A Grain of Salt With Your Tradeable Fishing Quotas
I'm not against ITQs as part of a broader fisheries management strategy, but I think we need to take a bit of caution in extolling their virtues and viewing them as the solution to overfishing. There are two key shortcomings of ITQs from an environmental perspective: they don't privatize everything, and they're only as good as the quotas they're based on.
Let's discuss the adequacy of the quotas first. Scientists are increasingly realizing that ocean ecosystems are far more complex than we once thought. This makes it even more difficult to what level of fishing would be sustainable, and hence how many ITQs to issue. In theory a precautionary approach could be taken, but that would be politically unfeasible, as fishers who stand to lose out would demand proof that a higher quota would be detrimental. And even with a quota established, there are enforcement issues -- e.g. sneaking fish to market, or "highgrading" (catching more fish than you can keep, then throwing back all but the best however-many).
ITQs create an approximate privatization of the target species. However, many of the environmental impacts of fishing go beyond catching too many of the target species. For example, bycatch -- the useless fish and other sea creatures swept up by a fishing net -- can have big impacts on the marine ecosystem (including indirect harm to the target species). But because (unlike fencing off land) only the target species, not the ecosystem that produces it, is privatized, the impacts of decisions about taking steps to minimize bycatch are spread across the community of ITQ-holders for that fishery, retaining the tragedy of the commons. After all, in Hardin's original illustration, the sheep were privatized but the ecosystem (the pasture) was not.
ITQs are a comparatively recent invention. Looking at how other cultures (who lack the kind of population modeling that enables us to set quotas) have dealt with fisheries can give us ideas for additional measures that can offset the problems with ITQs. Around the world, societies have employed two major strategies for limiting overfishing -- reserves and technology limits.
Reserves are simply places where or times when nobody can fish. This creates a sheltered portion of the ecosystem, from which fish spill over into the fishable portions. Reserves thus play on the very fluidity of population movements that make fisheries impossible to privatize on the pasture-fencing model.
Technology limits constrain the ability of fishers to overexploit or to catch in ecologically detrimental ways. For example, traditional societies have net size rules that are callibrated to let fish of certain ages through, preserving future generations. An important advantage of technology limits is that they are more easily enforced.