Throughout any animal’s lifetime, exposure to pollutants changes from place to place and from time to time. Exposure impacts different species to varying degrees too. Orcas tend to live for longer than porpoises, for example, so they have more time to accumulate chemical pollutants in their bodies. They also feed at a higher level within the food chain, so they are eating bigger fish and other marine mammals which themselves contain contaminants. Deaville explains that POPs are stored within the blubber. When an animal’s nutritional state deteriorates, either seasonally or as a result of illness perhaps, those toxic contaminants are released into the blood as the fat layer thins. Toxic chemicals are also passed on to calves via breast milk and so a mother can unwittingly offload toxic pollution onto a firstborn calf, sometimes resulting in death.
Rosie Williams, a toxicologist who works alongside Deaville, found in a 2021 study that high levels of PCBs in blubber samples from 267 stranded harbour porpoises are associated with reduced testicle size in male harbour porpoises. These pollutants are having a direct impact on fertility and potentially affecting the future breeding success of the species. PCBs are also known to suppress the immune system so porpoises with higher contaminant levels are more likely to die from infectious disease.
Of course, marine mammals are not experiencing just one chemical at a time. Since it began in 1990, the UK Cetacean Strandings Investigation Programme has carried out more than 4000 necropsies on porpoises, dolphins and whales and tested for a wide range of pollutants, from the widely known banned pesticide DDT to anti-foul paint residues and flame retardants added to textiles. “An animal may be living in a heavily fished area, with lots of bycatch, less prey, heavily contaminated, lots of noise, all those things are happening in tandem all at once – that’s how the animal lives its life,” Deaville says. “There’s a cumulative impact. It’s a complex thing to get a handle on.”
In the case of the stranded porpoise being dissected in the lab, chemical pollution may well have played a part. In a few months’ time, toxicology reports will show more about its specific chemical burden. Ultimately, for Deaville and Williams, it’s about noticing changing trends, which could then inform policy and change how pollutants are managed. Plastics are high up on the agenda too – a 2019 study by the University of Exeter found microplastics within all 50 marine mammals sampled that had stranded on UK shores. As yet, the actual impacts of those plastic particles on cetacean survival remains inconclusive.
Experts say that rather than having to cope with threats one at a time, animals experience the combined pressures of multiple forms of pollution within the marine environment. Marine mammals are living in a “soup” of pollution issues, says Deaville. The effects change depending on location or depth, and the animals’ season or stage of life, he adds.
So much of the damage and disturbance human activity causes is accidental and preventable, says Tom Mustill, filmmaker and author of a new book called How to Speak Whale. By designing things differently and considering how something might affect other animals, especially whales and dolphins that use such sophisticated communication, we can drastically reduce those negative consequences, he says. Enforcing speed limits on shipping vessels could drastically reduce collisions, for example – on the east coast of the US, proposals to introduce ‘dynamic speed zones’ could help protect endangered North Atlantic right whales in areas where these marine mammals are detected.
Slower ship speeds give the animals more time to adapt their navigation accordingly and reduce the risk of ship strikes. “This science of discovering what the sensory lives of these animals are like, how they perceive the world and how they communicate within it, allows us to understand how we might impact that and modify our impacts to that. That could be transformative,” says Mustill.
As stomach-wrenching as this porpoise post mortem was to witness, it’s clear that each stranding provides scientists with invaluable insight into how these cetaceans live and die, and how our actions are impacting them. The next step is to adapt strategies and policies accordingly so that by reducing pollution, marine wildlife and ocean health are better protected.
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