All this coming from a fella who keeps tilapia...
And you say you have done courses regarding introduction of pest fish in our waterways. Well then you should know about the devestation that tilapia have caused, oh, and surely you must know they are ILLEGAL!
It's done allready, you can't take it back, so why blast him with all this garbage that is of little use now.
Now....on a more higher note
I went fishing again today and caught two gudgeons, no idea what type, a freshwater clam, and catch of the day, a tiny eel, about 9cm long and 7mm thick. No idea what species, but obviously he's a native.