There is nothing in the fish's genetics that would make subsequent generations of fish less fit (not able to grow as well) for some years or limit their growth rates. Its in how the pond is managed mostly enough food and adequate harvest.

For example if there is no pellet feeding for the first few years while the BG and LMB grow then the LMB start to get numerous and you start pellet feeding the 2 and 3 year old BG will start to get bigger than the initial stockers because there will be fewer of them and more food. Its in the pond's mgt.

CJ accurately describes what happens with the initial stockers absent mgt. Man is the ultimate mgt tool.