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BiologyThe 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).