Ecological diversity has been reported for killer whales (Orcinus orca) throughout the North Atlantic but patterns of prey specialization have remained poorly understood. We quantify interindividual dietary variations in killer whales (n = 38) sampled throughout the year in 2017–2018 in northern Norway using stable isotopic nitrogen (δ15N: 15N/14N) and carbon (δ13C: 13C/12C) ratios. A Gaussian mixture model assigned sampled individuals to three differentiated clusters, characterized by disparate nonoverlapping isotopic niches, that were consistent with predatory field observations: seal‐eaters, herring‐eaters, and lumpfish‐eaters. Seal‐eaters showed higher δ15N values (mean ± SD: 12.6 ± 0.3‰, range = 12.3–13.2‰, n = 10) compared to herring‐eaters (mean ± SD: 11.7 ± 0.2‰, range = 11.4–11.9‰, n = 19) and lumpfish‐eaters (mean ± SD: 11.6 ± 0.2‰, range = 11.3–11.9, n = 9). Elevated δ15N values for seal‐eaters, regardless of sampling season, confirmed feeding at high trophic levels throughout the year. However, a wide isotopic niche and low measured δ15N values in the seal‐eaters, compared to that of whales that would eat solely seals (δN‐measured = 12.6 vs. δN‐expected = 15.5), indicated a diverse diet that includes both fish and mammal prey. A narrow niche for killer whales sampled at herring and lumpfish seasonal grounds supported seasonal prey specialization reflective of local peaks in prey abundance for the two fish‐eating groups. Our results, thus, show differences in prey specialization within this killer whale population in Norway and that the episodic observations of killer whales feeding on prey other than fish are a consistent behavior, as reflected in different isotopic niches between seal and fish‐eating individuals.

Killer whales (Orcinus orca) are at risk from high levels of biomagnifying pollutants, such as polychlorinated biphenyls (PCBs) and mercury (Hg). Previous toxicological risk assessments for the Norwegian killer whale population have assumed fish as the primary prey source, and assessed the population as below established effect thresholds. However, some individuals have recently been identified to also feed on seals. This study is the first to quantify levels of pollutants in seal-eating killer whales from northern Norway, and to measure Hg levels in the skin of killer whales worldwide. We found higher levels of all pollutants in seal-eating than fish-eating killer whales, including the emerging brominated flame retardants pentabromoethylbenzene (PBEB), pentabromotoluene (PBT) and hexabromobenzene (HBB). Sum polychlorinated biphenyls (ΣPCBs) in the blubber of seal-eaters (n = 7, geometric mean = 46 µg/g l.w.) were four times higher than fish-eaters (n = 24, geometric mean = 11 µg/g l.w.), which pushed all seal-eating individuals above multiple thresholds for health effects. Total Hg levels in skin of seal-eaters (n = 10, arithmetic mean = 3.7 µg/g d.w.) were twice as high as in fish-eaters (n = 28, arithmetic mean = 1.8 µg/g d.w.). Our results indicate that by feeding on higher trophic prey, the Norwegian killer whale population is at higher risk of health effects from pollution than previously assumed. Pollutants, such as polychlorinated biphenyls (PCBs) and mercury (Hg), have the potential to cause harmful effects in wildlife due to their persistent, bioaccumulative and toxic properties 1,2. PCBs are a type of organohalogen contaminant (OHC): anthropogenically produced pollutants that include a range of pesticides and industrial products. Hg is a naturally occurring element, however the concentrations measured in wildlife have increased by 3-4 orders of magnitude over the last 150 years due to anthropogenic activities 3. The killer whale (Orcinus orca) is a species particularly vulnerable to biomagnifying pollutants due to its position as an apex predator, long life span, and thick blubber layer in which lipophilic OHCs can accumulate 4. High recorded tissue levels of PCBs and Hg led to the killer whale being named the most exposed species by the Arctic Monitoring and Assessment Programme (AMAP) 5 , and over 50% of killer whale populations worldwide have been predicted to collapse within the next 100 years due to PCB exposure 6. The levels of pollutants in a killer whale population, and thus its risk of health effects, are higher in populations feeding at high trophic levels due to increased exposure from biomagnification in prey 6,7. For example, in the northeast Pacific, marinemammal eating killer whale populations have higher levels of PCBs than fish-eating populations from the same geographic area 8,9. Variations in Hg levels according to dietary preference have not yet been recorded in killer whales, although higher Hg levels would be expected in groups feeding on marine mammal prey, as Hg also biomagnifie...

