The essential problem that most biologist see with hatchery fish is inbreeding. Just like you wouldn't want to start with a brother and sister and create an entire population, you don't want to start with just two fish to create an entire population.

In the wild, there is an equal amount of chance that any fish will breed with another fish. This is what keeps genetic variation and a healthy population. The next generation from this mating has a relatively low survival rate in the wild, lets say that perhaps 5 fry from each mating make it into the ocean. On the other hand, a hatchery starts with a set small amount of males and females, which are specificly bred. Because the fry are raised in a controlled environment with virtually no predators or food competition, perhaps 2000 fry from each mating will make it into the ocean. This is how hatcheries work, a small amount of fish are taken in, bred, and a large amount of offspring are set loose.

So imagine if a wild fish has 5 offspring, the odds of any two of those five coming back and breeding with eachother(inbreeding), is very low. If you think about the hatchery fish, which may had have 2000 fry survive from one set of parents, it is much easier to imagine that two of those 2000 would be chosen by the hatchery personal and bred again to create another 2000 offspring. This goes on and on, with each generation reducing genetic variation and allele frequencies.

This all may be clearer if related to human populations. Of the 2000 kids from the first set of parents, 2 of them, brother and sister, mate to create another 2000 kids, 2 of those, brother and sister again breed to create another 2000 kids, and so on. Everyone has seen the problems with disease in the royal families of europe from inbreeding, do we want this same sort or thing to happen to the fish we eat?

David Bayes
University of Montana Biology Major