Research by the Institute for Nature and Forest Research (INBO) has found that exotic fish species are gaining ground in the River Maas. The research also showed that the round goby and the Kessler goby also have increased in number during the past year. Some native species, such as European catfish and eels were also found in the river. However, as in recent years, they were in the minority.
Since 2013, an INBO team has conducted an annual survey of the fish species that are present in the waters of the River Maas in Limburg Province. From the banks of the river between Kinrooi and Meeswijk the team counts, measures and weighs the various fish it catches. The fish census is carried out at 8 250-metre stretches of riverbank on the Belgian side of the Maas that forms the border between Belgian and Dutch Limburg.
This year for the first it was also carried out on the Dutch side of the river.
New method
INBO has been working using electrofishing technics for several years now. "We create a mild electric shock in the water, which stuns the fish and they almost automatically end up in our net," INBO’s Hugo Verreycken told VRT News. "95% percent of the fish do not suffer any permanent ill-effect from this. The fish currently have less oxygen in the water due to the heat and become stressed more quickly, so occasionally they do not survive."
Using the electrofishing method the researchers are able catch fish of up to 60cm in length. This year for the first they are taking a different approach. "We place traps at night and the larger fish get entangled in them. This is a net with a big opening, but as the fish swim further into the net, the opening gets smaller and smaller and they can't get out."
This has ensured that unlike in recent years the researchers have caught sever “whoppers”.
"We placed traps at 4 points deeper in the water and we caught a 1.6-metre catfish and one that was 1.5 meters. We also caught several eels there."
However, the number of catfish and eels caught was negligible compared it with the number of exotic fish that were caught.