Isotopic Niche Analysis of Long-Finned Pilot Whales (Globicephala melas edwardii) in Aotearoa New Zealand Waters

The quantification of a species' trophic niche is important to understand the species ecology and its interactions with the ecosystem it resides in. Despite the high frequency of long-finned pilot whale (Globicephala melas edwardii) strandings on the Aotearoa New Zealand coast, their trophic niche remains poorly understood. To assess the isotopic niche of G. m. edwardii within New Zealand, ontogenetic (sex, total body length, age, maturity status, reproductive group) and spatiotemporal (stranding location, stranding event, and stranding year) variation were investigated. Stable isotopes of carbon (delta C-13) and nitrogen (delta N-15) were examined from skin samples of 125 G. m. edwardii (67 females and 58 males) collected at mass-stranding events at Onetahua Farewell Spit in 2009 (n = 20), 2011 (n = 20), 2014 (n = 27) and 2017 (n = 20) and at Rakiura Stewart Island in 2010 (n = 19) and 2011 (n = 19). Variations in delta S-34 values were examined for a subset of 36 individuals. General additive models revealed that stranding event was the strongest predictor for delta C-13 and delta N-15 values, whilst sex was the strongest predictor of delta S-34 isotopic values. Although similar within years, delta C-13 values were lower in 2014 and 2017 compared to all other years. Furthermore, delta N-15 values were higher within Farewell Spit 2017 compared to any other stranding event. This suggests that the individuals stranded in Farewell Spit in 2017 may have been feeding at a higher trophic level, or that the nitrogen baseline may have been higher in 2017 than in other years. Spatiotemporal differences explained isotopic variation of G. m. edwardii in New Zealand waters better than ontogenetic factors.

Automated summary

The quantification of a species' trophic niche is crucial for understanding its ecology and interactions with the ecosystem. This study investigated the isotopic niche of long-finned pilot whales in New Zealand, finding that stranding events and sex were important predictors of their isotopic values. Spatiotemporal differences better explained the isotopic variation than ontogenetic factors.