The genus Cottus continues to undergo taxonomic revision in Europe, with a number of species described or revalidated since the turn of the 21^st century (Freyhof et al. 2005, Sideleva et al. 2015, 2022).

As a result, much of the extensive published literature which refers to Cottus gobio or "European Bullhead" no longer applies to the current concept of the taxon, but is instead relevant to other members of the genus.

This includes C. perifretum, which has continued to be widely cited as C. gobio since its description, despite its status as a valid taxon being strongly supported by subsequent studies (e.g., Fast et al. 2017).

Global and European regional assessment: Least Concern (LC)
EU 27 regional assessment: Least Concern (LC)

Although this species' population trend may be decreasing due to habitat degradation, the rate of decline is not understood to approach the minimum threshold for Vulnerable under Criterion A (≥ 30% over the longer of 10 years or three generations). It does not approach the range thresholds for Vulnerable under Criterion B (extent of occurrence (EOO) < 20,000 km², area of occupancy (AOO) < 2,000 km²) or D2. The population size is believed to exceed 10,000 individuals, hence it does not approach the thresholds for Criteria C or D. There exists no quantitative analysis which would permit application of Criterion E.

Therefore, the Common Bullhead does not currently approach the thresholds for any Red List criteria, and it is assessed as Least Concern both globally and for the EU 27 member states.

This species is native to Great Britain (England and Wales), and continental Europe from the Scheldt River system (Belgium, France, Netherlands) southward to the Charente River (France).

In Great Britain, introduced subpopulations are established in the rivers Clyde, Forth and Tweed (Scotland).

In continental Europe, it has become established in the rivers Meuse and Rhine (Belgium, France, Germany, Netherlands) after entering through a network of artificial canals constructed during the 19^th century. As a result, it has hybridised extensively with the congeneric Rhine Sculpin (Cottus rhenanus), and hybrid subpopulations now exist throughout the Meuse and lower Rhine. This hybrid lineage is reportedly tolerant of a broader suite of environmental conditions than the native species, and continues to spread upstream in the main Rhine channel and some of its tributaries.

This species' population size is unknown, but is understood to exceed the minimum threshold for Red List criteria (< 10,000 mature individuals). The current population trend has not been quantified, and the number of subpopulations is unknown.

A well-documented and widespread population size reduction occurred during the 20^th century. Site-scale declines continue to be reported, particularly at locations where certain non-native species have become established (see 'Threats').

However, it remains present in suitable habitat throughout its range, and its abundance is reportedly increasing in some countries, e.g., Belgium, France (see 'Conservation').

This small-bodied, cryptic, bottom-dwelling species inhabits stretches of streams and smaller rivers containing cool, clear, running water and coarse stony substrata such as gravel or small cobbles. It has also colonised some post-glacial lakes where it has been recorded at significantly greater depths.

Adult individuals are territorial and employ visual threat displays and agonistic sounds to defend their chosen sites. This leads to spatial segregation, with larger individuals occupying the most favourable sites. It is understood to be relatively sedentary, with a low dispersal ability.

This species is a visual predator and feeds on a wide variety of benthic macroinvertebrates. It is believed to be largely crepuscular, hunting at dusk, dawn and perhaps also at night.

Adult individuals mature at age 2-4+, and the annual reproductive period extends from March to April. The reproductive system is polygynous, and females apparently select males based on their size.

The adhesive eggs are deposited in a compact clutch on the ceiling of a small cavity, typically located beneath a rock or other hard structure. The male guards and tends the eggs until they hatch, and a single male may be simultaneously responsible for several clutches should he successfully spawn with multiple females.

The eggs hatch in 20-30 days, depending on temperature, and the newly-hatched larvae absorb their yolk sacs within c. 10 days, at which point they are able to disperse.

Studies suggest that growth rate and longevity are strongly influenced by local environmental conditions including altitude and water chemistry.

This species is threatened by pollution from agriculture and industry, which has led to widespread eutrophication of its favoured habitats and in some cases direct mortality through poisoning.

Changes in river and stream channel morphology, particularly canalisation and barrier installation, have also driven declines in the extent and quality of suitable habitat by interfering with flow and sedimentation regimes.

Research carried out in the United Kingdom has revealed that the presence of non-native crayfish species, particularly the Signal Crayfish (Pacifastacus leniusculus), can cause significant recruitment failure in Common Bullhead subpopulations. The crayfish outcompete the bullheads for shelter, leading to increased egg mortality and predation risk.

In continental Europe, it is also threatened by the ongoing expansion of invasive, non-native fish species in its native river systems, particularly the Bighead Goby (Ponticola kessleri), Western Tubenose Goby (Proterorhinus semilunaris), Monkey Goby (Neogobius fluviatilis), and Round Goby (Neogobius melanostomus). Experimental research has shown that these benthic predators are able to outcompete native sculpins for available food and habitat resources, consume smaller individuals, and may even displace them entirely over time.

This species is not used or traded.

The species is included (originally as Cottus gobio) in Annex II of the European Union Habitats Directive, and is of specific national conservation concern in Belgium and the Netherlands.

Numerous subpopulations occur within the boundaries of protected areas, some of which are included in the European Union's Natura 2000 network.

Conservation efforts have been particularly intensive in the region of Flanders, northern Belgium, where actions have included installation of artificial spawning substrates, habitat restoration through reforestation of riparian zones, enforced changes in land use, and both permanent and temporal measures to reduce erosion from surrounding agricultural land. In the Demer River system, where the species was once abundant but is now extremely scarce, a pilot scheme involving ex situ breeding and subsequent reintroduction has been implemented.

In the Netherlands, hybrid subpopulations (see 'Distribution') are considered to be native and have since the 1990s benefitted from general conservation activities focussed on improving the ecological quality of waterways. This may also have occurred elsewhere within its range subsequent to member states' adoption of the European Union's Water Framework Directive in 2000.