Diet variability in Norwegian killer whales (Orcinus orca): Evidence from quantitative fatty acid signature analysis (QFASA) and mercury levels

Understanding the diet of apex predators is essential to assess trophic interactions, food web dynamics, population structure, and contamination pathways. Consequently, this is important to inform species conservation efforts and management plans, and establish regulations and guidelines to reduce the threat of contaminants. Killer whales (Orcinus orca) are apex predators in northern Norway and their diet has previously been analyzed using various methods including predatory observations, analysis of stomach content and fecal samples, as well as the use of stable isotopes and anthropogenic contaminant concentrations as dietary markers. Such research identified Atlantic herring (Clupea harengus) as their primary prey. Lumpfish roe (Cyclopterus lumpu) and mackerel (Scomber scombrus), along with marine mammals including harbour seals (Phoca vituline) and harbour porpoises (Phocoena phocoena), have more recently been identified as additional prey types, and their contributions are known to vary seasonally. Whales observed feeding on marine mammals in addition to fish have been a priori classified as ‘mixed-diet’ individuals, while those with no such observations have been classified as ‘fish-diet’. Elevated concentrations of anthropogenic contaminants have been measured in mixed-diet killer whales due to feeding at higher trophic levels. Dietary markers used to date identify the relative trophic position and carbon source of Norwegian killer whales, but do not identify specific prey and their quantitative contributions to the diet of these apex predators. Quantitative fatty acid signature analysis (QFASA) is a method to estimate diet composition of animals based on their fatty acid signatures. By matching fatty acid signatures of prey types to predators, the relative contributions of different prey types to the predator’s diet can be quantified. This method has not been applied to Norwegian killer whales using a comprehensive prey library with a priori knowledge of the sampled individuals. Therefore, the aim of the present study was to quantify prey composition and investigate dietary variations of Norwegian killer whales using QFASA modelling and, thus, understand contaminant exposure in these apex predators. Fatty acid signatures were derived from killer whale blubber biopsies (n = 78) and a comprehensive library of both confirmed and hypothetical prey collected from 2018 – 2022 in northern Norway (n = 10 prey types; n = 63 total prey samples). Nitrogen stable isotopic ratios (ẟ15N) and mercury concentrations previously measured in the skin of the same individuals were used for comparative purposes to QFASA estimates. Results presented here support great dietary variation both within and between individuals. Herring was identified as the primary diet contributor across sampled killer whales (mean proportion ± standard error: 61.9% ± 3.9), followed by megrim (Lepidorhombus whiffiagonis; 11.5% ± 2.5) and lumpfish roe (11.5% ± 1.7). Relative proportions of fatty acids indicative of mammal- versus fish-prey (i.e., ‘indicator’ fatty acids) were not different between killer whales a priori assigned to different diet groups (i.e., mixed- versus fish-diet), with the exception of 16:1n7 which was reported to be higher in mixed-diet killer whales. Similarities were found in proportions of marine mammals estimated across killer whale diet groups, and no relationship was detected between estimated marine-mammal proportions and both ẟ15N and mercury levels. These results are most likely owing to large overlap in diet between the two groups and indicates that the risk of contaminant exposure is likely equal in the population. Patterns of dietary seasonality between sampling seasons was seldomly detected, even for repeat-sampled killer whales, and remain unconclusive. Elevated proportions of herring detected in those killer whales sampled in spring may indicate fatty acid turnover rates of up to six months in the outer blubber layer. Variability in diet estimates may be explained by changes in prey availability, however, several limitations must also be considered when interpreting diet estimates (e.g., fundamental requirements of QFASA modelling and unknown fatty acid turnover rates in killer whale blubber). This is the first study to quantify the diet of Norwegian killer whales by applying QFASA using a comprehensive prey library and a priori knowledge of sampled individuals. This research contributes as a complementary method to better understand variations in feeding habits of Norwegian killer whales and, thus, contaminant exposure to these apex predators. Results here further emphasize the complexity of Norwegian killer whale diets.