Is the Sharp Decline Harbour Seal Populations Around Scotland Caused by Killer Whales?

By Harriet Bolt, University of Aberdeen.

Despite conservation efforts, aerial surveys have shown widespread declines of harbour seals around the UK between 2001 and 2006, with the most severe declines in the declines around the Northern Isles. The widespread declines in harbour seal populations around Britain suggest a cause affecting a large part of the North Sea, while the exaggerated declines around the Northern Isles hint at an additional, localized cause. The frequent sightings of killer whales around Shetland and observations of them preying on harbour seals, has raised the possibility that killer whales predation may be contributing to the declines in this area

Researchers from Aberdeen University have used opportunistic sightings data to investigate the potential for killer whale predation on harbour seals to be a driver of declines around Shetland. There was a strong peak in sightings during June-July (1991-2006) coinciding with the harbour seal pupping season.

Killer Whales Most Common During Seal Pupping Season
Although based on opportunistic sightings of killer whales, results show a clear correlation with harbour seal declines around Shetland, especially during the pupping season. There was no trend of increasing killer whale presence or estimated predation that would explain why declines only started in the late 1990’s.

To fulfil the dietary needs of the number of killer whales sighted over the past decade, it is estimated that the whales would need to kill approximately 1,648 seals if they take a random mix of ages and sexes, however if the killer whales specialise in killing young pups, they would require some 3,829 seal pups, more than double the number.

Killer Whales a Significant Factor in Scottish Seal Decline
Annual estimates of harbour seals consumed by killer whales suggest that this could have contributed significantly to the observed harbour seal declines. This is particularly the case if killer whales are preferentially predating seal pups.

An additional factor not quantified are sub-lethal predator effects such as stress leading to reduced fecundity or reduced foraging time by prey species, due to predator presence leading to reduced resource consumption.

Further Analysis required
The analysis presented here highlights that killer whale predation is a potential factor in harbour seal declines around Shetland and deserves further consideration. It also raises questions for future research. Focused research on killer whales around Shetland is needed to provide abundance estimates and to quantify the diet of Shetland killer whales. This information can then be used to generate more sophisticated models and population viability projections of Shetland harbour seals incorporating killer whale predation as a factor.


Map of North Scotland showing killer whale sightings and harbour seal decreases.