Best to talk to someone who works locally with Medio Ambiente.
Yes, there was a discussion in Madrid a while ago.
But they are not going to kill the fish here, even though they don't want them.
It has always been that Catfish should not be in the river here?
So they have wanted the anglers to kill them.
Like I posted before, they did try to catch / net several to get rid of some a few years ago. Impossible now.
Like you say there are monsters.