Killer whales blamed for decline of Scottish seals

A A A

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.

News
Young Winstone: His ‘tough-guy’ image is a misconception
people
Sport
Adnan Januzaj and Gareth Bale
footballManchester United set to loan out Januzaj to make room for Bale - if a move for the Welshman firms up
Arts and Entertainment
Ellie Levenson’s The Election book demystifies politics for children
bookNew children's book primes the next generation for politics
News
Outspoken: Alexander Fury, John Rentoul, Ellen E Jones and Katy Guest
newsFrom the Scottish referendum to the Ice Bucket Challenge, our writers voice their opinions
PROMOTED VIDEO
Have you tried new the Independent Digital Edition apps?
News
i100
Sport
Yaya Sanogo, Mats Hummels, Troy Deeney and Adnan Januzaj
footballMost Premier League sides are after a striker, but here's a full run down of the ins and outs that could happen over the next month
News
Nigel Farage celebrates with a pint after early local election results in the Hoy and Helmet pub in South Benfleet in Essex
peopleHe has shaped British politics 'for good or ill'
Arts and Entertainment
Pharrell Williams' “Happy” was the most searched-for song lyric of 2014
musicThe power of song never greater, according to our internet searches
Sport
Tim Sherwood raises his hand after the 1-0 victory over Stoke
footballFormer Tottenham boss leads list of candidates to replace Neil Warnock
Voices
Strictly Come Dancing was watched by 6.9m viewers
voicesIt has been hard to form generally accepted cultural standards since the middle of the 19th century – and the disintegration is only going to accelerate, says DJ Taylor
Arts and Entertainment
Roffey says: 'All of us carry shame and taboo around about our sexuality. But I was determined not to let shame stop me writing my memoir.'
books
News
Danielle George is both science professor and presenter
people
News
i100
News
Caplan says of Jacobs: 'She is a very collaborative director, and gives actors a lot of freedom. She makes things happen.'
people
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
iJobs Job Widget
iJobs General

Selby Jennings: VP/SVP Credit Quant- NY- Investment Bank

Not specified: Selby Jennings: VP/SVP Credit Quant Top tier investment bank i...

Ashdown Group: Senior Marketing Executive- City of London, Old Street

£40000 - £43000 per annum + benefits: Ashdown Group: Senior Marketing Executiv...

Ashdown Group: Marketing Manager

£40000 - £43000 per annum + benefits: Ashdown Group: An international organisa...

Ashdown Group: Internal Recruiter -Rugby, Warwickshire

£25000 - £30000 per annum: Ashdown Group: Internal Recruiter -Rugby, Warwicksh...

Day In a Page

War with Isis: The West needs more than a White Knight

The West needs more than a White Knight

Despite billions spent on weapons, the US has not been able to counter Isis's gruesome tactics, says Patrick Cockburn
Return to Helmand: Private Davey Graham recalls the day he was shot by the Taliban

'The day I was shot by the Taliban'

Private Davey Graham was shot five times during an ambush in 2007 - it was the first, controversial photograph to show the dangers our soldiers faced in Helmand province
Revealed: the best and worst airlines for delays

Revealed: the best and worst airlines for delays

Many flyers are failing to claim compensation to which they are entitled, a new survey has found
The stories that defined 2014: From the Scottish independence referendum to the Ice Bucket Challenge, our writers voice their opinions

The stories that defined 2014

From the Scottish independence referendum to the Ice Bucket Challenge, our writers voice their opinions
Stoke-on-Trent becomes first British city to be classified as 'disaster resilient' by the United Nations

Disaster looming? Now you know where to head...

Which British city has become the first to be awarded special 'resilience' status by the UN?
Finally, a diet that works: Californian pastor's wildly popular Daniel Plan has seen his congregation greatly reduced

Finally, a diet that works

Californian pastor's wildly popular Daniel Plan has seen his congregation greatly reduced
Say it with... lyrics: The power of song was never greater, according to our internet searches

Say it with... lyrics

The power of song was never greater, according to our internet searches
Professor Danielle George: On a mission to bring back the art of 'thinkering'

The joys of 'thinkering'

Professor Danielle George on why we have to nurture tomorrow's scientists today
Monique Roffey: The author on father figures, the nation's narcissism and New Year reflections

Monique Roffey interview

The author on father figures, the nation's narcissism and New Year reflections
Introducing my anti-heroes of 2014

Introducing my anti-heroes of 2014

Their outrageousness and originality makes the world a bit more interesting, says Ellen E Jones
DJ Taylor: Good taste? It's all a matter of timing...

Good taste? It's all a matter of timing...

It has been hard to form generally accepted cultural standards since the middle of the 19th century – and the disintegration is only going to accelerate, says DJ Taylor
Olivia Jacobs & Ben Caplan: 'Ben thought the play was called 'Christian Love'. It was 'Christie in Love' - about a necrophiliac serial killer'

How we met

Olivia Jacobs and Ben Caplan
Bill Granger recipes: Our chef's breakfasts will revitalise you in time for the New Year

Bill Granger's healthy breakfasts

Our chef's healthy recipes are perfect if you've overindulged during the festive season
Transfer guide: From Arsenal to West Ham - what does your club need in the January transfer window?

Who does your club need in the transfer window?

Most Premier League sides are after a striker, but here's a full run down of the ins and outs that could happen over the next month
The Last Word: From aliens at FA to yak’s milk in the Tour, here’s to 2015

Michael Calvin's Last Word

From aliens at FA to yak’s milk in the Tour, here’s to 2015