- Scientific name: Pholis gunnellus
- Also know as: Rock Gunnell
- Size: Up to 25cm, but typically 8-15cm
- UK minimum size: N/a
- UK shore caught record: 1oz 2dr
- IUCN status: NE (Not Evaluate)
- Distribution: Found in coastal, inshore waters all around the UK and Ireland.
- Feeds on: Molluscs, marine worms and small crustaceans.
- Description: Very small eel-like fish which has a long, slender body. Mouth is small and upturned with noticeable lips. Colour is usually mottled brown to tan, although can be reddish. Single long dorsal fin which runs along the whole length of the body and small, almost circular, tail fin. Distinctive features include a series of black spots circled in white running along the top of the body/bottom of the dorsal fin and a black line running down from the eye to the edge of the mouth.
Butterfish are a small, slender and slimy fish, which has no scales present on its body. Butterfish can often be found in the inter-tidal zone, especially during the summer, but in winter they will move into water as deep as one hundred metres, moving back into inshore waters as the seas warm in the spring. Butterfish generally feed on small creatures such as sea slaters, small hermit crabs, molluscs and marine worms. They spend the time when they are not actively feeding hidden away in cracks and crevices to avoid predators. Butterfish which have been searching for food in shallow water will often find themselves stranded by the outgoing tide. However, this is not a problem for the butterfish as, unusually for a fish, they can breathe air through their gills and will simply find an area under a rock or within heavy seaweed and wait for the tide to come back in. In some areas anglers searching around the inter-tidal zone for peeler crabs will also sometimes find butterfish.
Breeding takes place in the winter with hundreds of eggs being laid in an empty shell or within a sheltered rock area. Unusually, both the male and female will guard the nest until the eggs have hatched. Due to their small size butterfish are rarely caught by sea anglers, although the increasing popularity of LRF (Light Rock Fishing) means that this species may now be caught more often by anglers specialising in catching very small fish. Anyone handling this species will soon see how they get their name as they are extremely slimy and slide out of the hands as soon as they start to wriggle. Butterfish are not eaten by humans and are not widely used as bait, although some anglers have had success livebaiting with butterfish for predatory species such as bass. The butterfish is occasionally confused with a blenny. The easiest way of telling them apart is by the black spots with white outline (which only the butterfish has) or look for feelers located on the top of the head – only the blenny has these. The butterfish also has a distinctive black line that runs downwards from the eye.