I'm not in the UK but I know about the Nile Perch. It was introduced to provide a more steady and reliable food for the people to live on only it is ruining the Lake by decimating all the other life in it. One of the dangers of introducing non-native species to a local habitat.
The previous guy getting killed, I have no idea but I'm guessing either he was attacked by an animal or shot by poachers/wrong place wrong time.