Beneath the Swell: How Kelp Forests Shelter Young Fish

Stand above a Cornish cove and you might see a dark patch moving beneath the swell. Get closer and the patch resolves into ribbons of brown seaweed, bending with each surge. For a fish small enough to sit in the palm of your hand, that patch is a huge and safe forest.

Kelp builds a habitat from the seabed up. Its holdfast, the tangle that anchors it to rock, has crevices for tiny animals. Higher up, young fish can feed among the fronds and slip behind them when a hunter passes and looking for a meal. Dense and shaded cover can break a predator’s line of sight and make a chase harder, though larger fish still come here to hunt. A young fish simply has more places to disappear and hide.

From Cornwall to Hampshire, I’m interested in what makes one patch of underwater habitat better for young fish than another. One thing we look at is how hard a baby fish has to swim to hold position. Dense kelp can slow the current, and a fish sheltering behind the fronds will have less swimming to do and therefore will use less energy and be less stressed. To do this we count fish tail flicks, giving us a clue about the effort involved. Over a whole tide, small differences in effort could matter to a fish with lots of growing to do. This is called a mechanistic effect – and I will explain about other ‘mechanisms’ that kelp deliver to enhance the life of baby animals in the ocean.

The next time you see a dark patch beneath the swell, watch it for a moment. Somewhere between those fronds, a young fish might be waiting for the tide to turn.