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TUNA fishing that鈥檚 dolphin-friendly may save the mammals at the expense of the young tuna needed to keep the fisheries sustainable. Protecting the dolphins leaves the tuna more prone to overfishing, a theoretical study has shown.

Fishing boats often locate schools of yellowfin tuna by setting their nets around floating objects or free-swimming schools of fish. This eco-friendly strategy has gradually replaced the setting of nets around schools of dolphins, because too many dolphins got caught in the tuna nets. But dolphin-safe methods tend to catch younger, smaller tuna. The old method, by contrast, picked off the biggest tuna, the only ones capable of keeping up with the fast-swimming dolphins.

Fish ecologist Tim Essington of Stony Brook University, New York, and his team used standard fisheries models to estimate the impact of catching different sized tuna on the sustainability of the fisheries. If all fishers set their nets round dolphins, the yellowfin fishery in the eastern Pacific Ocean could sustain an annual harvest of 370,000 tonnes without driving populations into a downward spiral.

By contrast, if all fishers used dolphin-safe methods, the fishery could sustain a harvest of 200,000 tonnes鈥攕ubstantially less than the current take of 270,000 tonnes. 鈥淵ou鈥檙e fishing them before they鈥檝e grown up. If you let them grow a bit and then catch them, you can get a higher yield,鈥� says Essington.

The disparity could have wider ecological effects. Tuna are important predators in the ocean ecosystem, and every young fish caught potentially removes a lifetime鈥檚 worth of preying on other species. Essington鈥檚 models predict that fishing off floating objects, which catches the youngest fish, would reduce tuna predation four times as much as fishing off dolphins. He is now looking at real fish populations to see whether the model鈥檚 predictions play out in the oceans.

But critics say it鈥檚 already clear that neither the dolphin-friendly method nor its alternative is satisfactory. The solution is to go back to fishing tuna with a pole and line, says Carl Safina, vice-president for ocean conservation at the New York-based National Audubon Society. 鈥淵ou catch the big fish, you don鈥檛 catch the dolphins, and you create a lot of jobs.鈥