OFF Chile's Pacific coastline, between December and February, divers take to the chilly waters in search of a predatory sea-snail, the loco (Concholepas concholepas). Boiled and daubed in mayonnaise, it is a local favourite. Asian gourmets prefer it stir-fried or as sashimi. The snail's popularity rocketed when Chile opened its markets in the late 1970s. This was nearly its undoing: it fetched $15,000 a tonne at export and a snail rush ensued. In 1980 Chilean fishermen landed 25,000 tonnes; by 1989, when the fishery was officially closed, stocks had collapsed. That is a sad and familiar tale. But it has an inspiring sequel. On reopening the fishery, the government changed the rules. Small groups of artisanal fishermen, registered as co-operatives, had exclusive rights to harvest loco and other benthic creatures in a defined area of seabed. They were also encouraged to chase away illegal snail-gatherers. This has been good for man and snail. Chile's 50,000 artisanal fishermen now produce a steady, and lucrative, 2,500-5,000 tonnes of loco a year. Such hopeful stories are urgently needed, as new research from Chris Costello and Steve Gaines of the University of California, Santa Barbara, indicates. Using an array of catch and fish life-cycle data, they have devised a new statistical method to gauge the health of fish stocks, and applied it to over 7,000 fisheries. Most, probably making up 80% of the global catch, had previously been subject only to sketchier estimates compiled by the UN's Food and Agriculture Organisation, based on official figures. The new study (under peer review for the journal Science) reckons that those unassessed fisheries are gravely depleted, with on average about half the fishy biomass they need to maintain their maximum annual yield (the usual definition of sustainability). Another analysis, using the same method, of around 1,500 fisheries last year reached similar results. Both suggest the least-known fisheries are the most damaged. This conflicts with the sunnier analysis of industrial fishing fleets, based largely on expensive studies of a few hundred fisheries, mostly in European and American waters. These are depleted but generally recovering, thanks to recent reforms. That these turn out to be exceptional cases is unsurprising. The rapacious habits of fishermen and perverse effects of the subsidies some extract from governments are well known. Sometimes overfishing stems from ignorance and sometimes from short-termism, exacerbated by the belief that whatever they don't take, others will. The cost is enormous. Besides harbouring millions of species, fisheries provide the primary source of protein for a billion people and livelihoods for hundreds of millions, most of them poor. The World Bank reckons that benefits (such as income and food) lost by overfishing between 1974 and 2008 amount to $2.2 trillion. More happily, the new assessment reckons that only 2% of fisheries have so far collapsed - defined as a fall to less than a tenth of the historical biomass. Previous estimates were higher, at up to 30%. Yet incidences of collapse are rising, the researchers stress; and once collapsed, fisheries do not necessarily recover. Shoals of northern cod have not yet returned to the Grand Banks fishery off Newfoundland, which collapsed in 1992. On February 24th, at a conference in Singapore hosted by The Economist, the World Bank's outgoing head, Robert Zoellick, will announce plans for a new assault on the sea's troubles, which also include acidification (a result of climate change) and pollution by human and agricultural waste. The ambitious aims are to raise an additional $1.5 billion for marine management; to rebuild at least half of the fish stocks identified as depleted; to double the number of marine protected areas; and to halve the economic losses in fisheries, partly by scrapping ruinous subsidies. Above all, fishermen need better incentives to manage stocks properly. This has been tried in several ways. In Iceland and elsewhere they have a tradable share of a scientifically determined quota. Or they can be given long-term rights - akin to property rights - over an expanse of sea, as Chile's loco snailers have. According to a 2008 study by Messrs Costello, Gaines and others, this approach works: fisheries where such rights are in force are only half as likely to collapse as the average fishery. Yet the spread of such schemes has been woefully slow; only a few hundred mainly rich-world fisheries have adopted them so far. Though sensible in theory, rights-based schemes are hard to get right in practice. Getting locals on board can be difficult and is a slow process at best. If the target species is especially valuable or slow-growing, overfishing (at least in narrow economic terms) is rational. That may make monitoring and enforcement impractically costly. Rights-based fisheries are not the only answer to overfishing. In particular, a lot more of the sea needs protecting from any fishing at all. Yet there are few better ways to make fishermen control themselves. Chile's predatory snails, and their human predators, will vouchsafe that.