Stone Moroko

Identification

The topmouth gudgeon is a small, slender fish of 5 to 10 cm with a distinctive dark lateral stripe. The back is grey to olive green, the flanks silvery. The mouth is small and upward-pointing. Males darken during spawning and develop small tubercles on the head.

Behavior & ecology

The topmouth gudgeon is an invasive exotic from East Asia, probably accidentally introduced with grass carp shipments in the 1980s. Extremely successful: rapid reproduction, broad water quality tolerance, few natural predators. The greatest danger is as a carrier of the rosette agent, a parasite lethal to native cyprinids. The topmouth gudgeon itself is largely immune.

In the Netherlands

Since first being recorded around 1990, it has spread rapidly. With 625 records, the species now occurs in ponds, urban waters, and polder ditches throughout the country. Listed as an EU invasive alien species. It threatens native small fish like the sunbleak and bitterling.