The bitterling (Rhodeus amarus) is a small, high-backed freshwater fish with flat sides. It reaches a maximum size of 7-8 cm and is a member of the carp family (Cyprinidae). Its back is grey-green in colour, the sides are silvery and shiny and there is a blue-green stripe along its middle. Its belly is whitish to slightly reddish. The males intensify their colours only at spawning time. The fish owes its name to its bitter taste.

The bitterling is found in central Europe, to the north of the Alps. Its range stretches from eastern Europe to France. But it is not found in Scandinavia, Denmark and large parts of the United Kingdom. The bitterling prefers vegetation-rich bank areas with well-aerated, muddy substrate. It is found in shallow, slow-flowing or standing waters and feeds on animal and plant plankton. Due to its reproductive behaviour, it is tied to the presence of mussels that dwell in the same habitat. At spawning time in April to June, the female forms an ovipositor that is approx. 5 cm long, with the help of which it places its eggs between the gills of pond or river mussels. One to two of the total 40 - 100 eggs that are laid are placed in each mussel. The males’ sperm reach the interior through the water that the mussels breathe, and thus fertilise the eggs. Until they can swim, the larvae stay in the mussels, which gives them a certain degree of protection against predators. The mussels also benefit from this sort of brood care. Their larvae adhere to the fish and are spread in this way.

Because of this specialised reproduction biology, the bitterling is now highly endangered in Germany. In many places, water pollution and water maintenance measures are affecting the mussels. Because of this, their populations have declined there, or have actually disappeared. In NRW, however, the conservation status of the bitterling is currently good. Including in the Bienen Old Rhine, where it is still very common. The FFH area ““Bienener Altrhein, Millinger and Hurler Meer and Empeler Meer” was therefore also identified to protect the bitterling.