Attacks by killer whales may be helping to drive the sudden and mysterious decline of seals around the northern coasts of Scotland, new research suggests.
British populations of harbour seals (also known as common seals) are falling steeply, with numbers in Orkney and Shetland dropping by 40 per cent in the five years to 2006.
So far, the declines are unexplained, but a new theory is that killer whales, or orcas, the bulky, black-and-white predators which are in fact the largest members of the dolphin family, have increased their taking of seals to such an extent that it may be causing populations to shrink.
The harbour seal is one of Britain's two native seal species, the other being the bigger grey seal. But while grey seal populations remain buoyant, harbour seal numbers are tumbling, especially in the Northern Isles, where nearly half of them live. Surveys in Orkney and Shetland in 2001 found 12,635 animals, but when the counts were repeated in 2006, they showed that numbers had plunged to 7,277.
Several possible causes have been put forward for the precipitate decline, including viral epidemics, shootings, drownings in illegally set nets and the disappearance of sandeels, the small fish that form an important component of the seals' diet. But research led by Andrew Foote from the University of Aberdeen presented at a conference of the European Cetacean Society has turned the spotlight on to killer whales, suggesting that they may be preying heavily on harbour seal pups at the pupping season in June and July (grey seals reproduce in the autumn).
Orcas are among the fiercest animals on Earth, but in contrast with sharks and terrestrial predators such as tigers and lions, there is no record of them ever attacking people. One of the most gripping pieces of natural history footage ever shot was the scene in David Attenborough's The Trials of Life where killer whales hurled themselves out of the sea and up on to a beach in Patagonia to catch unsuspecting sea lion pups.
The new study is based on analysing inshore sightings of killer whales around Scotland, and in Shetland especially. It showed a clear correspondence between the seal pupping season and peak orca sightings.
The researchers constructed a bio-energetic model of the killer whales to get an idea of how many seals might be consumed by the orcas sighted to meet their daily energy requirements. Over the decade from 1997 to 2006, this worked out at 1,648 harbour seals, if the killer whales were preying on all age and sex classes evenly, and 3,829 if they were specialising in naive pups. These figures, likely to be an underestimate, would be adequate to drive a population decline.
Recent declines of Steller sea lions in the Central Aleutian Islands in the North Pacific could have been caused by fewer than 40 killer whales, according to one estimate, and five killer whales alone could have caused a reduction in Alaskan sea otter numbers in the same area. In Hood Canal, a fjord off Puget Sound in Washington State, two incursions lasting 59 and 172 days by two killer whale pods, 17 whales in total, were estimated to have removed 1,800 seals. In the Southern Ocean, they may also be driving declines in elephant seals and minke whales.
But why would the predator-prey relationship get out of balance? Five years ago, American biologists put forward a theory that orcas were driving declines in several Pacific marine mammals, including harbour seals, fur seals, sea lions and sea otters, because of a phenomenon known as "feeding down the food web" – a 50-year chain reaction set off by commercial whaling. They suggested that killer whales which once preyed extensively on sperm, fin and sei whales, had switched to smaller prey when great whale populations were decimated by humans. The mass slaughter in the Pacific of more than 500,000 great whales between 1949 and 1969, they said, forced the orcas to switch to the smaller mammals, causing their numbers to plunge in turn.
However, this theory is not universally accepted and Mr Foote does not think it explains orca predation on seals in the Northern Isles. "It's more likely that the killer whales have moved into the area following the movements of fish stocks, such as mackerel, and have just found there are lots of seals there, and are staying around feeding on them," he said.
Killer whale facts
Killer whales or orcas are among the largest carnivores on Earth, with the males growing to more than 32ft long and attaining a weight of more than 11 tonnes (the females are smaller). The male's enormous dorsal fin can be six feet high. They are found all over the world, from pole to pole, especially in colder waters, and are at the top of the marine food chain. They hunt in packs or "pods", and will attack anything in the ocean, up to and including the blue whale, the world's largest animal – with the remarkable exception of people. They have never been targeted by commercial whaling ships, although many of the animals were captured when the fad for dolphinaria came along and kept for display in pools, where they can be trained to perform, like other members of the dolphin family to which they belong.