Thirty of the ocean’s largest predators, dead. A mass stranding of Sperm Whales, Physeter macrocephalus, in 2016, has led to one of the most extensive investigations of a Sperm Whale mortality event ever, with twenty-seven of the thirty whales examined. The results are out.
The stranding occurred in the Southern North Sea, an area normally not frequented by these prodigious predators. The whales were all sub-adult males aged between 10 and 16 years old.
Causes of death are still unclear, however; man-made trauma, infection, chemical contamination and pollution, harmful algal blooms, sea surface temperature rise and marine earthquakes were all investigated, yet all ruled out.
Marine debris, e.g. plastics, were found in one third (nine out of twenty-seven) of the whales, yet did not seem to have caused any blockages or obstructions leading to their starvation.
The shallow waters of the North Sea are perilous grounds for such a deep diving species, which could explain the stranding. Sperm Whales dive to depths of 3000 m, use their bio-sonar to locate prey in the deep and consume up to one ton of squid per day.
However, Boreoatlantic armhook squid (Gonatus fabricii), the whale’s preferred prey species, does not live in the Southern North Sea and it was found that the whales had last eaten in the waters off Norway, 1300 km away.
With an average depth of 700 m, the North Sea is an “acoustic dead zone” to the Sperm Whales, according to co-author Rob Deaville, Project Manager of ZSL’s Cetacean Strandings Investigation Programme (CSIP).
Deaville goes on to say that the exact causes of death are still uncertain. However, when such a deep diving species can’t find their way back out of the shallow waters of the North Sea “stranding is inevitable”.
The food these whales rely on depends on well-managed fisheries. The UK government is now deciding the future of our ocean in the new Fisheries Bill through a consultation.