>The "full cost" wouldn't need to be replacing each fish caught. It would be a fee/tax to ensure a sustainable fish population - this could still result a reasonable market for fish. //

In the past we fished at sustainable levels because we were incapable of fishing to extinction. WWF (http://wwf.panda.org/about_our_earth/blue_planet/problems/pr...) say we're taking 2-3 times the volume of fish from the oceans as is sustainable.