Over the past decade or so, most commonly targeted fish in the Costa Rican longline fishery, mahi mahi, has also been the most common species caught. Mission accomplished. But, a research team including Drexel’s environmental science expert James Spotila, found that the second-most-common catch is not a species of fish at all. It’s sea turtles, and vulnerable populations of them at that.

The researchers, comprised of experts from Drexel, the Costa Rican nonprofit conservation organization Pretoma and a U.S. nonprofit working in Costa Rica known as The Leatherback Trust, estimated that more than 699,000 olive ridley and 23,000 green turtles were caught during the study period, which ranged from 1999 to 2010.

These findings and more were reported in a recent study in the Journal of Experimental Marine Biology and Ecology. The researchers argue that time and area closures for the fisheries are essential to protect these animals as well as to maintain the health of the commercial fishery.

The researchers used data from scientific observers on longline fishing boats who recorded every fish and other animal caught by the fishermen from 1999 to 2010 and the locations of the captures and fishing efforts.

“It is common to see sea turtles hooked on longlines along the coast of Guanacaste in Costa Rica. We can set some free but cannot free them all,” says Spotila. “The effect of the rusty hooks may be to give the turtles a good dose of disease. No one knows because no one holds the turtle to see if it gets sick.”

The researchers also noted that even a few deaths of reproductive females may have a significant toll — particularly when longline operations are conducted close to nesting beaches. They reported that declines in olive ridley nesting populations in Ostional, where massive synchronous nesting occurs, were associated with these captures.

The researchers also observed that longlines caught large numbers of silky sharks, stingrays, sailfish and yellowfin tuna. And many small blacktip sharks were captured in an area near the Osa Peninsula, indicating that fishing was occurring at a nursery ground for that species.

Based on their findings, the researchers caution that populations of fish affected by the Costa Rican longline fishery may be in danger of collapse and that there are insufficient scientific data to predict whether and when such a collapse will occur and in what species.

To better manage the fishery and protect the threatened and endangered species of sea turtles in Costa Rica, the researchers argue that policymakers in Costa Rica must enforce time and area closures for longline fishing.

Also, there is a need to establish protected marine areas safe from longlines. They also recommended targeted seasonal closures to longline fishing in coastal waters close to the main turtle nesting beaches.

Spotila, who is also chairman of the board of The Leatherback Trust, said that successful recovery of the fish and turtle populations depends on enforcement of the laws that have been already passed by the national legislature.

“What is being done up until now obviously is not working,” he says.

CATCH_AND_RELEASE

Researchers observed large numbers of sea turtles, including green turtles like this one, captured in Costa Rica’s longline fisheries.