big_pond, I think that what you are refering to are hybrids, particularily hybrid bluegills. While bluegills can hybridize with other species in the sunfish family, we normally think of the cross with green sunfish. If you do a search, you will find much information about this.
This first cross is normally called the F1 generation and it does indeed show hybrid vigor. Later crosses, the F2, F3 etc. generations show hybrid depression. If you're familiar with hybrid seed corn, you know that the companies that produce it must make this F1 cross every year.
Hybrid depression tends not to happen with pure species. Exceptions are with highly inbreed species or the extremely unlucky. Genetic problems in wild species usually cause the individuals with these problems die very quickly leaving only healthy individuals.
Again, there are many posts that get much more technical that this. Hopefully this helps.
