The butterfish: a rock pool handful

The butterfish, a small species rarely exceeding 25cm (10in) in length, has a narrow, slender eel-like body. Its real name is in fact the gunnel, from the Latin Pholis gunnellus — but because of its incredibly slimy, mucus-covered skin it has been dubbed the butterfish.

Its dorsal fin runs from the back of its head to the base of its tail and is composed of short, sharp spines. The pelvic fins are tiny but they too contain a single sharp spine and the fish’s anal fin, also spiny, is about half the length of the dorsal fin. Its head is rather small with fleshy, almost pouting, lips.

butterfish Pholis gunnellus

The butterfish’s colouring is characteristic – usually warm brown or green brown, with dusky bars along its sides and a dark stripe down each cheek. Its most distinctive feature is the row of white-ringed black spots running along the base of the dorsal fin that serve to frighten predators. There are usually about 12 of these spots or ‘eyes’, but the number varies from 9 to 15. On some butterfish smaller black spots, caused by parasitic worms, can appear on the skin.

Seaweed shelter

The butterfish is common on rocky shores and in shallow waters around the British Isles, where it spends the daylight hours in crevices under boulders or among clumps of seaweed. Many are often found sheltering under rock-pool debris, keeping their skin damp while the shore is exposed to the sun.

It is often found below the low tide level but is very difficult to see or catch – except occasionally in prawn traps. On sandy and muddy bottoms individual butterfish are sometimes caught in shrimp trawls.

The populations are widely distributed and usually appear at similar depths throughout the northern North Atlantic. It can be found along the rocky shores of the White Sea, all around the Norwegian coast, Iceland and southern Greenland, and is abundant on the north east coasts of Canada and the USA.

Buttery feeding

Spawning takes place in January and February when the eggs, laid in a rounded clump little larger than a walnut, are placed among stones or even inside the shells of molluscs.

They are guarded by the female until they hatch in early March and disperse in the plankton. Young butterfish grow very slowly – about l-3cm a year – and only become sexually mature after three years. During the winter the female butterfish doesn’t even venture away from the eggs to find food.

The butterfish is a general feeder, taking sandhopper-like amphipods, sea lice, young shrimps, crabs, and marine worms such as ragworm and lugworm. These creatures are particularly common on the shore, but also occur in deeper water and probably form much of the fish’s diet below the low-tide line.

As a small, shore fish no one could pretend that it has .any angling or commercial value. It is however common on so many of Britain’s coasts that there are plenty of places where it can be caught.

Like a number of the small blennies and gobies, which also live in rock pools, it can be fun to catch on a hot summer’s day.