Killer whales (Orcinus orca) have been documented preying on either fish or marine mammals in several regions, suggesting that this odontocete species has the ability to specialize on different types of prey. Off Norway, killer whales have been shown to rely on the Atlantic herring (Clupea harengus) as a main prey resource. Infrequent observations have revealed seals as an additional component of their diet, yet the extent of predation on marine mammals has remained largely unknown. Here, we present the findings of 29 years of photographic and observational data on seal-feeding killer whale groups identified in Norwegian coastal waters. Four groups have been observed preying and feeding on seals over several years, taking both harbor (Phoca vitulina) and grey (Halichoerus grypus) seals. These stable groups are shown to adopt small group sizes, were typically observed in near-shore areas and were not encountered on herring wintering grounds. Behavioral and social traits adopted by these groups are similar to those of pinniped-feeding killer whales from other regions. The potential ecological reasons and the extent of such prey specializations are discussed.

Killer whales (Orcinus orca) in Norwegian waters have long been known to rely on 28 Atlantic herring (Clupea harengus) as a main prey resource. However, research almost 29 exclusively conducted at seasonal herring grounds may have biased studies away from 30 detecting other potentially significant prey species. Since 2013, dedicated research efforts 31 have focused on monitoring killer whale occurrence and foraging ecology throughout the year 32 in northern Norway. This study presents results on site-fidelity of photographically identified 33 individuals, predation records and behavioral patterns from five spring seasons (March-April) 34 in 2014-2018 in Andfjord, northern Norway. A minimum number of 75 adult and subadult 35 killer whales (out of a catalogue of 971 individuals) returned seasonally to the study area for 36 foraging and residency for up to six weeks. Lumpfish (or lumpsucker, Cyclopterus lumpus) 37 was the only type of prey identified (based on molecular or visual identification) on 22 38 predation events from 2016 (n=4), 2017 (n=2) and 2018 (n=16). Spatial group cohesion 39 observed when foraging was a potential adaptation for efficiently hunting this prey species. 40 These whales were also encountered at herring wintering grounds the same years, but with 41 different group sizes. Such behavioral adaptations suggested intra-annual switching between 42 prey resources and foraging strategies. 43

Little is known of the movement or presence of unregulated, emerging contaminants in top predators. The aim of the present study was to conduct the first screening of legacy and emerging contaminants in multiple tissues of killer whales (Orcinus orca) from Norway and investigate tissue partitioning and maternal transfer. Blubber was collected from 8 killer whales in 2015 to 2017, in addition to muscle from 5 of the individuals, and kidney, liver, heart, and spleen from a neonate. We screened for 4 unregulated brominated flame retardants and found pentabromotoluene (PBT) and hexabromobenzene (HBB) at low levels in the blubber of all individuals (median PBT 0.091 ng/g lipid wt, median HBB 1.4 ng/g lipid wt). Levels of PBT and HBB (wet wt) were twice as high in the blubber than the muscle for each individual, confirming preferential accumulation in lipid-rich tissues. Perfluoroalkyl substances and total mercury levels were lower in the neonate than adults, suggesting less efficient maternal transfer of these substances. Polychlorinated biphenyl levels in blubber exceeded the threshold for onset of physiological effects (9 µg/g lipid wt) in 7 of the 8 whales, including the neonate. The presence of PBT and HBB in the neonate is the first evidence of maternal transfer of these unregulated contaminants in marine mammals. Our results are relevant for the continued environmental monitoring of contaminants in the Arctic. Environ Toxicol Chem 2021;00:1-11.