Topmouth gudgeon quickly spread through Europe and were introduced to the UK via the aquatic trade. They are extremely invasive, causing great harm to local ecosystems. They breed successfully in large numbers, out-compete native fish and predate on invertebrates and the eggs and larvae of other fish. They are highly territorial, with males guarding the eggs, and have been recorded in densities of 60+ fish per square metre.
Appearance: round, elongated body, and silver in colour with shimmering purple sides. They have a characteristic up-turned lower jaw and mature males have a prominent hump behind the head.
Typical size: 4-8cm
Lateral line scale count: 34-38 (this is the dark row of scales along the central length of the fish's body)
Lifespan: two to four years