http://news.bbc.co.uk/1/hi/sci/tech/8574775.stm

Well that looks like the tuna is screwed. As they get scarcer the rpice will rise to encourage more fishing of them. Then one day they are gone.

The real problem with preserving fish stocks is that if it's in International Waters and not Coastal Waters, then you need an international agreement/treaty to govern the limits, otherwise anyone can take whatever they like. I remember watching in the commentary on "South Pacific Islands" from the BBC that New Zealand in particular was getting pissed off with foreign fishing vessels raping the South Pacific.