Bullhead (Cottus gobio)

GenusCottus (1)
SizeMaximum length: 18 cm (2)

The bullhead is classified as Least Concern (LC) on the IUCN Red List (1). Listed on Annex II of the EC Habitats Directive (3).

The bullhead (Cottus gobio) is the only freshwater member of the family Cottidae that is native to the UK. It is a small fish, with a large mouth (4), large pectoral fins, prominent eyes (5) and a wide flattened head; hence the common names 'bullhead' and 'miller's thumb' (4). They are brownish in colour with mottling or barring, and pale undersides (6). During the spawning period males become black in colour with a white-tipped dorsal fin, and females become plump (4).

The bullhead is widely distributed in England and Wales but in Scotland is only known from the Forth and Clyde catchments (7). Elsewhere, the species is found in Europe, but it does not have such a favourable status there, and so it is listed in Annex II of the Habitats Directive (4).

Occurs in freshwater streams, rivers and lakes with hard stony substrates and shows a preference for fast flowing, shallow water bodies (7) (8). The large pectoral fins enable bullheads to hold their position in fast flowing water (5).

The bullhead is crepuscular; it spends the day under stones or in vegetation and emerges at dusk to feed on small bottom-dwelling invertebrates such as insect larvae and crustaceans, as well as the eggs and larvae of other fish (5) (8). They are occasionally cannibalistic, particularly of eggs in other nests (5). Bullheads are visual, ambush predators, and are good swimmers, moving quickly in short, sharp darts along the bed (5).

The spawning season occurs between February and June, and fertilisation is external (8). The eggs are laid underneath stones or in a pit, and then guarded and cared for by the male who fans them to ensure that they receive enough oxygen (9). The eggs hatch two to four weeks later, and the larvae feed on their yolk sac for a further two weeks before dispersing. Maturity is reached within two years (10).

Bullheads often behave aggressively towards one another, and competition for shelter and foraging space can be intense; research is currently being carried out by the University of Southampton into this area (5). Being small, bullheads are vulnerable to a wide range of predators, particularly brown trout, pike, grey heron, kingfisher and dippers (8).

The reasons for the poor status of the bullhead in Europe are unclear.

A number of areas have been selected as candidate Special Areas of Conservation (SACs) (7) in order to reflect a variety of ecological situations and the geographical range of the bullhead (7). This should help to secure a strong conservation status for this fish, but it is thought that further measures will be required to maintain the UK population (7). The life in UK Rivers Project is helping to conserve this species (8).

For more on the bullhead: 

Information authenticated by Andrew Davey, University of Southampton.

  1. IUCN Red List (April, 2011)
  2. Maitland, P.S. (2000) Guide to Freshwater Fish of Britain and Europe. Hamlyn, London.
  3. Carter, M.G., Copp, G.H. and Szomlai, V. (2004) Seasonal abundance and microhabitat use of bullhead Cottus gobio and accompanying fish species in the River Avon (Hampshire), and implications for conservation. Aquatic Conservation: Marine and Freshwater Ecosystems, 14: 395 - 412.
  4. Environment Agency. (1998) Species Awareness leaflet Number 3: Spined loach and Bullhead. Environment Agency, Peterborough.
  5. Davey, A. (2002) Pers. comm.
  6. Miller, P.J. and Coates, M.J. (1997) Fish of Britain and Europe. Harper Collins Ltd, London.
  7. JNCC (September, 2008)
  8. Tomlinson, M.L. and Perrow, M.R. (2003) Ecology of the Bullhead. Conserving Natura 2000 Rivers Ecology Series No. 4. English Nature, Peterborough. Available at:
  9. Cihar, J. (1991) A Field Guide in Colour to Freshwater Fish. Silverdale Books, Leicester.
  10. Newdick, J. (1979) The Complete Freshwater Fishes of the British Isles. A & C Black, London.