fishes. We showed effects of reduced dietary intake of n-3 LC-PUFA induced by co-existence with an invasive species; however, other anthropogenic factors such as climate change can further reduce the availability of dietary n-3 LC-PUFA for stream dwelling fishes and other consumers. This emphasises the need for further research on how dietary availability of n-3 LC-PUFA in prey affects behaviour and physiology of animals in the wild.