North of the Arctic Circle, off the coast of northern Norway, one of Earth’s most colossal natural spectacles occurs in winter.

Killer whales have always followed herring to its wintering grounds, but the appearance of large numbers of humpback whales and even fin whales is a new phenomenon

A mass gathering of ravenous marine giants – orcas (also known as killer whales), humpbacks and fin whales – arrive to gorge on overwintering herring in an ice-free expanse of the North Atlantic Ocean.

Over a billion spring spawning herring congregate here – an enormous fish feast for any predators that can get to them.

“Killer whales have always followed herring to its wintering grounds, but the appearance of large numbers of humpback whales and even fin whales is a new phenomenon, which started about five years ago,” says Dr Tiu Similä, who has studied the behavioural ecology of Norwegian killer whales for nearly 30 years.

These gatherings have given rise to a previously unrecorded behaviour, which has been filmed for the first time as part of a new BBC / National Geographic coproduction series.

In recent years, humpback whales (Megaptera novaeangliae) have seen been following orcas (Orcinus orca) as they hunt herring, and cashing in on the smaller marine mammals’ hard work.

These 60ft (18m) giants have migrated 5,000 miles (8,000km) across the Atlantic to feed for a few weeks before making their long journey back to the Caribbean.

“We have several observations of killer whales herding herring to a tight school towards the surface. And just as they start feeding the humpback whales come and ‘take over’ the herring schools and the killer whales move away,” says Dr Similä.

“This does not always happen of course.

“At times we can see humpback and killer whales feeding in the same area without any signs of competition. I believe that in these situations there is not a school but a vast shoal of herring in relatively shallow waters.”

What makes the humpbacks’ behaviour even more unusual is that in the Atlantic and Pacific oceans, orcas are known to predate humpbacks, preying on young calves.

[It] looks kind of like ‘whale soup’ with whales all over the place [and] seagulls taking their pick

North Atlantic orcas are known for their incredible “carousel” method of hunting. The predators work together to break away a group of herring into a smaller, more manageable “ball”. They encircle the herded herring, chasing the fish tighter together and upwards. At the water's surface, herring try to escape by leaping into the air, creating a “boiling” effect.

But the six-tonne orcas slap down their powerful tails, sending a shockwave that stuns their underwater prey. They then easily pick off the immobilised fish, leaving the water glistening with scales.

During the feeding season hundreds of orcas arrive from the open ocean to Norway’s coast and offshore waters, travelling in pods of about 30 animals.

Northern Norway is the only place in the world where humpbacks have been seen apparently taking advantage of orcas’ hunting technique.

Experts are still investigating the predatory behaviour of these newer arrivals.

This winter, for example, research teams will study the behaviour of overwintering herring and interactions between humpback whales and orcas, to try to understand more about what’s happening at this unique feeding site.

Away from the orcas, humpbacks have their own astonishing and unique way of catching fish or krill. They create “bubble nets” where a single whale or a group blow a ring of bubbles out of their blowholes, encircling their prey. The whales swim through their bubble net with their vast mouths open, scooping up their catch.

Humpbacks are not the only whales to barge in on the herring fish feast, or even the biggest.

Endangered fin whales (Balaenoptera physalus), also seen at the herring's overwintering site in recent years, reach up to 85ft (26m) in length and can swallow 3,000 herring in one go. These incredible creatures sometimes feed in groups alongside other whales including humpbacks. But orcas are known to predate this species too.

Norwegian spring spawning herring stay in an area of ocean off northern Norway from November to February before swimming to spawning grounds further south.

The sea doesn't freeze over in winter here thanks to warm waters carried from the Gulf of Mexico by the Gulf Stream, one of the world’s strongest ocean currents.

The herring change their overwintering ground every 20-30 years, explains Dr Similä. Since 2006 they have been overwintering in offshore waters from the island of Andøya to the city of Tromsø.

From 1986 to 2005 the main wintering ground was in the Vestfjord fjord sytem, says Similä.

“In 2006 the majority started to winter a bit north of this fjord system, and since 2010 some of the herring are also in coastal waters.”

“In the situations where killer whales and humpbacks feed in the vicinity of each other, there is a lot of herring. It does not look like a ‘frenzy’, other than there is a lot of whales as well,” says Dr Similä.

“[It] looks kind of like ‘whale soup’ with whales all over the place [and] seagulls taking their pick.”

Atlantic: The Wildest Ocean on Earth begins in the UK on BBC Two, Thursday July 30 at 21:00 BST for viewers in England and Scotland.

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