Catfish feed mainly on non-native species, not on migratory fish

Between October 2023 and August 2025, 152 catfish were caught, most of which were smaller than 80 cm. By tagging catfish and later recapturing them, the researchers were able to estimate population size. This indicates that several hundred catfish are present in the study area.

“We see that the catfish population near Lith mainly consists of smaller individuals,” says ecologist Jacco van Rijssel (Wageningen Marine Research). “Large catfish (over 1.5 metres) were rarely caught during this study and were also infrequently detected by sonar. This suggests that potential predation pressure on larger migratory fish was limited during this period.”

The stomach contents of 90 catfish were examined. Food was found in only 27% of the stomachs. The diet consisted mainly of non-native crayfish and Ponto-Caspian gobies. Catfish appeared to preferentially feed on these non-native species, while species such as the Chinese mitten crab were consumed far less frequently. Among migratory fish, only a single eel of approximately 35 cm was found in a stomach.

“This reflects the food availability around the complex,” says Van Rijssel. “During the study period, few migratory fish passed through the area, including species that are important from a nature conservation and management perspective. As a result, catfish had little opportunity to prey on them.”

Of the 30 tagged catfish, data from 20 individuals were sufficient for analysis. Seven of these catfish were also detected in the downstream river area, up to 70 kilometres downstream of Lith. This showed that the species is less site-faithful than previously assumed.

Catfish presence at the weir and lock complex was highest in spring and summer and declined sharply in winter, when many individuals left the area. Water temperature proved to be the most important factor determining catfish presence at the complex.

Downstream of the complex, some catfish were found to respond to the switching on and off of the hydropower plant. During these switching moments, individuals moved noticeably between the weir and the hydropower plant. When the plant was switched on, the fish moved away; when it was switched off, they returned.

Catfish are difficult to study using standard capture methods. For this research, the team therefore collaborated with professional and recreational fishers, and fish-friendly techniques were used to study the diet and behaviour of the animals.

The impact of catfish on migratory fish was limited in this study. However, the researchers emphasise that the low availability of migratory fish, the relatively small size of most catfish and the limited number of observations of large individuals mean that predation on migratory fish by large catfish cannot be ruled out.

“This study provides a picture of catfish diet and behaviour at a single location,” says fish ecologist Maximiliaan Claus (Waardenburg Ecology). “Because catfish are highly adaptable to their environment, effects may differ at other locations or when higher numbers of migratory fish are present.”

The researchers point out that catfish numbers are likely to increase under warmer conditions. With larger numbers of both catfish and migratory fish, the likelihood of interactions between the two may therefore increase, as has already been observed in other European waters.

The report identifies several open knowledge questions, including:
•    greater insight into long-term catfish behaviour and space use;
•    improved understanding of distribution and population development across the Netherlands;
•    the genetic origin of catfish populations;
•    more knowledge about the diet and behaviour of larger catfish.

The study was carried out by Wageningen Marine Research in collaboration with Waardenburg Ecology and Sportvisunie, commissioned by the Ministry of Agriculture, Fisheries, Food Security and Nature (LVVN) and Rijkswaterstaat.