Manmade chemicals at levels below the toxic threshold for marine animals are now linked to a rise in the number of porpoises killed by disease.
Europe banned polychlorinated biphenyls (PCBs) in 1987 because of their believed toxicity to people and wildlife. These chemicals have a history of use in electrical equipment, coatings and paints.
But the chemicals, often still found in electrical products and likely to be in landfills, still enter the environment and foodchain carried in water.
Ecotoxicologists at Brunel University London and ZSL (Zoological Society London) measured PCBs found in blubber from porpoises washed up off UK waters. Their study published recently in Environmental Science and Technology found a rise in PCB blubber levels of 1 mg per kg of lipid matches a 5% rise in risk of death from infectious disease. That’s more than double the 2% risk rise estimated in 2006.
“Our study shows that despite being banned over three decades ago PCBs are still a threat to harbour porpoises,” said Brunel researcher Rosie Williams.
“Our findings have serious implications for the management of PCB contamination in the UK and reinforce the need to prevent PCBs entering the marine environment to ensure that levels continue to decline.”
The team looked at samples from 814 porpoises found stranded between 1990 and 2017. By 2007, their PCB concentrations were below the official cut-off for toxic effects (9 mg per kg of lipid), Yet in 2016 and 2017, 39% tested had more than this safe limit.
“That we are still finding PCB concentrations at these levels is evidence that they are being transferred from generation to generation and continuing to enter the environment,” said Williams.
The picture is different in different parts of the UK, the team found. Levels in West England and Wales, where PCBs were made, are falling more slowly than elsewhere and may still be above the toxicity threshold. This may mean areas are being newly contaminated and PCBs enter the environment in these places at a higher rate.
The 209 types of PCBs take longer to degrade than many other pollutants and stay inside marine life, like dolphins, porpoises and whales, for different lengths of time. Scientists think the toxins damage the immune system, some more so than others.
“We need to be cautious about using thresholds to decide if species are at risk or not,” said Williams. “Thresholds that are applied across a group as wide as ‘marine mammals’ are unlikely to be very accurate. Therefore, it’s important to have studies like this which we can use alongside toxicity thresholds to assess population level risks.”