LNHS Talks – CSI of the Sea – What Causes Cetacean Strandings? By Rob Deaville

Dear Readers, while the subject of cetacean strandings is not a happy one, it is something that concerns many people. Are whales stranding themselves more frequently, and if so, why? Rob Deaville is the Project Manager for the Cetacean Strandings Investigation Team (CSI), and has been researching this subject for the past 21 years. He began by defining exactly what a stranding was: it’s

when a live or dead marine mammal swims or floats onto shore and becomes ‘beached’ or incapable of returning to the sea

Strandings can occur with individual or groups of whales, and they have been recorded throughout history. CSI looks at why strandings occur, and looks at whether the reasons are natural, or manmade.

Deaville started out with some historical context. Since 1324, when whales were described as ‘royal fish’, the monarch technically owns any stranded cetaceans that appear around the UK coast. In 1911 there was a mass stranding of over 50 pilot whales at Penzance in Cornwall, though the reaction of the local people was to chop the whales up, presumably for meat. However, it did alert the Natural History Museum (NHM) to the possibilities that strandings presented to learn more about cetaceans, which are very difficult to study in the wild. From 1913 onwards the NHM collected data on 3,949 strandings, and collected material for skeletal preparations from the dead animals (including one whale who was shipped across from Kings Cross on and open-bedded truck and dissected on the front lawn of the Museum). The modern stranding programme that Deaville is involved with started in 1990.

Deaville moved on to talk about 3 whale strandings in the London area. In 1658 a North Atlantic Right Whale stranded on the Thames and was dragged ashore and butchered by the local population for its oil. The skeleton of this whale, still showing the damage from the butcher’s knives, was found in Greenwich in 2010.

In 1954 ‘Jonah’ the whale was hunted off the coast of Iceland, embalmed and then taken round the country as a travelling freak show.

However, in 2006 a Northern Bottlenose Whale was seen in the Thames, as far upstream as the Houses of Parliament. Deaville was involved in the rescue attempt to try to return this whale to the open sea, although the animal unfortunately died in transit. As he says, we have gone from a nation of butchers and gawkers to a nation of conservationists – the response of the public to the rescue attempt was heartfelt and sympathetic.

Deaville then moved on to look at strandings more generally in the UK. He showed a chart that showed how strandings of fin whales and humpback whales had increased in the UK since the moratorium on whale-hunting in 1986, and how this is not necessarily a bad thing, as it can indicate that numbers of the whales are increasing, and hence we are starting to see them stranding (there were no humpback strandings in the UK prior to the ban on hunting).

Over the past 30 years there have been 17,000 strandings in the UK. 88% of these are where the cetaceans are already dead when they appear. By far the largest numbers of strandings are of smaller whales, with 8,615 harbour porpoises and 2915 common beaked dolphins comprising the bulk of the numbers, along with over 2,000 unidentified cetaceans who were presumably too decayed or damaged for the species to be ascertained. 7% of strandings are live animals, and the remainder are whales found entangled or floating at sea. In total, 22 species of whales have been identified from UK strandings, which is a quarter of all the whale species in the world, which reflects the diversity of our coastline, from the shallow coastal areas where harbour porpoises strand to the deeper, pelagic areas where sperm whales and humpbacks come to grief.

Deaville moved on to mass stranding events, which have occurred largely in Northern Scotland with one event in Cornwall. Organisations such as the RSPCA and local diving and rescue groups managed to rescue large numbers of the cetaceans involved (usually dolphins, porpoises or pilot whales) but many of the animals died. In two of the cases there seemed to be a direct link to human noise (probably use of sonar but Deaville didn’t go into detail on this).

Looking at the number of strandings, there seems to have been a marked increase during the period 2016-2019. There was also a reduction in strandings by cold-water species of cetacean, and an increase in strandings by warm-water species, which points to the impact of climate change.

Deaville then moves on to talk about the cause of death of the animals that he sees. A major reason for the demise of harbour porpoises and common dolphins is ‘by-catch’, whereby the animals get entangled in fishing nets. Harbour porpoises seem to be being killed all around the UK, but common dolphins are clearly most frequently killed around the south-west, particularly Cornwall. Deaville spoke of a ‘by-catch season’ for common dolphins from December to April, where they wash up dead but with evidence of damage from fishing nets.

Entanglement is a very specific form of by-catch that affects larger whales such as minke whales. Sometimes they drown because of a sudden entanglement in ship’s ropes, sometimes they die from starvation over a period of time when ropes and nets prevent them from hunting. Most of the cases are in Scotland where there’s a lot more creel-type fishing, using roped baskets to catch lobsters and crabs.

