Described as the first global synthesis of existing data, the study found especially high rates of "bycatch" in the Mediterranean and eastern Pacific.
Six of the seven sea turtle types are on the Red List of Threatened Species.
Writing in the journal Conservation Letters, researchers advocate much greater use of gear safe for turtles.
These include circular hooks rather than the conventional J-shaped hooks on long fishing lines, and hatches that allow the reptiles to escape from trawls.
Turtles must come to the surface to breathe.
When they are caught in a net or on a fishing hook, they cannot surface, and drown.
Lead researcher Bryan Wallace said the state of the world's turtles was an indicator of the wider health of the oceans.
"Sea turtles are sentinel species of how oceans are functioning," he said.
"The impacts that human activities have on them give us an idea as to how those same activities are affecting the oceans on which billions of people around the world depend for their own well-being."
Dr Wallace works in the global marine division of Conservation International and at Duke University in the US.
Off target
The raw material from the study came from records of bycatch - incidental catches in fishing gear - from different regions of the world.
Over the period 1990-2008, records showed that more than 85,000 turtles were snared.
However, those records covered a tiny proportion of the world's total fishing fleets.
"Because the reports we reviewed typically covered less than 1% of all fleets, with little or no information from small-scale fisheries around the world, we conservatively estimate that the true total is probably not in tens of thousands, but in the millions of turtles taken as bycatch in the past two decades," said Dr Wallace.
Three types of fishing gear are identified in the survey - long-lines, gillnets and trawls.
Modern long-line boats trail strings of hooks that can be 40km long, usually in search of high-value species such as tuna and marlin.
Gillnets are usually stationary, and use mesh of a set size in an attempt to target certain species of fish.
The researchers suggest that several areas of the world account for particularly high levels of bycatch - the Mediterranean Sea and the eastern Pacific Ocean for all types of gear, together with trawling operations off the west coast of Africa.
Catches cut
Modifying fishing gear can have a dramatic impact on the size of bycatch.
Shrimp trawls fitted with turtle excluder devices (TEDs) catch markedly fewer of the reptiles.
A grid prevents anything large from entering the back portion of the net, and a hole above the grid allows accidentally snared animals such as turtles to escape.
A number of countries now require that shrimp boats must use nets fitted with TEDs.
The circular long-line hooks also reduce bycatch of birds such as albatrosses.
However, some fleets have resisted adopting selective gear because fishermen believe it will reduce their catch.
In many parts of the developing world, the gear is not available.
Marine turtles face other significant threats.
Debris in the oceans, such as plastic bags, can also cause drowning, while development in coastal regions can affect nesting and reproduction.
Some turtles are still targeted for meat, and their shells used for tourist souvenirs.
Numbers of adult leatherbacks - the largest species, growing to more than 2m long and capable of journeys that span entire oceans - are thought to have declined by more than 75% between 1982 and 1996.
Commercial Fishing Estimated to Kill Millions of Sea Turtles
ScienceDaily 6 Apr 10;
The number of sea turtles inadvertently snared by commercial fishing gear over the past 20 years may reach into the millions, according to the first peer-reviewed study to compile sea turtle bycatch data from gillnet, trawl and longline fisheries worldwide.
The study, which was published online April 6 in the journal Conservation Letters, analyzed data compiled from peer-reviewed papers, government reports, technical reports, and symposia proceedings published between 1990 and 2008. All data were based on direct onboard observations or interviews with fishermen. The study did not include data from recreational fishing.
Each dot on the map represents a previous study that was included in this analysis. (Credit: Conservation International)
Six of the world's seven species of sea turtles are currently listed as vulnerable, endangered or critically endangered on the IUCN Red List of Threatened Species.
"Direct onboard observations and interviews with fishermen indicate that about 85,000 turtles were caught between 1990 and 2008. But because these reports cover less than one percent of all fleets, with little or no information from small-scale fisheries around the world, we conservatively estimate that the true total is at least two orders of magnitude higher," said Bryan Wallace, lead author of the new paper.
Wallace is the science advisor for the Sea Turtle Flagship Program at Conservation International and an adjunct assistant professor at Duke University's Nicholas School of the Environment. Most of his co-authors are researchers at Duke's Center for Marine Conservation.
Their global data review revealed that the highest reported bycatch rates for longline fisheries occurred off Mexico's Baja California peninsula, the highest rates for gillnet fishing took place in the North Adriatic region of the Mediterranean and the highest rates for trawls occurred off the coast of Uruguay.
When bycatch rates and amounts of observed fishing activity for all three gear types were combined and ranked across regions, four regions emerged as the overall most urgent conservation priorities: the East Pacific, the Mediterranean, the Southwest Atlantic, and the Northwest Atlantic.
"Although our numbers are estimates, they highlight clearly the importance of guidelines for fishing equipment and practices to help reduce these losses," Wallace said.
Effective measures to reduce turtle bycatch include the use of circle hooks and fish bait in longline fisheries, and Turtle Excluder Devices (TEDs) in trawling. Many of the most effective types of gear modifications, Wallace noted, have been developed by fishermen themselves.
Wallace said the Hawaiian longline fishery and the Australian prawn fishery have significantly reduced bycatch through close working relationships between fishermen and government managers, use of onboard observers, mandatory gear modifications and innovative technologies. TurtleWatch, a real-time database that provides daily updates on water temperatures and other conditions indicating where turtles might be found, has guided fishermen to avoid setting their gear in those areas.
Other approaches, such as the creation of marine protected areas and use of catch shares, also reduce bycatch, preserve marine biodiversity and promote healthy fish stocks in some cases, he said.
"Fisheries bycatch is the most acute threat to worldwide sea turtle populations today. Many animals die or are injured as a result of these interactions," Wallace said. "But our message is that it's not a lost cause. Managers and fishers have tools they can use to reduce bycatch, preserve marine biodiversity and promote healthy fish stocks, so that everyone wins, including turtles."
The study stems from work Wallace began in 2005 as a postdoctoral research associate at the Duke University Marine Lab, where he helped develop the first global bycatch database for longline fisheries. That work was part of a three-year initiative called Project GloBAL (Global By-catch Assessment of Long-lived Species).
Co-authors on the new study -- all of whom were part of the Project GloBAL team -- are Rebecca L. Lewison of San Diego State University; Sara L. McDonald of Duke's Center for Marine Conservation; Richard K. McDonald of the Center for Marine Conservation and the University of Richmond; Connie Y. Kot of the Center for Marine Conservation and the Marine Geospatial Ecology Lab at Duke's Nicholas School of the Environment; and Shaleyla Kelez, Rhema K. Bjorkland, Elena M. Finkbeiner, S'rai Heimbrecht and Larry B. Crowder, all of the Center for Marine Conservation. Crowder is director of the center and the Stephen Toth Professor of Marine Biology at the Nicholas School. Lewison formerly was a research associate at the Duke Marine Lab.
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