Foraging ecology and niche overlap in pygmy ( Kogia breviceps) and dwarf ( Kogia sima) sperm whales from waters of the U. S. mid- Atlantic coast

A complementary approach of stomach content and stable isotope analyses was used to characterize the foraging ecology and evaluate niche overlap between pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales stranded on the U.S. mid-Atlantic coast between 1998 and 2011. Food habits analysis demonstrated both species were primarily teuthophagous, with 35 species of cephalopods, and 2 species of mesopelagic fishes represented in their overall diets. Pianka's Index of niche overlap suggested high overlap between whale diets (O-n = 0.92), with squids from the families Histioteuthidae, Cranchidae, and Ommastrephidae serving as primary prey. Pygmy sperm whales consumed slightly larger prey sizes (mean mantle length [ML] = 10.8 cm) than dwarf sperm whales (mean ML = 7.8 cm). Mean prey sizes consumed by pygmy sperm whales increased with growth, but showed no trend in dwarf sperm whales. Significant differences were not detected in N-15 and C-13 values of muscle tissues from pygmy (10.8 parts per thousand +/- 0.5 parts per thousand, -17.1 parts per thousand +/- 0.6 parts per thousand), and dwarf sperm whales (10.7 parts per thousand +/- 0.5 parts per thousand, -17.0 parts per thousand +/- 0.4 parts per thousand), respectively. Isotopic niche widths also did not differ significantly and dietary overlap was high between the two species. Results suggest the feeding ecologies of the pygmy and dwarf sperm whales are similar and both species occupy equivalent trophic niches in the region.