Discrimination between freshwater and marine fish using fatty acids: ecological implications and future perspectives

Fatty acids (FA) are a major source of nutrients and energy in aquatic food webs, as well as serving as the main components of all cell membranes. Increasing anthropogenic impacts (e.g., climate change) are predicted to selectively alter the production of these critical compounds, with potential cascading effects reaching higher trophic level organisms, including humans. To provide a more comprehensive assessment of these potential effects, we synthesized and systematically explored differences in the abundance and distribution of FA in fish, due to their pivotal role in aquatic ecosystems and value to humans. An extensive data set consisting of 1382 fish FA profiles was analyzed to identify the main differences in FA composition of freshwater vs. marine fish, taking into account the effects of taxonomic, geographic (i.e., latitude zone), and functional (i.e., feeding mode) factors. Freshwater fish had relatively high contents of 18:2n-6 (linoleic acid, LNA) and 20:4n-6 (arachidonic acid, ARA) indicative of freshwater algae and terrestrial dietary sources. In contrast, 20:1n-9 and 22:1n-11, well known as marine zooplankton biomarkers, typified the FA composition of marine fish. Using this result, we constructed a new metric (i.e., a specific C-18-C-22 unsaturated FA ratio) that we propose can assist in characterizing the feeding habitat (freshwater vs. marine) and, to some extent, the diet of fish. Our analysis also revealed that carnivores from high latitudes had higher contents of 20:5n-3 (eicosapentaenoic acid, EPA) and 22:6n-3 (docosahexaenoic acid, DHA), thus making them an excellent source of essential FA for human consumption. In parallel, unsaturated FA contents were higher overall at temperate and polar latitudes than in the tropics. The latter two trends may be driven by water temperature variation across latitudes, which is known to affect FA composition of cell membranes in ectotherms. Last, a strong retention of DHA was observed in both freshwater and marine fish. We conclude that our discrimination between freshwater and marine fish provides a quantitative tool that is applicable to a broad range of research and conservation initiatives.