Scientists from the UK’s Cetacean Strandings Investigation team are trying to determine the cause of the biggest mass stranding in a century

Slicing cleanly through two inches of skin and blubber, Rob Deaville considers the possible causes of death of the sea mammal on his dissecting table. “It’s a female, juvenile, stranded in north Devon,” he says. “No signs of parasite infestation. It looks healthy. It may have just come too close to shore.”

This porpoise, in the process of being dismembered with small parts of its vital organs tested for disease and pollutants, is one of hundreds that come to the labs in the Zoological Society of London each year, awaiting a post-mortem – a necropsy, in the scientific term – that will help to establish how the animal lived and why it died.

In recent weeks the team’s expertise has been called on to investigate a highly unusual series of events. A mass stranding of sperm whales has puzzled scientists, with a total count of six now having washed up on British beaches, the biggest in the century since Zoological Society of London (ZSL) has been making a count, and taking custody of the bodies.

This is part of a much bigger event, as at least 29 have now been found on the coasts of the UK, the Netherlands and Germany. It is impossible to tell whether all the whales were members of the same pod, or a clutch of pods, but it seems likely that the strandings are related. Sperm whales tend to live in groups of females with their young, while adult males roam further afield singly.

Deaville, project manager of the Cetacean Strandings Investigation programme – CSI for whales, dolphins and their relatives if you like – is reluctant to make guesses as to what the cause of the deaths may be. While people naturally want to have answers as soon as possible, the need for close examination and the inherent caution of the scientific method mean this is not realistic. “We just do not know yet, and I don’t want to speculate without the data,” he says.

Several potential explanations have been put forward. Disease may be a factor, or changes related to climate change, or the overfishing of some of the sea areas where the whales tend to gather. Most recently, concerns have been raised over the lingering effects of now-banned chemicals, called PCBs, polluting European waters.

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PCBs are lipophilic, notes Deaville, meaning that they are found in concentration in the animals’ thick layer of blubber, which is why extensive samples of it are taken in the labs. The liver is also a key source of samples, as it will reveal the levels of heavy metals – such as lead, cadmium and mercury – that the creature has absorbed, mostly from its fish diet.

These are the early warning systems of the seas. Heavy metals are now so concentrated in fish that pregnant women and small children are advised to eat no more than two portions of affected fish in a week.

Another likely cause of death is that the whales simply got lost. The North Sea is one of the shallowest in the world, so much so that archaeologists are only now discovering the remains of human settlements buried on the seabed from the last Ice Age, when “Doggerland” was above sea level and inhabited.

For whales, which navigate by echo-location in a similar way to bats, shallow water is a trap, as it confounds their ability to use sound. When out of the breeding grounds of their usual prey, such as squid, they can quickly become starved and dehydrated, because they obtain their water from their food. Once in shallow water, the enormous weight of their bodies is no longer so buoyant, and can crush them. The recently found dead whales may simply have strayed too far from their usual haunts and been unable to find a way out. Whether climate change, which has caused cold water species to move north and brought normally tropical fish to UK waters, has played a role is still unclear.

Counterintuitively, the strandings may actually be good news for the species. While it is hard to count marine populations accurately, strandings “can be a proxy” says Deaville. “The bigger the population, the more likely it is that some will be stranded,” he explains.

The whales, and any more that are found, will be extensively examined in the ZSL labs and definitive results, with any clear conclusions that can be drawn from them, are likely to become available in the next few months. However, even those findings may still leave the mystery unsolved. “We don’t know whether we will find an answer,” says Deaville.

Facebook Twitter Pinterest Cetaceans Strandings Investigation team inspects inspects the carcass of a sperm whale on the beach in Hunstanton in Norfolk, England. The whale is the 29th to get stranded in Europe in in recent weeks. Photograph: Alan Walter/Reuters

Cetacean Strandings Investigation programme in brief

The CSIP has a curious history. Under a 13th-century law enacted by Edward II, all whales, sturgeon and porpoises are regarded as “royal fish”, so that when caught by UK fishermen or stranded around the coast they are the property of the crown. The Queen no longer exercises her right to have this bounty hauled on to her dinner table or cut up to make corsets, but the CSIP fills in, building on work done at the Natural History Museum since 1913 when formal records of strandings began.

The information gleaned from investigating the circumstances of strandings and performing necropsies on the corpses helps to inform conservation efforts, and acts as an early warning system in case of emerging diseases or other hazards for marine mammals.

Based at the ZSL, next to London zoo, the CSIP recently celebrated its 25th birthday, and in that time Deaville has examined more than 3,500 specimens, adding greatly to our knowledge of the wildlife that dominate the UK’s seas.