Trophic ecology of Carolina Sphyrna gilberti and scalloped S. lewini hammerheads in the southeastern USA

ABSTRACT: Carolina hammerheads Sphyrna gilberti and scalloped hammerheads S. lewini are cryptic species with an overlapping distribution, and young-of-the-year (YOY) use similar coastal and estuarine nursery areas along the US Southeast coast. The diet of scalloped hammerheads has been widely studied throughout their global distribution; however, little is known about their diet in the western North Atlantic Ocean. Carolina hammerheads have only been recently described, and as such, their trophic ecology is largely unstudied. Stomach content analysis of genetically identified YOY Carolina and scalloped hammerheads revealed both species to be generalist feeders with diverse diets and no apparent resource partitioning between them. By contrast, multi-tissue (muscle and plasma) stable isotope analyses showed significant interspecific differences in signatures, particularly in muscle δ¹³C values for the youngest hammerheads. Due to slow tissue turnover rates, YOY muscle signatures were assumed to reflect maternal isotopic signatures, suggesting habitat or resource partitioning between mature female Carolina and scalloped hammerheads. In particular, the data are consistent with mature Carolina hammerheads inhabiting more offshore waters or consuming a higher proportion of pelagic prey relative to scalloped hammerheads. YOY muscle and plasma δ¹⁵N values decreased drastically across the sampling season, reflecting a loss of the maternal signal as the YOY hammerheads fed and grew rapidly in these productive nursery habitats.