An introduction to Bullhead Ecology

 

The exceptional camouflage of a Bullhead on the river bed 

The Bullhead Cottus gobio, is the only freshwater cottid found in Britain, a family more frequently found along the coast whilst rock pooling. Perfectly adapted for a predatory life benthically in flowing waters, with its huge head and flattened body. The large mouth has teeth on both jaws as well as at the front of the vormer bone. Bullheads lack a swim bladder to reduce buoyancy and have a double fluid filled cornea to protect the eyes from suspended particles in fast flowing water. These features are combined with its exceptional camouflage on the stony river bed.

Bullheads unusually for British fish in that they show parental care within a large nest area. Bullheads spawn from February to June ranging from once up in cold headwaters, to as many as 4 times in more lowland catchments. Males are territorial and protect the nest site, but females may mate with several partners. Males can be distinguished at spawning time by their dark colouration, cream coloured dorsal fin edge and protruding genital papillae. Males excavate a nest scrape beneath a rock to attract the female, and can give out an audible 'knocking' sound to advertise its territory. The female lays up to 400 eggs on the underside of the rock.

The male will then protect these eggs from predators, particularly invertebrates like caddisfly larvae, as well as fanning them to aid oxygen supply. The eggs hatch after 20-30 days depending on water temperature and stay in the vicinity for around 10 more days until the egg sack is absorbed at this point they start to disperse, often downstream in a planktonic phase.  

Bullheads are predominately a benthic predator or anything which will fit in their mouths this usually include gammarid shrimps and insect larvae, and I have also found them feeding on small fish on several occasions. Typically a crepuscular hunting but they will also foraging into the night, with daytime activity generally low to avoid predatory birds. Other key predators include large stocked brown trout which can be detrimental if maintained in large numbers, also the non-native invasive Signal Crayfish feeds on eggs, juvenile fish and in high densities removes prey.

Populations are often very high in hard water lowland streams, like chalk streams. Bullheads mature rapidly (breed in 1st year) and are short lives (rarely exceeding 4 years old), so most of these habitats contain around 4 age classes of fish within the population. In order to maintain population size recruitment needs to be annual and successful. By comparison colder upland soft water streams support lower population densities. Growth rates are also slower with sexual maturity reached later (as old as 3 years), with fish living longer and upland streams often showing 5 age classes.

Bullhead are found sympatrically with Brown Trout as share similar requirements for high levels of dissolved oxygen in the water, which may be soft, hard or neutral. Bullheads may tolerate low-moderate levels of pollution but high levels of nitrates and dissolved carbon leading to eutrophication ultimately makes habitats unsuitable, combined with warmer temperatures, slower flows, higher amounts of basal sediments (unsuitable for breeding) and lower oxygen levels, making lower stretches unsuitable. Bullheads may occur around the edges of larger upland lakes near inflows in non eutrophic waterbodies.

An interesting species but one which faces many threats in British rivers through pollution, agricultural change, fishery management and invasive non-native predators like Signal Crayfish.