Characterization of forage fish and invertebrates in the Northwestern Hawaiian Islands using fatty acid signatures: Species and ecological groups

The fat content and fatty acid (FA) composition of 100 species of fishes and invertebrates (n = 2190) that are potential key forage species of the critically endangered monk seal in the Northwestern Hawaiian Islands were determined. For analysis, these species were classified into 47 groups based on a range of shared factors such as taxonomy, diet, ecological subsystem, habitat, and commercial interest. Hierarchical cluster and discriminant analyses of the 47 groups using 15 major FAs revealed that groups of species with similar FA composition associated into 5 functional groups: herbivores, planktivores, carnivores (which also included piscivores and omnivores), crustaceans, and cephalopods. Discriminant analyses performed on the 4 main functional groups separately revealed that herbivores, planktivores, and crustaceans could be readily differentiated on the basis of their FA signatures, with 97.7, 87.2, and 81.5% of individuals correctly classified, respectively. Classification success was lower within the carnivores (75.5%), which indicates that some groups of carnivorous species likely exhibit highly similar diets and/or ecology, rendering their FA signatures harder to differentiate. Despite a relatively broad overlap, most groups were reasonably well classified (>75% success). Five groups had <60% classification success; these taxa were better explained by separately considering individual factors such as species, diet, or ecology rather than the full range of shared factors. The agreement of the FA signatures with ecologically based groupings indicates they can be used to characterize the fish and invertebrate community in this highly complex subtropical ecosystem. These data provide crucial information on the distinctions and overlaps in the diets of reef fish and invertebrates.