I'm only posting this because I took the time to write it. A.P.'s post above links to what is undoubtedly a better scientific summation. With that...

Fishyfeller/Swifty27:

Remember that all hatchery fish initially came from wild parents. When those parents are not native to the river for which they are bred, a different range of issues exists, but ignore that for now. While the first hatchery broods probably had high levels of fitness and productivity, rearing in a hatchery environment does little to select for the most fit specimens (no predation, consistent water conditions, and plenty of food to go around), so a lot of less fit specimens survive to smolt and be released than would survive being reared in the wild environment. Furthermore, there is no competition for spawning among returning adult hatchery fish, so the gene pool ends up with increasing representation by the less fit recruits with each generation. As natural selection has less and less influence on a given stock, that stock becomes less and less fit to survive in the wild environment.

If it's true that hatcheries are better at producing numbers of returning fish (in many cases it's not), it's because so many more hatchery smolt are released than wild stock survive to smolt. In terms of returning adults per smolt migrating out, the wild smolt outperform their hatchery counterparts significantly.

It's a tough nut to crack. When all the costs of producing hatchery fish are considered, hatcheries are probably among the least cost effective expenditures our governments undertake. The trouble is that salmon are an important commercial food source, and while I'm confident wild stocks could recover to a level that would support solid recreational opportunities on their own, the same can't be said for the commercial market. That's the real reason we need hatcheries. Because commercial harvest takes such a big chunk of the wild returns and the hatchery returns, our sport fishing opportunities are also dependent on hatchery "supplementation," if they're to be meaningful anyway.