Ship strike, where animals are killed by propellors or being hit by a ship, largely affects harbour porpoises and common dolphins, and occurs, not surprisingly, where there are ports and a lot of shipping activity.

Not all causes of death are man-made, however. A significant cause of death is attack by bottle-nosed dolphins on other cetaceans, particularly harbour porpoises – the dolphins strike the much smaller porpoises, and often bite them with such force that they kill them. It’s still not clear why this happens, but all the observed cases have been by sub-adult male dolphins, so the current theory is that it has some link to sexual behaviour. I had no idea when I used to watch Flipper on television that dolphins could be such violent animals!

In 2015, a Cuvier’s Beaked Whale live-stranded in Scotland, and in spite of several attempts to refloat the animal it eventually had to be euthanised. An underwater necropsy was carried out, and it was found that the poor creature had a large volume of plastic sheeting in its stomach. This particular animal seemed to have some other health problems, which might have brought it closer inshore and resulted in aberrant feeding behaviour. While it might be the case that the ingestion of plastic debris is an increasing problem for cetaceans, Deaville found that there were only 3 cases out of almost 4000 where death was a direct result of an animal eating debris, and 35 cases where debris had been ingested though it was not the direct cause of death. Deaville did flag up that certain species of deep-diving suction-feeding whales could well be more directly affected by plastics, however, but these are whales that are least likely to be recovered for autopsy. He also said that the effect of microplastics was not yet fully understood in cetacean strandings.

He then moved on to infectious disease, which is by far the largest cause of death in cetacean strandings across all species. Deaville has been extensively studying the effect of PCBs (Polychlorinated biphenyls) on cetaceans – although these chemicals are now banned, they continue to persist in the environment, and they accumulate in animals that are at the top of the food chain such as whales. They are immunosuppressant, which might make the whales more vulnerable to infectious disease, and they also affect reproductive success. In studies of PCB levels, striped dolphins, bottlenosed dolphins and killer whales were all found to have toxic levels of PCBs in their blubber, with killer whales being most affected.

Deaville went on to speak about Lulu, a member of the last remaining resident Killer Whale pod in the UK. She was found dead on a beach in Tiree, Scotland, following becoming entangled in creel nets. However, her PCB levels were the highest ever found in any mammal, and her ovaries were non-functional. Lulu’s pod have never had a calf in the 30 years that they’ve been studied, and there is no doubt in Deaville’s mind that that pod will become extinct, and there will be no more coastal killer whales in the UK.

Finally, Deaville went on to talk about cetacean strandings in the Thames. The vast majority of strandings are harbour porpoises (probably about 450 out of the 500 strandings recorded in the past 30 years). However, pelagic species such as minke whales do sometimes find themselves in the shallow seas of the estuary, and unfortunately for them there’s usually only a fatal outcome. However, the Thames is a much healthier environment that it has been historically, and lots of marine mammals make their way up and down the river without coming to any kind of harm.

To conclude, Deaville talked about two high-profile  strandings. In 2019 a humpback whale was seen in the Thames. The animal was clearly unwell, and died two days after its first appearance. On investigation the whale was seen to be a juvenile female, and she had been severely injured in a ship strike probably 48 to 72 hours previously. During 2019 there were 3 cases of entanglement and one of ship strike involving humpback whales, and as the population of this species increases it’s likely that there will be more events of this kind.

In 2021 a small female minke whale stranded in the lock at Richmond. An attempt was made to move her but she escaped, and was later found at Teddington lock, the furthest up the Thames that a whale has ever been sighted. She was eventually euthanised, and necropsy showed that she was extremely thin, and probably hadn’t fed for some time – it’s possible that she hadn’t been completely weaned from her mother, and, having wandered into the Thames, wouldn’t have been able to find any alternative food. Again, this is a species which is increasing, so more strandings might reflect more animals.

So, to conclude:

  • CSI has investigated over 18,000 strandings and has conducted 4500 necropsies.
  • Although its work is opportunistic (inasmuch as it relies on being able to retrieve stranded animals), it is a cost effective method of looking at threats to the marine environment, and its work is used to create policy, to further the science of cetacean strandings, and to educate and inform.
  • CSI looks at the anthropogenic causes of strandings, such as bycatch, entanglement, ship strike, noise, climate change and marine debris, and non-anthropogenic causes such as bottlenose dolphin attack.
  • However, CSI consider that PCB exposure is the biggest single conservation concern for some species, and is a threat to the existence of some populations.

So, an absolutely fascinating talk, and lots that I didn’t know. If you’d like to listen to the whole talk, you can find it here.



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