The world’s populations of killer whales (Orcinus orca) are projected to decline within a century because of contamination from manufactured toxins, according to a new modelling study published in the journal Science.
Polychlorinated biphenyls (PCBs) were used widely during the nineteenth and early twentieth centuries for a variety of industrial and commercial applications. Production was steadily phased out from the late 1970s amid mounting evidence of their toxic effects on humans and the environment.
Animals at the top of the food chain are particularly hard-hit, accumulating higher levels of the contaminants through a process called biomagnification. Studies from multiple species show conclusively that these pollutants impair reproduction, suppress immunity and cause cancer.
For apex predators such as killer whales, the toxic legacy of PCBs may have implications for long-term population trajectories.
“Our model predictions suggest that over 50% of the populations we investigated had tissue PCB levels that are negatively influencing population growth, and in the worse cases, potentially leading to population collapse,” explains lead author Jean-Pierre Desforges of Aarhus University in Denmark.
“These results highlight the significant risk that PCBs still pose to killer whales, decades after they were banned.”
To create this world-first assessment, Desforges and colleagues modelled how different levels of contamination would affect whale immunity and reproductive rates relative to an unexposed control. The model incorporated published data on PCB tissue levels collected from over 350 individual whales in 19 geographically-distinct populations.
Even at low levels, the toxins were shown to cause delays in population growth. According to the simulations, nine populations – generally those furthest from industrialised countries – will continue to grow, but at reduced rates relative to the control.
Killer whales in the 10 remaining populations face declines, with eight regions at risk of complete collapse due to PCB exposure within the next century: Brazil, Northeast Pacific, Canary Islands, Greenland, Hawaii, Japan, Strait of Gibraltar and the United Kingdom.
These impacts may be exacerbated as killer whales switch their food sources from low to high-PCB contaminated species (from fish to seals, for example), as has been documented for some populations in the North Atlantic.
Although sobering, the findings insights nevertheless advance understanding of how exposure to manufactured toxins may affect populations of long-lived marine apex predators, the authors write.
“This study goes beyond previous descriptive work to actually model the potential effects of PCBs at the population level, which is the scale we need to consider for the protection of this species,” explains Desforges.
The results may also help stimulate efforts to afford killer whales a global protected status, which is currently not possible, because of insufficient data.
Desforges notes that a worldwide clean-up will require considerable political will and financial resources. Legislation to address environmental contamination by PCBs and other pollutants has set remediation targets for 2025 and 2028, but few countries appear to be meeting these deadlines.