Intra-specific correlations between fatty acids and morphological traits are consistent across fish species

We explored intraspecific relationships between biochemical and morphometric traits among sixteen fish species from the Canadian waters of the Laurentian Great Lakes. We focused on hypotheses linking two morphometric descriptors (fish length and condition factor, CF, the latter derived from species-specific allometric-growth equations) to lipid content and polyunsaturated fatty acid (PUFA) composition of dorsal muscle tissue. Each population from each of the lakes was assumed to display different relationships. All correlations were better represented by linear than non-linear relationships. Interspecific and among lakes differences accounted for most of the registered variability of fatty acid (FA) composition. Once these two covariates were parsed out, there was a discernible intraspecific coupling between fish length, CF, and fish biochemical traits, including positive correlations with muscle lipid and PUFA composition. The sign of the correlations was consistent among different populations, aligned with the theoretical expectations, and with interspecific trait correlations. However, for some species, correlations could be positive in one lake but negative in another, suggesting that local environmental variations could affect biochemical rates. We also found three trends associated with the rate at which different fatty acids accumulate in fish muscle tissue that are potentially related to food availability. These relationships between CF and FA composition provide a promising starting point to establish efficient, cost-effective, field-based estimates of essential FA descriptors. In this vein, we present a user-friendly application for the estimation of lipid content and PUFA composition, based solely on weight and length, that any angler and/or citizen-science participant can easily obtain.