Ecotype and geographical variation in carbon and nitrogen stable isotope values in western North Pacific killer whales (Orcinus orca)

Abstract

Killer whales are top predators in marine trophic chains, and therefore their feeding preferences can substantially affect the abundance of species on the lower trophic levels. Killer whales are known to feed on many different types of prey from small fish to large whales, but a given killer whale population usually focuses on a specific type of prey. Stable isotope analysis is widely used to study whale diets, because direct observations are often impossible. Killer whale feeding habits in the western North Pacific are poorly studied, and the large‐scale stable isotope analysis provides a unique opportunity to gain insights into the trophic links of this top predator. In this study, we compare the δ13C and δ15N stable isotope values from killer whale skin samples obtained in different areas of the western North Pacific from fish‐eating (R‐type) and mammal‐eating (T‐type) killer whale ecotypes. The effect of ecotype was highly significant: both carbon and nitrogen stable isotope values were lower in R‐type whales than in T‐type whales. The geographical variation also affected killer whale stable isotope values due to both the differences in killer whale diet and the variation in baseline stable isotope values across the study areas.

Trophic ecology of Angolan cold-water coral reefs (SE Atlantic) based on stable isotope analyses

“…However, sponges exhibited the highest δ 15 N values (from 17.33 to 21.90 ‰) of all samples analyzed in this study. These results are in the range of, and sometimes higher than, values usually attributed to top predators in marine trophic webs 119 , 120 . According to TP calculations following Post, 72 , the 15 N-enriched ratios of sponges correspond to a mean TP of 5.69 ± 0.53 (Table 1 ), being inconsistent with the expected filter feeding behavior in the trophic web.…”

Too Close for Comfort? Isotopic Niche Segregation in New Zealand’s Odontocetes

“…In New Zealand, killer whales are documented to regularly feed on stingrays [ 106 ] and sharks [ 107 , 108 ], with marine mammal predations also recorded [ 109 ]. In locations where prey specialisation occurs, stable isotope analysis has been able to quantify inter-individual dietary variation, showing higher δ 15 N values for individuals feeding on marine mammals compared to those feeding on fish [ 104 , 110 , 111 , 112 ]. Interestingly, the isotopic niche of killer whales in this study was the smallest of all mesopelagic species, suggesting little isotopic variety in their diet and limited inter-individual differences, which is further confirmed by the low Layman metrics values.…”

Revised taxonomy of eastern North Pacific killer whales ( Orcinus orca ): Bigg’s and resident ecotypes deserve species status

“…Prey specialization was the primary evidence initially used to identify the fish-eating resident and mammal-eating Bigg’s ecotypes in the eastern North Pacific [ 11 , 34 , 42 ]. Multiple methods have been used to determine dietary specialization and seasonal diet changes for residents and Bigg’s, including behavioural observations and examination of prey remains [ 11 , 42 ], molecular prey species identification from faeces [ 45 , 84 ], stable isotopes [ 41 , 47 , 85 , 86 ], pollutants [ 41 , 47 , 87 , 88 ] and fatty acids from skin and blubber biopsies [ 41 , 47 ]. Because of their higher trophic level, the marine mammal-eating Bigg’s exhibit higher δ15 N and δ13 C isotope levels, lower proportions of omega-3 long-chain mono-unsaturated and poly-unsaturated fatty acids (evidence of fish consumption) and higher levels of persistent organic pollutants than fish-eating residents [ 89 ].…”