Trophic ecology, based on stable isotope values, of long-finned pilot whales Globicephala melas stranded on the Icelandic coast

In Iceland, occurrences of long-finned pilot whales Globicephala melas and strandings in coastal waters are irregular and thought to be related to prey movements; however, little is known about the population’s trophic ecology, making it difficult to assess its role in the regional ecosystem. To investigate the trophic niche of the species, we measured δ 13 C and δ 15 N values from skin and muscle samples collected from 16 strandings (12 single and 4 mass strandings) spanning 33 yr (1988-2021). We tested the effects of sex, age class and body length on δ 13 C and δ 15 N values within the species. We found that δ 13 C values did not differ between sexes or age classes, nor did they vary with body length. Values of δ 15 N did not vary with sex or body length, but varied slightly between adults and juveniles, albeit not sufficiently to support a difference in trophic levels. Whales from different mass stranding events showed similar stable isotope niche widths (approximately 0.20‰ 2 ) and overlapping isotope niches (generally >45% of the standard ellipse area corrected for small sample size). Overall, pilot whales in this region showed a narrow isotopic niche width and low intraspecific variation in isotopic niche. Future analyses using complementary chemical tracers and stomach contents will be important to identify the factors driving these patterns. The first measurements of the stable isotope niche of pilot whales presented here provide a baseline for future studies to determine niche overlap between cetaceans in Iceland, contributing to their management and conservation.