This is a problem well understood in CONSERVATIVE economic theory. It is called "the tragedy of the commons." When something, like the high seas, belong to all takers, there is no counterbalance to the taking except devastation of an apparently not unlimited resource. The long term economic solution, if it is one, is apparently fish farming where private property rights can secure a return to private capital investment. The only question is whether the oceans must remain depleted of fishlife in order for this to occur. Unfortunately for libertarians this is the kind of issue that can only be addressed by public policy - unless we want to turn the oceans into private property - just as the commons were enclosed in England.