On Tuesday, October 8, a humpback whale that was previously spotted in the Thames was found dead by the British Divers Marine Life Rescue (BDMLR).
The humpback, lovingly nicknamed Hessy, was a rare breed thought to have died due to being nutritionally compromised. However, the Zoological Society of London (ZSL) conducted a necropsy and found the primary cause of death was a wound caused by a ship strike on its jaw. This examination, as well as the postmortem, has also shown scars of ‘historic entanglement’ on its dorsal fin.
The 30-metric ton, juvenile female, was found motionless on mudflats along the river at Greenhithe, Kent by the BDMLR (the average lifespan of a humpback whale is 45 – 50 years). It had previously been predicted that the whale would only be in the river for about a week, before swimming out with the next high Spring tide.
The humpback whale, however, at only 27 feet, did not have a lot of fat reserves, and Julia Cable from the BDMLR said, “It is swimming up and down the river, so it is using more reserves all the time.” The charity was reluctant to send out boats to monitor Hessy, as they feared it could stress the mammal.
Humpback whales have the longest migration pattern of all living mammals. They move in late Autumn to winter breeding grounds and calving grounds in warmer tropical waters before returning south in the Spring.
This particular whale likely arrived in the Thames due to navigational error from the North Sea during the Spring tides last week, when water levels rise significantly. Nevertheless, warming waters have caused whale species to have to migrate much further to reach their feeding grounds, decreasing the time they have to forage for food.
The Thames is by no means an optimal feeding ground for humpback whales, whose diet consists primarily of crustaceans, plankton, and small fish. Humpbacks are deep divers. When found, Hessy was nutritionally compromised and had no evidence of recent feeding.
Less than two weeks after the humpback whale’s death, another whale was found dead in the Thames at Denton, near Gravesend, on October 18. Unlike Hessy, there were no sightings of the second whale and Port of London Authority was surprised to find it. The second whale, 32 feet in length, had turned up just miles from the first.
ZSL’s cetacean strandings investigation said, “There is no reason to assume the two recent whale strandings in the Thames are in any way linked.”
After the post-mortem examination by the ZSL, the second whale was found to be ill because of the parasitic worms, Bolbosoma turbinella, inside its intestinal tract. Scientists found that it was a sei whale, a species endangered since 1970. This was only the seventh sei whale to be recorded in the UK.
Like the first, this whale also had no microplastics in their stomach and intestinal tracts. However, unlike Hessey, the second whale had no evidence of previous entanglement, ship strike or traumatic injuries and was in a moderate nutritional condition.
There was, however, evidence of blood accumulation in the liver of the second whale, which is proof of its health being compromised due to being stranded outside its natural habitat.
Sei whales migrate to mate in the Northern Hemisphere in Autumn months and their diet, like humpbacks, mainly comprises crustaceans and krill. Different to humpbacks however, whose populations are currently increasing in free waters, Sei whales are constantly depleting due to their being a favoured species in commercial whaling.
Global warming, increasing pollution, ship strikes, and entanglement in fishing gear are all threats to the life of whales. In order to ensure the flourishing of whale populations, it is important to support charities and organizations fighting for conservation.