Unexpected Trophic Enrichment in Δ15Naa for killer whales

Sam Rossman

Elementar

Peggy Ostrom

Michigan State University

Killer whales (Orcinus orca) are unrivaled apex carnivores currently experiencing dramatic population declines due to the anthropogenic depletion of salmonid species in the Pacific Northwest, USA. Thus, understanding how killer whales capture and consume prey is critical to conservation efforts and management strategies. While stable isotopes are a useful tool to understand cryptic foraging, bulk isotope analysis of cetaceans with well-known diets suggest trophic enrichment factors (TEF) are well below common mammalian values c.a. 3.4 ‰. To examine the unusual TEF for bulk as well as establish TEF values for compound specific isotope analysis we conducted a controlled feeding trial using killer whales. Killer whales were fed a diet of four common fish species based upon their dietary needs; diet was consistent for at least one year prior to the study. Fish came from a single large lot (i.e. same geographic and temporal collection) to reduce isotopic heterogeneity. Blood samples were taken at the onset of the study (June, 2016) and again five months later (October, 2016). Bulk isotope values of individuals did not change over the course of the study. Plasma TEF for bulk nitrogen showed a low mean enrichment of 1.9 ‰ (Fig. 1) while, unexpectedly, δ15Nphe showed an enrichment of 1.5 ‰ and δ15Nglu was 2.7 ‰ lower in the killer whales compared to the diet average (i.e. d15Ndiet killer whale glu-d15Ndiet glu = -2.7 ‰). The 15N depletion in killer whale glu may be the consequence of differences in the Glu metabolism between killer whales and their prey.