The answer is physically fit older people. this is because ageing causes changes to the brain size, vascalature and cognition. Brains shrinks with increasing age and there are changes at all levels of molecules morphology.
Invasive species are successful in novel environment due to lack of natural predator or competition. Therefore if the non-native share a niche with other fish in the lake ecosystem, there will definitely be competition for resources. This could lead to competitive exclusion of one species or resource partitioning between the species consequently, the non-native fish species will not invade the new environment unchecked. I hope this helps
The purpose is to produce gametes or sex cells that will help later during sexual reproduction