I think people are mixing two different things up here, if we let the seal population manage itself then of course some would starve but it would quickly even out, that's what nature does, this isn't cruel it's just, well, natural.

As for clubbing them, if we choose to regulate the population or even harvest them we should do so in a controlled fashion which should also take into account economic issues as these aren't divorced from environmental issues. I would reckon, and I'm no clubbing expert that a trained hunter/clubber/seal killer, call them what you will, can make clubbing if not humane by PETA standards at least quick and relatively painless, and I say this as an animal lover but also a meat eater.