CORVALLIS, Ore. - Efforts to reduce the number of loggerhead turtles killed as bycatch by commercial fisheries should focus on protecting those with the highest "reproductive values," according to the authors of a newly published study.
This new holistic look at the ecology of sea turtles acknowledges not only the issue of protecting the species, the authors point out, but the importance of prioritizing research and management efforts to reduce the mortality rates that have the largest impact on the population. Those efforts can include gear modifications to reduce turtle mortality while maintaining productive fisheries.
The study was just published in the Journal of Applied Ecology.
"There are a lot of things that kill sea turtles and fisheries bycatch is just one of them," said Selina Heppell, an associate professor of fisheries and wildlife at Oregon State University and a co-author on the study. "Different fisheries catch different sizes and numbers of turtles, so they have different impacts. Instead of harping on a particular fishery at a particular time, it is important to look at the issue of population declines more holistically and figure out the best ways of protecting the species, not just the individuals."
Loggerhead turtles are hard-shelled, slow-growing sea turtles found in the Pacific and Atlantic oceans and the Mediterranean Sea. The Pacific populations originate from nesting beaches on Japanese islands, and travel across the ocean to feed off Baja. They rarely range far to the north because of the cold water, though one turtle was found off Oregon last November and died despite efforts to resuscitate it.
Atlantic loggerheads nest mostly in Florida, which has experienced a 50 percent decline in nesting females in the past 10 years.
Listed as threatened under the Endangered Species Act, the loggerhead turtle faces threats from the loss of nesting habitat as well as bycatch. The building of new seawalls in Japan, to protect against encroachment from rising sea levels, has resulted in significant beach erosion that has destroyed prime nesting grounds for Pacific populations, Heppell pointed out.
The biggest contributor to juvenile and adult mortality, however, is from incidental bycatch by commercial fisheries. Turtles get tangled up in drift and trawl nets and are trapped underwater, where they drown. Other fishing methods also catch a lot of turtles but can be less lethal, the authors say, such as long lines with hooks that allow turtles to swim to the surface and breathe.
"The use of circle hooks by fishermen has decreased the mortality as well," Heppell said, "because the turtles tend to get hooked in the mouth instead of the gut so when they're hauled on board, they don't get ripped up. A lot of fishing boats in the U.S. and Canada are carrying shank cutters to cut the circle hooks so the turtles can be released more easily back into the water.
"We're trying to get the use of 'J-hooks' discontinued because when they're swallowed they're much more likely to be fatal," she added. "But circle hooks are not a panacea - they can get stuck in the roof of the mouth and the turtles can be stressed - but they seem to be helping and mortality is down."
What the authors really advocate for turtles and other long-lived species is more effort aimed at protecting individuals that have the highest reproductive values - adult breeders and sub-adults that are approaching maturity. Reproductive value provides a "currency" to compare the impacts of fisheries that catch turtles of different ages, Heppell pointed out. For example, protecting 100 adult females of breeding age or 100 large juveniles that are nearly mature is more important than protecting 100 small juveniles that only have a one in 30 chance of surviving until breeding age.
Loggerheads grow slowly and don't reach maturity until roughly 25 years of age - so the loss of breeding-age adults to bycatch is particularly damaging. Analysis of where these breeding adults feed and migrate is critical to reducing the spatial overlap between the turtles and the fishing fleets that may be in those areas.
The authors' research suggests that more loggerheads with high reproductive values were caught in trawl gear than in long lines - a probable reflection of the overlap between those fisheries operations and the habitat use by the turtles.
"If we can establish where the animals with the highest reproductive values are and limit their interaction with gill and drift nets, while continuing to reduce lethal interactions with long-lines, trawls and other gear, we may be able to find effective compromises," Heppell said. "It would be nice to save all of the turtles. But we also need to have sustainable fisheries.
"Focusing on saving the most valuable individuals of the species may be our best bet."
The concept of attaching reproductive values to individuals as a way of evaluating human impacts on a species' long-term viability could also be applied to seabirds, mammals and sharks, the authors say.
The journal article was written by Heppell, lead author Bryan Wallace of Duke University and his colleagues Shaleyla Kelez and Larry Crowder; and Rebecca Lewison of San Diego State University.
The research was funded by the Pelagic Fisheries Research Program, based at the University of Hawaii.
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