Levels of banned chemicals in UK-stranded orcas are 30 times over the toxic threshold, uncovers new ZSL-led research. The finding is just one alarming discovery from the investigation into the scale at which chemical pollution threatens the future of marine mammals.   

Published on 27 November 2023, the first-of-its-kind study, reveals that levels of six chemical pollutants remain at highly toxic levels within UK marine mammals – exceeding the safe limits in half of animals investigated – more than two decades after the use of many of them was restricted or banned.   

Collected over 30 years, the data behind the findings comes from one of the world’s largest marine mammal toxicology datasets, created by partners across the UK – including the Centre for Environment, Fisheries and Aquaculture Science (Cefas) and ZSL’s Cetacean Stranding Investigation Programme (CSIP), a Defra-funded programme which investigates strandings of marine mammals, marine turtles and sharks around the English and Welsh coast to understand the threats they face.  

By analysing post-mortem records and associated tissue samples from over 1,000 animals spanning 11 species of whales, dolphins and seals stranded in the UK, the research uncovered how concentrations of these dangerous chemicals were highest in long-lived species at the top of the food chain – orcas, bottlenose dolphins and white-beaked dolphins. The scientists behind the research call for urgent action to protect marine wildlife from the lethal impacts of chemical pollution. 

The team identified that in recent years, the average concentrations of banned toxic chemicals known as polychlorinated biphenyls (PCBs) in orcas were thirty times the amount at which marine mammals would start to face negative health effects. Once used widely – including in paints, adhesives and motor oil – PCB’s were internationally banned in 2004. However, the report highlights how many of these chemicals continue to impact the natural world.  

Dr Rosie Williams, lead author and postdoctoral researcher at ZSL’s Institute of Zoology explained: “It’s been over 20 years since several of these chemicals were banned globally, yet we still see concerningly high concentrations in wildlife. Chemical pollutants such as PCB’s pose one of the biggest threats to marine mammal health. Although concentrations of the pollutants appear to be declining, our findings reveal that, in many species, they are still present at levels associated with negative effects on the immune and reproductive systems.”  

The research identified that in the most recent five years of the study (2014-2018), almost half of the animals studied had pollutant levels exceeding toxic thresholds. In bottlenose dolphins, the concentrations of polybrominated diphenyl ethers (PBDEs) – a chemical used as a flame retardant in several products including electronics and soft furnishings– was on average 200 per cent above the threshold for the onset of negative health effects. Like PCBs, PBDEs are known to weaken the immune system, increasing disease susceptibility.  

Rosie explained why pollutant levels are so high: “These toxins are initially taken up by plankton at the bottom of the food chain. Unable to be broken down or excreted, these persistent chemicals increase in concentration the further up the food chain they go, in a process known as ‘biomagnification’. As apex predators, many marine mammal species consume large amounts of toxins every time they feed, making them some of the most contaminated wildlife species.”  

The study identified the rates of decline of most pollutants has slowed – indicating that concentrations are approaching a constant level and raising concern about the potential for ongoing pollution. The situation threatens to worsen further with climate change, as around 10,000 landfills in Europe are situated on coasts at risk of climate change, sea-level rise, flooding or erosion with the potential to release their contaminant load directly to the marine environment. 

Rosie added: “This is a huge wake-up call. We rely on this same ecosystem for some of our own food – so these findings ring alarm bells not only for the future of marine life, but indicate a risk to human wellbeing. For example, PBDE exposure in humans has been linked to impaired development in children. The global costs associated with environmental chemical exposures – such as the costs of medical treatment – are estimated to be around 10% of global GDP– so there’s a huge economical incentive to address this problem now. Tackling the issue of chemical pollution will not only protect the environment and wildlife but will also protect humans too.”  

Professor Susan Jobling, senior author and Aquatic Ecotoxicologist at Brunel University London, commented: “Legacy pollutants like PCBs may be out of sight, but they are still invisibly poisoning marine mammals, undoubtedly contributing to global population declines through their multiple effects on immune, reproductive and neurological systems. Moreover, newborn whales and dolphins fed cocktails of these chemicals via the milk of their mothers soon after birth, at the time when they are still developing, are more highly exposed than their parents and at risk of permanent neurological and reproductive damage that may only become visible decades later.”