A critically endangered Hawksbill sea turtle swims in coral around St John, US Virgin Islands
Credit: Caroline Rogers, seaturtle.org
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A new report confirms that already endangered sea turtles species are dwindling further as they become collateral victims of fishing.
A database, the first ever to characterise the effects of global fishing bycatch on marine turtles, suggests that aggressive conservation action may be needed.
The research shows that over the past 20 years at least 85,000 sea turtles have fallen victim to bycatch - the unintentional capture of an animal while fishing for something else. But, this number is based on data collected from between just 1 and 5 per cent of the total fishing fleets.
The report, a joint project between Conservation International and the US institutions Duke University, San Diego State University and University of Richmond, suggests that the turtle bycatch total could actually be in the millions.
The Mediterranean, Eastern Pacific, and both the Southwest and Northwest Atlantic were identified as the most high-risk regions for the turtles. Five of the world's seven sea turtle species (see end of article) are listed as either Endangered or Critically Endangered on the International Union for Conservation of Nature's Red List of Threatened Species.
"Bycatch is a really big, complicated problem affecting marine biodiversity, marine ecosystem function, and the viability of fisheries globally," said Bryan Wallace, an ecologist with Conservational International and lead author on the paper.
"We wanted to be able to provide managers, scientists, and industry with the best, most up-to-date perspective on bycatch of at least one [marine animal] to assist in implementation of conservation strategies and management prioritisation."
Wallace and his team published their work last week in the American journal Conservation Letters.
Vulnerable turtles
Many other marine animals are beleaguered by fishing. Bycatch threatens marine mammals, seabirds, sharks and rays, and is partially responsible for their population declines.
But, sea turtles are a particularly apt gauge of the problem, says Wallace, because they are a good mark for "how oceans are functioning in many parts of the world" due to their wide distribution throughout different habitats.
Wallace and his team considered bycatch data from major fishing techniques, including longline, gillnets, and trawls, for all ocean regions worldwide from 1990 to 2008.
Turtles are vulnerable to the fishing gear because they are air-breathers; if they get caught on hooks or entangled in nets or lines and are unable to surface, they can drown.
While bycatch is not the only threat to sea turtle populations, it is the most serious one in most regions.
"Sea turtles are slow-growing, late-maturing and long-lived," said Wallace, making them vulnerable to bycatch throughout their lives. Their slow-maturity and late reproduction make it difficult for populations to recover from the trauma of bycatch.
Bycatch could be reduced by altering the fishing gear, like including trapdoors in trawls or using circular hooks, which both allow the turtles to escape. Another tactic is to use bait that is less likely to attract them in the first place.
Bycatch over-estimate?
"It is extremely difficult to compile and summarise global data on bycatch that encompasses the potential effects of multiple [gear types] and disparate regional practices," said Ronald Kneib, an ecologist at the University of Georgia Marine Institute, US, who is an editor in journals covering both marine turtles and bycatch.
"Undoubtedly, the authors have made a significant contribution in this regard."
But, Kneib said, the bycatch totals suggested by the research should be taken as a "worse-case scenario" until more data can be collected, as the research did not consider the difference in mortality rate between fishing gear types and the fact that the 1 to 5 per cent of observations may not perfectly represent the total number of fishing vessels.
And, said Kneib, who was not part of the reserach, it important to note that bycatch does not necessarily equal death.
Wallace's next step is to focus on the impacts of bycatch on individual sea turtle populations in specific regions and fisheries to get a closer look at the issue.
The world's sea turtle species:
Flatback (Natator depressus) - Data Deficient
Green turtle (Chelonia mydas) - Endangered
Hawksbill (Eretmochelys imbricata) - Critically Endangered
Kemp's ridley (Lepidochelys kempii) - Vulnerable
Leatherback (Dermochelys coriacea) - Critically Endangered
Loggerhead (Caretta caretta) - Endangered
Olive ridley (Lepidochelys olivacea) - Endangered
