Costa Rica's Longline Fisheries Threaten Sea Turtles and Sharks

Costa Rica's longline fisheries threaten the survival of sea turtles and sharks living in the eastern Pacific, a study in the Journal of Experimental Marine Biology and Ecology suggests.

According to the report, the second most common animal caught on the country's longline fisheries in the last decade were not a commercial fish at all, but the olive ridley sea turtle, which is internationally classified as vulnerable. The lines also caught more green turtles than most species of fish during this time, according to the international team of researchers.

The data used in the study came from scientific observers on longline fishing boats. They were responsible for recording every animal caught by fishermen between 1999 and 2010, in addition to the locations of the captures.

In all, an estimated 699,000 olive ridley and 23,000 green turtles were caught during this period. And while roughly 80 percent of captured sea turtles were released from longlines, long term impacts are not adequately understood, the researchers say.

"The effect of the rusty hooks may be to give the turtles a good dose of disease," James Spotila, a professor of environmental science in the College of Arts and Sciences at Drexel and co-author, said in a statement. "No one knows because no one holds the turtle to see if its gets sick."

Even the loss of a few reproductive females could have a significant toll, the researchers note. This is particularly true when longline operations are carried out in shallow waters near nesting beaches.

Sharks in the region have undergone a reduction in size and numbers through the last 11 years, according to the study: the average fork length of silky sharks was 97 cm in 2010, compared to the average 144 cm for mature adults. This decrease in average size can be attributed to a decrease in the number of adults found in the population, the report explains.

Furthermore, the researchers note that it's not clear whether the commercial fish, and in particular mahi-mahi, can withstand the rate at which they are currently being fished given a lack of available data.

To counteract this threat, the study calls for well-enforced marine protected areas where turtles and fish are safe from longlines and seasonal closures in coastal waters close to main turtle nesting beaches.

According to Randall Arauz, president of the non-profit conservation group Pretoma, "There is still time to save both the fishery and the turtles if action is taken soon."