Comparative feeding ecology of shortfin mako, blue and thresher sharks in the California Current

摘要

This study describes the feeding ecology of three pelagic shark species in the California Current: shortfin mako (Isurus oxyrinchus); blue (Prionace glauca); and thresher (Alopias vulpinus) sharks. Stomach contents of sharks collected from 2002 to 2008 were identified to the lowest taxonomic level and analyzed using univariate and multivariate methods. Of 330 mako sharks sampled (53 to 248 cm fork length [FL]), 238 stomachs contained 42 prey taxa, with jumbo squid (Dosidicus gigas) and Pacific saury (Cololabis saira) representing the most important prey based on the geometric index of importance (GII). In addition, 158 blue sharks were sampled (76 to 248 cm FL) and 114 stomachs contained 38 prey taxa, with jumbo and Gonatus spp. squids representing the most important prey. Lastly, 225 thresher sharks were sampled (108 to 228 cm FL) and 157 stomachs contained 18 prey taxa with northern anchovy (Engraulis mordax) and Pacific sardine (Sardinops sagax) identified as the most important prey. Overall, mako sharks had the most diverse diet based upon Simpson’s diversity index (1/D) (8.43 ± 1.16), feeding on many species of teleosts and cephalopods, followed by blue sharks (6.20 ± 2.11) which consumed a wide range of prey (primarily cephalopods), while thresher sharks were most specialized (2.62 ± 0.34), feeding primarily on coastal pelagic teleosts. Dietary overlap was lowest between blue and thresher sharks (Sørensen similarity index = 0.321 and Simplified Morisita Horn index = 0.006), and seasonal variability in diet was greatest for blue sharks (Simplified Morisita Horn index = 0.260, Analysis of Similarity (ANOSIM) p < 0.001). In addition, size class, and subregion were significant factors that affected diet of each species differently (ANOSIM p < 0.001). Despite similarities in life history characteristics and spatial and temporal overlap in habitat, diets of these three common shark species are distinct in the California Current.