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In some areas, killer whales eat mainly marine mammals and large fish such as tuna and sharks, and are then threatened by PCBs. In areas where killer whales eat mainly small fish such as herring, they are less threatened. Credit: Audun Rikardsen
More than 40 years after the first initiatives taken to ban the use of PCBs, chemical pollutants remain a deadly threat to animals at the top of the food chain. A new study, just published in the journal Scienceshows that current levels of PCBs could lead to the loss of half of the world's population of killer whales in the most contaminated areas in only 30 to 50 years.
Killer whales (Orcinus orca) are the last link in a long food chain and are among the richest mammals of PCBs (polychlorinated biphenyls) in their tissues. The researchers measured values as high as 1300 milligrams per kilogram in the fat tissue of killer whales. For comparison, a large number of studies show that animals with PCB levels as low as 50 milligrams per kilogram of tissue can show signs of infertility and serious impacts on the immune system.
With colleagues from a wide range of universities and international research institutions, researchers from the University of Aarhus have found that the number of killer whales is rapidly decreasing in 10 of the 19 populations of killer whales studied and that a few decades.
Killer whales are particularly endangered in heavily contaminated areas such as the nearby waters of Brazil, the Strait of Gibraltar and surrounding areas of the United Kingdom. Around the British Isles, researchers estimate that the remaining population has fewer than 10 killer whales. Along the east coast of Greenland, killer whales are also affected by the high consumption of marine mammals such as seals.
PCBs accumulate in the food chain
The killer whale is one of the most prevalent mammals on Earth and is found in all the oceans of the world, from one pole to the other. But today, only populations living in the least polluted areas include a large number of individuals. Overfishing and artificial noise can also affect the health of animals, but PCBs can have a dramatic effect on the reproduction and immune system of killer whales.
The diet of killer whales includes seals and large fish such as tuna and sharks that accumulate PCBs and other pollutants stored at successive levels of the food chain. It is these populations of killer whales that have the highest concentrations of PCBs and it is these populations that present the greatest risk of collapse of the population. Killer whales that feed mainly on small fish such as herring and mackerel have a significantly lower PCB content and are therefore less exposed to effects.
When foreign hazardous substances enter the marine environment, they are assimilated to the first link in the food chain, phytoplankton. Phytoplankton are eaten by zooplankton, which are eaten by smaller fish, and so on. The chemicals accumulate in each link of the food chain, which means that the killer whales that feed the survival of the species is threatened. Killer whales that feed mainly on small fish are not threatened in the same way. Credit: Graphic: University of Aarhus
PCBs have been used worldwide since the 1930s. More than a million tons of PCBs have been produced and used, among other things, in electrical components and plastics. With DDT and other organic pesticides, PCBs have spread to the world's oceans.
During the 1970s and 1980s, PCBs were banned in several countries and in 2004, through the Stockholm Convention, more than 90 countries committed to eliminating and eliminating large stocks of PCBs. PCBs only decompose slowly in the environment. In addition, PCBs are transmitted to orc offspring by the mother's high-fat milk. This means that dangerous substances remain in the bodies of animals instead of being released into the environment where they settle or degrade.
Global survey of killer whales
"We know that PCBs deform the reproductive organs of animals such as polar bears, so it was natural to look at the impact of PCBs on the world's scarce killer whale populations," says the professor. Rune Dietz of the Department of Biological and Arctic Sciences. Research Center, University of Aarhus, which initiated studies on killer whales and co-authored the article.
The research group, which includes participants from the United States, Canada, England, Greenland, Iceland and Denmark, reviewed all existing literature and compared all data with their own most recent results. Recent. This has provided information on PCB levels in more than 350 killer whales worldwide, the largest number of killer whales ever studied.
By applying models, the researchers then predicted the effects of PCBs on the number of offspring as well as on the immune system and killer whale mortality over a 100-year period.
More than 50% of threatened populations
By collecting data from around the world and loading them into population models, researchers can find that 10 out of 19 populations of killer whales are affected by high levels of PCBs in their bodies. PCBs particularly affect the reproduction and immune system of whales. The situation is worse in the oceans around Brazil and the United Kingdom where the model predicts that populations were cut in half in the first few decades since the use of PCBs has become widespread. Here, the models predict a high risk of extinction of the species during a period of 30 to 40 years. The line indicates the median values, while the shaded field indicates the variation. Credit: University of Aarhus
"The results are surprising.We find that more than half of the studied populations of killer whales in the world are severely affected by PCBs," said postdoctoral researcher Jean-Pierre Desforges of the University of Toronto. Aarhus, who led the investigations.
The effects result in fewer animals over time in these populations. The situation is worse in the oceans around Brazil, in the Strait of Gibraltar, in the northeast Pacific and around the United Kingdom. Models show that populations have almost halved in the last 50 years.
"In these areas, we rarely see newborn killer whales," says Ailsa Hall, who developed with Bernie McConnell the models used by the Marine Mammals Research Unit in Scotland.
"As the effects have been recognized for more than 50 years, it is frightening to see that models predict a high risk of population collapse in these areas over a period of 30 to 40 years," explains Jean-Pierre Desforges.
A female killer whale can live for 60 to 70 years, and although the world has taken its first steps to eliminate PCBs more than 40 years ago, killer whales still have high levels of PCBs in their bodies.
"This suggests that efforts have not been effective enough to avoid PCB build-up in high-trophic species that live as long as killer whales, so there is an urgent need to launch others." initiatives than those of the Stockholm Convention, "concludes Paul D. Jepson, Institute of Zoology, Zoological Society of London, England, another expert on killer whales and co-author of the article.
In the Faroe Islands, Iceland, Norway, Alaska and Antarctica, the outlook is not so bleak. Here, killer whale populations are growing and models predict that they will continue to do so throughout the next century.
Explore more:
Chemicals threaten endangered orcas in Europe
More information:
J.-P. Desforges el al., "Predicting the collapse of the world's population of killer whales by PCB pollution" Science (2018). science.sciencemag.org/cgi/doi… 1126 / science.aat1953
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