Written by
ZSL logo


Zoological Society of London

28 November 2023

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 today (28 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.  

How do scientists study toxins in UK marine mammals?

Collected over 30 years, the data behind the shocking 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 1000 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.  

Two bottlenose dolphins swim alongside each other
Common seals on the shore in the UK

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.  

Why are levels of marine pollution high in marine mammals?

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. 

A common dolphin undergoing a necropsy from a scientist
© Greg Norman
Orca breaches surface of water

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.”  

The research was made possible by lab analysis from Cefas, along with partners from Brunel University London, The University of Glasgow/Scottish Marine Animal Strandings Scheme, the Natural History Museum, Cornwall Wildlife Trust, Cornwall Marine Pathology Team, Marine Environmental Monitoring and Atlantic Technological University.

What can we do to protect marine mammals from chemical pollution?

The scientists behind the alarming discovery are calling for urgent action to protect the marine environment from both historic and emerging pollutants - by improving current pollution control measures and encouraging governments to take a more proactive approach to regulation – such as phasing out all non-essential use of hazardous chemicals and preventing one harmful chemical from being replaced by a closely-related and equally damaging substitution.  

Rosie explained: “We’re still cleaning up the mess of historic pollutants that were banned over two decades ago. We need to act now, learn from our past mistakes and employ stronger, science-backed measures to curb pollution. Ambitious and urgent action is required and by taking action today we can start mitigating the profound impact that chemical pollution continues to have on marine life.”  

white-beaked dolphin swimming underwater

Rob Deaville, CSIP Project Manager and researcher at ZSL’s Institute of Zoology explained: “As highly mobile marine species living in the vast expanse of the sea, gaining a better understanding of the dangers that marine mammals face can be difficult. Our investigations give a unique insight into these threats – including exposure to chemical pollutants. Members of the UK public can continue to directly help us by reporting any strandings they see, so we can further investigate these threats and try to improve the future for these iconic species found in our waters.”

Rosie added: “This isn’t just a UK problem and chemical pollution is just one of multiple threats facing marine ecosystems worldwide. We are putting our ocean under an ever-increasing amount of stress as climate change, overfishing and plastic pollution all continue to worsen. As our understanding of these interconnected issues grows, world decision makers must act now to protect our oceans from further harm.”

Members of the public can support CSIP’s vital research by reporting dead strandings in England and Wales through calling 0800 652 0333. 

Join the fight to save our living world at ZSL

Climate change and human activity have pushed our precious planet to its limit, causing the devastating loss of so many habitats and species. From lab to field, hands on and behind the scenes, we’re leading the future of conservation, shaping agendas and influencing change to support better life, health and living for people and wildlife.