Predicting trophic position in sharks of the north-west Atlantic Ocean using stable isotope analysis

Abstract

Trophic positions (TP) were estimated for the blue shark ( Prionace glauca ), shortfin mako ( Isurus oxyrinchus ), thresher shark ( Alopias vulpinus ), and basking shark ( Cetorhinus maximus ) using stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N). The basking shark had the lowest TP (3·1) and δ 15 N value (10·4‰), whereas the thresher shark had the highest values (4·5, 15·2‰). Mako sharks showed considerable variation in TP and isotopic values, possibly due to foraging from both inshore and offshore waters. Thresher sharks were significantly more enriched in δ 15 N than blue sharks and mako sharks, suggesting a different prey base. The δ 13 C values of thresher sharks and mako sharks varied significantly, but neither was significantly different from that of blue sharks. No statistical differences were found between our TP estimations and those derived from published stomach contents analyses, indicating that stable isotope data may be used to estimate the trophic status of sharks.