Trying to wipe out a species ( wels catfish ) will not work but will balls up the fishing and ruin the economy of a lot of small business built round the fishing for years. The Americans have spent billions of dollars trying to wipe out the migration of Bighead ,Asian and Silver carp from getting to the Great Lakes from the Mississipi river system and are failing badly. Physical and electric barriers,netting , poisons, you name it. All have failed. The water companies in the UK tried to eradicate Zander years ago and also failed .The return of the native Barbel and other species would only be temporary as the Catfish and others at the top of the food chain are sure to return in the future.
To add insult to injury I am told the fish that are slaughtered are being frozen and sent to Romania as food . Not entirely wasted but a few euros in someones back  pocket.
 Why dont they just try and control the numbers ie put the big ones back ( the large ones dont taste nice at all so I'm told) and market the smaller ones.