Orcas are exposed to a variety of pollutants in their habitat that accumulate in their tissues. A new study now shows that the level of contamination depends mainly on which prey they specialise in, and less on the region in which they live.
Orcas, the undisputed hunters of the oceans, actually have no one to fear at the top of the food web. And yet their health is at risk. In addition to climate change and noise pollution from ships, persistent organic pollutants (POPs), which are characterised by their longevity and have been released into the environment for decades, can have serious consequences for the whales. An international research team has now found that the severity of the contamination does not depend on the region in which the orcas live, but rather on the food they eat.
In the current issue of the journal Environmental Science and Technology, the researchers report on their findings obtained by analysing blubber samples of orcas. They examined 162 orcas for POPs, which include industrial chemicals (e.g. PCBs), pesticides (e.g. DDT) and dioxins, as well as flame retardants and perfluorine compounds used in the manufacture of textiles. Many of the compounds have long been banned under the Stockholm Convention. The pollutants are nevertheless omnipresent in the environment and accumulate in the fatty tissue or organs of animals. They are passed on from the smallest organisms in the food web to the large predators, which are consequently the most contaminated – a process known as biomagnification.
The team collected the samples from free-ranging orcas in the North Atlantic from Canada through Greenland and Iceland to Norway, about which, in contrast to those in the North Pacific, there is little information so far. The researchers found that the samples from orcas from the western North Atlantic had ten times higher pollution levels (about 100 milligrams of PCBs per kilogram of fat tissue) than those from orcas from the eastern part (about 10 milligrams of PCBs per kilogram of fat tissue). In animals off Greenland and Iceland, PCB concentrations were around 50 milligrams per kilogram of fat tissue. The authors suspect that this is related to the different prey preferences of the various orca groups.
Orcas in the eastern North Atlantic, which feed exclusively on fish or on fish and seals, have the lowest risk of taking in pollutants. The more their diet is oriented towards other marine mammals, the higher their risk of contaminant exposure, as seals and smaller toothed whales are also high up in the food web, unlike herring for example.
In addition, sex plays a role, albeit to a lesser extent. The research team found that male orcas were significantly more contaminated with pollutants than females.
The authors describe their findings as worrying, especially in view of the fact that the whales are not only exposed to one pollutant, but to a cocktail of different pollutants, which may have stronger adverse health effects than individual chemicals. Previous studies have shown that POPs affect the immune system, the hormonal cycle and metabolic processes.
Thanks to numerous studies on the so-called Southern Residents – a group of three orca families in the waters off British Columbia – it is known that pollutants, especially PCBs, have for example a negative effect on reproductive success and early development. If Southern Resident females successfully give birth at all, only about half of the orca babies survive the first year. Juvenile and adult animals also repeatedly wash up dead on beaches.
In conclusion, the authors emphasise the urgent need for improvement in the management of legacy and emerging contaminants and their disposal “to reduce risks to the oceans’ most important predators”.
Julia Hager, PolarJournal
Featured image: Stefan Leimer
Source Anaïs Remili, Rune Dietz, Christian Sonne et al. Varying Diet Composition Causes Striking Differences in Legacy and Emerging Contaminant Concentrations in Killer Whales across the North Atlantic. Environmental Science & Technology, 2023; DOI: 10.1021/acs.est.3c05516.
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