Image copyright PRU Image caption The whales are hunting fish beneath the ice
Killer whales are becoming experts at plundering up to 900 tonnes of huge fish each year from the melting ice of the Arctic, say scientists.
Among their fishing prey are species that can grow to the size of double-decker buses, such as sockeye salmon and northern pike.
Killer whales in the Arctic are enjoying a bonanza, say the researchers who carried out the study.
Researchers say the research has provided invaluable information on the whale feeding habits of the region.
“What we were trying to do is find out how in an ecosystem with changing conditions there might be a way in which ice caps could offer a feature that we were not previously aware of, that is a material advantage,” lead researcher Prof Ross Williams, from the Natural History Museum in London, told BBC News.
Image copyright PRU Image caption Killer whales are becoming expert hunters
Co-author Dr Eric Lyon, of the University of St Andrews, said that ice melting was making prey-rich waters on both sides of the Arctic to increasingly available.
The researchers spent the last three years tracking killer whales in five summer months in the central and northern Arctic.
The first season, in 2015, was followed by another year in which they analysed the habits of 51 killer whales in 67 dives into the rivers and streams of northern Canada’s Hudson Bay.
In 2016, the research team carried out a third assessment of nearly 30,000 dives this year, again using the same model.
The team recorded the abundance of fish that the whales dived for as well as the fish that they killed and sampled the whale’s oral fluency: its ability to vocalise the food in its mouth.
Image copyright PRU Image caption Killer whales cruise upriver on the thermally great weather days
What the researchers found was the killer whales were doubling their fish-hunting activity, their catch doubling every year, with a fish-rich summer in 2016, followed by a hunger year in 2017 and this year of fish abundance, from June to August.
The growth in fish-hunting may even have continued into 2018, said Prof Williams.
“As the ice goes away and the watershed expands and gets more accommodated for fishing in terms of water cover, the prey is changing.
“It’s the same everywhere, there will be different species you can catch depending on the area you’re in. There are good predators in terms of their knowledge about what species are available.
“In the Arctic, the killer whales really do quite a bit of tailgating, they trot in front of the salmon, if there’s sufficient going on, they’ll take a spear or bill or tusk and then the salmon will knock it away and then come back to the whale.”
Image copyright PRU Image caption Killer whales gliding over ice deposits in Yamal Ocean north of Arctic Ocean
The information the scientists have collected about the behaviour of these big and potentially deadly creatures is invaluable, he said.
“What we’ve got is not a finished product, it’s still accumulating information, but there are going to be one or two really important variables in the next few years that we will be able to use, to feed back and work out how these animals are managing the temperature.”
The research paper, analysing the unique analysis of fishing patterns in the Hudson Bay whale feeding range and developed using a cutting-edge combination of technology and observational data, is published in the journal PLOS One.
Image copyright PRU Image caption Killer whales hunt a variety of fish, including hake, pygmy pygmy and rainbow trout
About 500 orcas are known to live in the western Hudson Bay ecosystem, with the majority of these being killer whales.
The whales forage in six broad feed areas around Hudson Bay, but the team found that the one specific diet area was suitable for the feast of prey that killer whales are known to enjoy.
This concentrated band – the area larger than a football pitch at its core – was found to produce 900 tonnes of fish per year.