There maybe endangered or vulnerable species in that pond that the catfish would eat.
The catfish might be carrying a virus or a bacterial infection that it, itself, is immune to but that the local wildlife has no immunity to which could kill them off.
Run off from that pond in heavy rain, or birds or other predators could then carry that infection to other bodies of water and infect them too.
As just one example, where I live, in the UK, we have had exactly that problem with people releasing American crayfish into our rivers; they carry a bacterial infection that has almost completely wiped out our native crayfish; they also outcompete them for food and have no natural predators here to keep their numbers down
Non native animals must never, ever be released; the risk to local wildlife is just too great.