Food preferences determine pollution levels in orcas | Polarjournal

Orcas are the top predators in the world’s oceans. These intelligent and adaptable marine mammals have adapted to a wide range of food sources and locations. Animals are also on the hunt in the Arctic every year, be it for fish, whales or seals. But the pollution of the oceans with persisten pollutants affects even the large dolphins. An international research team has now discovered that individual food preferences in orcas have an impact on the amount of pollutants that accumulate in each animal.

The team of Canadian, Icelandic and Danish scientists found up to 300 times higher levels of so-called PCBs (polychlorinated biphenyls) which originate, among other things, from the anti-fouling paints of ships, from exhaust gases, but also from industrial waste water, in 50 examined orcas from Iceland. Interestingly, however, the team notes, significant differences exist within groups of orcas, including between the sexes. After further analysis, the team concluded that the reason for this could be the different food preferences of the individual animals. Study leader Melissa McKinney, assistant professor at McGill University in Canada explains, “Orcas are the ultimate marine predators and because they are at the top of the food web, they are among the most contaminated animals on the planet.”

Orcas adapt their hunting strategies to their prey and the conditions. Their diet consists of fish and other marine animals, but also seals and whales. There are large differences both between families and within families in terms of exposure to pollutants. The highest values were found in seal hunters (blue), the lowest in fish hunters (red). There are also big differences between bulls and cows and even individually (green). Graph: Remili et al. Environ. Sci. Technol. 2021, 55, 8, 4923-4931

The research team examined skin and blubber samples taken from the animals using small arrows (minimally invasive procedure). They then analysed the concentration of various known pollutants on the one hand; on the other hand, they investigated what the animals head fed on. For this purpose, nitrogen isotopes were measured and compared. It was found that the orcas around Iceland consist mainly of two diet eco types: those that feed mainly on fish (herring) and those that prefer a mixed diet of fish and marine mammals. And just the latter had up to 9 times higher amounts of PCBs concentrations than the fish hunters. “The concentrations of PCBs that we found in the whales that ate a mixed diet exceeded all known toxicity thresholds and are likely to affect both their immune and reproductive systems, putting their health at risk,” Melissa McKinney continues. Among individual animals, the differences were even greater, with a 300-fold difference between the animal with the lowest amount and the one with the highest amount of PCBs in its body.

Orcas are very coordinated in their attacks on seals, whales and also schools of fish. Each clan member has his task and the animals even control the situation above water (spy-hopping). This makes orcas very successful hunters and the top predators in the Arctic Ocean. Picture: Michael Wenger

With respect to the established limit for PCB contamination, which is classically around 17 mg per kilogram of lipd weight many animals were well above these thresholds. This finding by the team is all the more surprising as it was previously assumed that, on the one hand, orcas around Iceland would have only low levels of exposure to such substances. On the other hand, it showed that the levels are similar to those found in orca populations in the North Pacific. It was also found that within food groups, females showed higher differences between concentrations than males. These were all in the similar ranges, regardless of the group. From this, the researchers conclude that in future, the individual lifestyle of the animals must be taken into account when conducting studies on pollution levels in orcas.

Orcas in the northern hemisphere are divided into residents and transients. While the former usually stay in one region, transients move from food source to food source. They can cover long distances in the process. In the case of the study from Scotland to Iceland. As a result, there is also a greater risk of swimming through heavily contaminated areas. Picture: Michael Wenger

“The next step for us is to assess the proportion of marine mammals in the diets of these Icelandic and other North Atlantic orcas,” says PhD student and first author Anais Remili. “We also plan to put together a large dataset of contaminants in orcas across the Atlantic Ocean to contribute to their conservation efforts by quantifying potential health risks.” PCBs and other substances found in orcas, such as DDT, have been banned in industrialized countries for some time. But they are so persistent that they continue to exert their harmful effects decades later, not only in animals but also in ourselves.

Dr Michael Wenger, PolarJournal

Link to the study: Remili et al (2021) Environ. Sci. Technol. 55, 8, 4923-4931 Individual Prey Specialization Drives PCBs in Icelandic Killer Whales; https://doi.org/10.1021/acs.est.0c08563

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