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Sea Turtles Suffer Collateral Damage From Fishing

More than 8 million sea turtles have died in the past 2 decades after being accidentally caught by fishing vessels, a global analysis suggests. The researchers identify the Mediterranean Sea and the eastern Pacific Ocean as particularly dangerous for sea turtles. But they also note that turtle deaths probably are widely underreported, particularly along coastlines with small-scale fishing.

Fishing is not a precise operation. Nets or hooks often trap many more kinds of animals than intended, such as sharks, dolphins, or turtles. Smaller studies have shown that this so-called bycatch can kill large numbers of turtles—in Baja California, for example.

Now researchers have compiled a global picture of the impact that various types of fishing gear have on turtles. A team led by conservation biologists Bryan Wallace of Conservation International in Arlington, Virginia, and Rebecca Lewison of San Diego State University in California reviewed the scientific literature for reports of turtle bycatch between 1990 and 2008. They came up with 85,000 fatalities over this period. But that's probably a negligible fraction of the real toll, Wallace says, in view of a previous study by Lewison and others estimating that 200,000 loggerhead turtles alone are killed each year just from fishing hooks. Given that only about 1% of fishing vessels worldwide are monitored, Wallace guesses that the real number is probably at least two orders or magnitude higher, or 8.5 million. "Small-scale fisheries are woefully underreported for bycatch," he notes, especially in the eastern Indian Ocean and off West Africa.

Deadly. Gill nets will drown turtles that get caught in them, such as these green sea turtles (Chelonia mydas).
Credit: Projeto Tamar Brazil - Image Bank.

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To see where the worst areas are, Wallace and Lewison then compared the relative impact of various kinds of fishing gear in different parts of the world. As their team reports online this week in Conservation Letters, two areas stood out for their intense fishing and high rates of bycatch. In the eastern Pacific, endangered leatherbacks and other turtles get caught on hooks baited for tuna. Loggerhead turtles die the same way in the Mediterranean, where boats from 21 countries hunt for tuna and swordfish. These two areas "are urgent conservation priorities," Wallace says.

"Bycatch is probably the most important threat to sea turtles," says Karen Bjorndal, a sea turtle ecologist at the University of Florida, Gainesville, who was not involved in the research. "This gives a much better picture of what these species are facing." Wallace Nichols, a turtle ecologist and research associate at the California Academy of Sciences in San Francisco, adds that even though many of the findings in the paper are intuitive to turtle biologists, a global analysis helps spur further research and could prompt governments to strengthen conservation efforts.

*This article has been corrected. It originally stated that endangered hawksbills and other turtles get caught on hooks baited for tuna. Endangered leatherbacks are caught, not hawksbills.