Stuart,
I thought we were already there as regards selective breeding of fish. Both carp and "rainbow" trout are far removed from their wild ancestors. What with sterile triploid trout, and now the mis-named F1 carp ( a crucian x carp cross) that is supposedly more resistant to disease, I'm not sure these are good moves.
Just like breeding of dogs and cats, you simply choose the ones with the characteristics that you think are "best", and breed from them. Do this through many generations, and in the case of dogs we get the many varieties of what are supposed to be the same species. With fish you take the fastest growing of each generation. Part of the problem with selective breeding like this is the danger of propagating genetic defects, partly through not knowing what defects are present, partly through the likelihood of in-breeding.
With rainbow trout, there is a wild ancestor, from the streams of California if my memory serves me right, that was then cross bred with Steelhead. Rainbow are naturally partially migratory like our own trout as are Steelhead (again from memory).
As for bigger fish, at the moment we can't even get a definitive reason for bigger fish in our rivers and lakes - longer growing season, warmer summers, hormonal pollution, less fish, lack of winter-kill, agricultural fertilizers, a mixture of the above?
