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What animals consume is perhaps one of the most fundamental questions in ecology. Limitations of stomach content and faecal analyses have led to the development of alternative bio- chemical methods, namely fatty acid (FA) signature analysis and stable isotope (SI) analysis. Propor- tional estimates of diet composition can be made at the level of the individual by statistically match- ing a consumer's FA signature to an inclusive prey database, after accounting for predator metabolism effects in a mixing model. Both of these approaches provide inferences about or quanti- tative estimates of diet that are time-integrated and readily applied to most taxa of interest, thereby alleviating many problems associated with conventional analysis. Moreover, analysis is done at the level of individual predators, thereby increasing our ability to detect differences amongst various demographic groups. However, it is difficult to validate diet estimates from either of these methods in the field, particularly for wide-ranging aquatic predators such as seals or seabirds consuming complex diets. Concurrent sampling of individual predators for both stable isotopes and fatty acids provide 1 such independent test. To that effect, we analyzed SI and FA for grey seals Halichoerus grypus (n = 75) and potential fish and invertebrate prey (n = 45). We show 2 lines of evidence for the convergence of stable isotope and fatty acid estimates of diet. First, in fish, invertebrates and grey seals, principal components derived from fatty acid signatures are correlated to the stable-carbon iso- tope (δ13C) signature of the same species. Second, both the estimated portion of pelagic prey and the trophic level of the diet derived from the quantitative analysis of fatty acid signatures of individual seals are positively correlated with the stable-carbon (δ13C) and stable-nitrogen (δ15N) signatures in those same individuals. The corroboration of independent measures of diet provides further validation of these biochemical methods to understand the foraging ecology of consumers.

Convergence of diet estimates derived from fatty acids and stable isotopes within individual grey seals