Paaiende beekforellen in de Geul: waarom gaat het mis?

Spawning brown trout in the River Geul: why does it go wrong? The River Geul is one of the few strongholds of brown trout (Salmo trutta fario) in the Netherlands. During the spawning season, spawning activity has been observed in the River Geul for many years, but frequent natural recruitment fails to occur. In order to increase knowledge of key factors, spawning beds have been located and subsequently monitored during the egg and larval development. Over a river length of 28 km, 66 spawning beds were found. Of these, five beds were further investigated by carefully collecting eggs with a rake and dipnet. Alive eggs were found in two spawning beds, dead eggs in two, and no eggs in one spawning bed. Subsequently, electrofishing was used to sample spawning beds and their surroundings for the presence of 0+ animals. This yielded only one individual. The evaluation of a reintroduction project of Atlantic salmon (S. salar) in the River Geul, where small fish of 1-2 cm are released in spring, shows that the salmon grow up to be healthy fish in good condition. Thus, the most likely scenario for the low recruitment of brown trout is low survival in the egg phase. During spawning, the eggs sink into the gravel bed and develop here during several months. The eggs in a gravel bed depend on a complex and permanent flow of oxygen-rich water flowing through. However, most likely due to erosion of fine sediment by agricultural activities, these gravel beds become clogged up which prevents the oxygen supply to the eggs. De beekforel is een kenmerkende vissoort van snelstromende beken met koel en zuurstofrijk water. Van oudsher is het Zuid-Limburgse stroomgebied van de Geul een van de bolwerken van beekforel in Nederland. Sinds enkele jaren wordt tijdens het najaar weer paaiactiviteit in de Geul waargenomen. Substantiële natuurlijke aanwas lijkt echter uit te blijven. In het kader van het herstel van de natuurlijke vislevensgemeenschap in de Geul, onderzocht Natuurbalans tussen 2019 en 2021 knelpunten en kansen hiervoor. Om beter grip op het verminderde paaisucces van beekforel te krijgen zijn paaibedden geïnventariseerd. Hierbij zijn diverse factoren onderzocht die betrekking hebben op de ei-ontwikkeling