The Miller’s Thumb

Just upstream and to the north of the Silver Street bridge, and downstream from the Beach, lies a short stretch of the Cale which is not easily accessible to humans. It is, however, the home of a sizeable number of one particular fish, endangered across Europe, which is doing very nicely thank you – the bullhead, also known by it’s nickname of “miller’s thumb.”

Known strictly as the European bullhead, to distinguish it from similar species in other parts of the world, this is the only member of its family found in English rivers. It is a small fish, not usually growing above seven inches in length, with a wide flattened head from which it gained its name, a wide mouth and very prominent eyes. It is brown in colour, usually with mottled markings or bars across its body, and the underside is pale. They have large pectoral fins, which enables them to hold their place against a fast-flowing current.

The bullhead spends the day nestled beneath a stone or hiding amongst vegetation, and emerges to feed towards dusk. It eats various small crustaceans, insect larvae, and eggs and larvae of other fish species, according to whatever might be found along the river bed. It will, occasionally, eat the eggs of its own kind from other spawning sites, as they are very competitive with each other for foraging space; but this is not believed to be commonplace behaviour.

Of course, the great food chain does not often end with fish, and they are themselves one of the favourite food supplies of the kingfisher, which can frequently be flying up and down from its nest nearer to Hawker’s Bridge, in order to feed on them. They are also predated by dippers along the Cale, as well as larger fish such as the brown trout .

The bullhead’s breeding, or spawning, season, occurs between February and June, happening earlier in warmer climates such as here in Somerset. During this time the males change their brown colours for black, with a white-tipped dorsal fin; and the females gain noticeably in plumpness. The eggs are laid on the riverbed, underneath a stone or in a shallow. The eggs are pinkish in colour, and take between two and four weeks to hatch. During this time they are guarded by the male, who will fan the eggs with his tailfins to ensure a steady supply of oxygenated water. One male may guard the eggs of several females in the same spawning site.

Once hatched, the young continue to feed on their yolk sacs for a further two weeks before dispersing along the river. After two winters, those young will be mature enough to breed themselves; and they are reckoned to live in the wild for up to ten years or so. In rivers with a high productivity of food supplies, they may produce several spawns in one season. Their plentiful presence in the Cale is a positive sign for the health of the river.

sources for this article:
http://www.arkive.org/bullhead/cottus-gobio/
http://eol.org/pages/1012522/details