I guess it depends on the job you're being interviewed for. If you're looking for a job in the ministry of environment, for example, I think you should say "much less than there were in the past! Overfishing is a serious problem and it must be addressed before it's too late". If you're applying for a job in, I don't know, accountancy, you can say "there are enough fish in the sea to assume their number is infinitesimal".
It's a tough question, yeah. Some random thoughts, hope it helps a bit.