Carousel Feeding: How Norwegian Orcas Hunt Herring

Why a herring shoal is a problem worth solving

A single herring is a small, fast, silver animal that is very difficult for a large predator to catch one at a time. It is also worth very little. An orca weighing several tonnes cannot make a living chasing individual fish around a fjord.

What makes herring worth eating is that there are unimaginable numbers of them in one place. A wintering shoal is measured in millions of individuals, packed into a volume of water that a group of whales can plausibly get around.

But a shoal is also a defence. Fish school precisely because a predator struggles to fix on any one target in a mass of identical moving bodies, and because thousands of eyes see danger sooner than two. Carousel feeding is the answer this population worked out: instead of breaking the shoal up, take its own defensive instinct and use it.

How the technique actually works

The pod finds a shoal and begins to work it from below and around the edges, swimming in tightening circuits. A frightened shoal responds by packing closer together and by moving away from the threat, which here means upward, because the whales are underneath.

The orcas keep the pressure on from every side but one. They turn on their sides to flash white bellies and white eye patches through the water, and the shoal shrinks away from the flashes. Some individuals release streams of bubbles, which work as a wall the fish will not cross.

The result is a herring ball: a dense, spinning sphere of fish held against the underside of the surface with nothing above it and whales below it. That is the carousel, and getting it built is most of the work.

Then one animal turns, accelerates, and strikes the ball with the flat of its tail. The blow is not aimed at any particular fish. The pressure wave stuns dozens of herring at once, and the whales collect the stunned individuals at leisure while the rest of the pod holds the ball in place. A hunt can run for a few minutes or the better part of an hour, with animals taking turns to strike and to herd.

Fins working in circuits rather than a heading

Not every orca does this

Carousel feeding is a local technique, not a species-wide behaviour. Orca populations that hunt seals stalk them silently in small groups. Populations that hunt sharks have their own methods. Populations that eat salmon chase individual fish and never build anything.

The technique exists where a fish-eating population has spent generations on dense schooling prey, which in practice means the herring grounds of northern Norway and Iceland. Move these whales to a different ocean and the behaviour would go with them and then, having nothing to use it on, would die out.

That is why it is described as culture rather than instinct. A calf learns it by spending years alongside adults doing it, in a family it will never leave. The knowledge is held by the group and transmitted down the generations, and it can be lost.

The uninvited guest

Building a herring ball takes a pod a considerable amount of effort. A humpback whale can undo that effort in one movement, and here they routinely do.

Humpbacks winter in the same fjords for the same fish. Having watched orcas concentrate a shoal into a tight sphere, a humpback simply swims up through it with its mouth open and removes a large part of the ball in a single lunge. The orcas have no realistic way of stopping an animal three times their size.

It looks like a fight and it is not. It is theft, it is frequent, and for anyone on a boat it means the best sightings are often two species at once: a working pod, a lunging humpback, and a cloud of gulls over the top of both.

Seeing it from the deck

Most of a carousel happens below the surface and you will not see the ball itself. What you see is the evidence of it, and knowing the signs turns an empty grey sea into a readable one.

Look for birds. Gulls and kittiwakes collect over a hunt because stunned herring drift up and float, and a tight swirl of birds working one patch of water is the most reliable indicator there is. A white-tailed eagle sitting over the same spot means the same thing.

Then look for the water itself: a churn or a boil where there is no wave to explain it, and fins moving in circuits rather than in a straight line. A pod travelling holds a heading. A pod feeding does not.

And listen. On a quiet boat with the engines down, the blows carry a long way over flat water, and you will usually hear a pod before anyone points at it.

The one thing not to do is stare at the middle distance waiting for a fin to appear in it. Whales surface where the fish are, the fish are where the birds are, and the birds are visible from much further away than anything else in this chain.

Before you book

How a herring ball is built

1. Find the shoal

   The pod locates a wintering herring shoal, usually deep, and moves in underneath it from below.

2. Push it upward

   Swimming tightening circuits, the whales apply pressure from every direction except above, and the shoal packs and rises.

3. Flash and bubble

   White bellies and eye patches are turned toward the fish, and streams of bubbles form walls the herring will not cross.

4. Hold the ball

   The shoal ends up as a dense sphere pinned against the surface, held there by whales circling below it.

5. Strike with the tail

   One animal turns and hits the ball hard with its tail. The pressure wave stuns dozens of fish at once.

6. Collect and repeat

   The stunned herring are picked off one at a time while the rest of the pod keeps the ball together, and another whale takes its turn.