Each individual stock may be alright, but the F2 fraction may not be as fit. One would expect deterioration. The population of concern is the population to be supplemented. One would have to use a local stock in these programs. Yes, one would have to keep infusing local wild stock into the hatchery gene pool.
Fitness is a function of the environmental gradients.
This is defined by relative fitness of local stock compared to that of a foreign stock in that local environment. Also, the F1 would in some way not be allowed to breed. One wouldn't see a two- or threefold increase in fitness. Unless the returns were to facilities right on salt water, one wouldn't get all F1 accounted for. But it would be difficult to maintain these.