ACIDIFICATION, warming, the destruction of coral reefs: the biggest problems facing the sea are as vast, deep and seemingly intractable as the oceans themselves. So long as the world fails to cut its emissions of greenhouse gases, cause of the global warming behind these troubles, they will grow. By comparison, overfishing, another great curse, should be easier to put right, especially in the coastal waters where most fishing occurs. And yet it goes on, year after year.

Fishermen have every reason to do something. Many fisheries are hurtling towards collapse; stocks of large fish have been reduced by up to 90%. When stocks are overfished, they yield a smaller catch. The cost of mismanagement, in lost economic output, is huge: some $50 billion a year, according to the World Bank.

One reason why the pillage continues is that knowledge of fish stocks is poor, especially in developing countries. A new statistical attempt at estimating the remaining shoals (see article), from the University of California, Santa Barbara, is therefore welcome—even if that is not true of its findings, that stocks are even more ravaged than previously thought. The study found that better-understood fisheries are likelier to be healthy. Another reason for overfishing is new technology (developed, aptly enough, for battlefields), which makes shoals easier to detect. As large boats and refrigeration have spread, fishing fleets have covered greater distances and hoovered up larger catches. Because technology lets fishermen fish with less effort, it disguises just how fast the stocks are depleting.

Fishermen generally understand the risks of overfishing. Yet still they flout quotas, where they exist. That is often because they take a short-term view of the asset—they would rather cash in now and invest the money in something else. And it is invariably compounded by a commons-despoiling feeling that if they don't plunder, others will.

In most fisheries, the fishermen would make more money by husbanding their resource, and it should be possible to incentivise them to do so. The best way is to give them a defined, long-term right to a share of the fish. In regulated industrial fisheries, as in Iceland, New Zealand and America, this has taken the form of a tradable, individual share of a fishing quota. Developing countries, where law enforcement is weak, seem to do better when a group right over an expanse of water is given to a co-operative or village fleet. The principle is the same: fishermen who feel like owners are more likely to behave as responsible stewards. The new statistical study confirms that rights-based fisheries are generally healthier.

The rights stuff

Yet only a few hundred of the oceans' thousands of fisheries are run this way, mainly because such schemes are hard to get right. Limiting access to a common resource creates losers, and therefore discord. Cultural differences affect success rates; not everyone is as law-abiding as Icelanders. Almost everywhere it takes time to convince fishermen, the last hunter-gatherers, to change their habits. But, barnacled by caveats though it may be, the rights-based approach is the best available.

In rich countries, satellite imagery will increasingly help, by making monitoring cheaper and better. In many poor ones, devolution is making it easier to form local organisations. Another promising idea is to incorporate rights-based fisheries with no-catch zones. These safeguard breeding-stocks and are easier to monitor than individual catches. Where stocks are recovering, as a result of these reforms, fishermen are likelier to see scientifically determined quotas as in their self-interest. In the end, that may be the only hope.