Why is it not a good idea to keep dolphins in captivity?

Dolphins are highly intelligent. In the wild they live in complex social groups. In captivity, dolphins live shorter lives than they do in the wild. This is significant given the fact that they are kept in an environment that is free of predators, pollution and other threats that they face in the wild. Wild dolphins can swim up to 100 miles a day but in captivity they have very little space in which to move around and so display unnatural behaviour. The captive environment can never replace their natural one.