Costa Rican Longline Fishery Kills Thousands Of Sea Turtles, Sharks
October 3, 2013

Costa Rican Longline Fishery Kills Thousands Of Sea Turtles, Sharks

April Flowers for - Your Universe Online

In the last decade, the second-most-common catch on Costa Rica's longline fisheries were olive ridley sea turtles, not a commercial fish species. These longline fisheries also caught more green turtles than most species of fish, according to a new study led by Drexel University.

The findings, published in the Journal of Experimental Marine Biology and Ecology, indicate longline fishing is a major threat to the Pacific populations of sea turtles and sharks. The research team included scientists from Drexel University, the Costa Rican non-profit conservation organization Pretoma and a US non-profit working in Costa Rica, The Leatherback Trust. They argue that time and area closures for fisheries are necessary for the conservation of these animals, as well as maintaining the health of the commercial fishery.

The data for this study was collected by scientific observers on longline fishing boats who recorded every fish and other animal caught by the fishermen from 1999 to 2010. They also recorded the locations of the capture and fishing efforts. This data allowed the scientists to perform a mathematical analysis of the fishery that resulted in maps of geographic locations and estimates of the total number of captures of sea turtles in the entire fishery.


Mahi mahi is the most commonly targeted fish in Costa Rica, and the researchers found it was the most commonly caught as well. They were surprised, however, to find that olive ridley sea turtles -- intentionally classified as vulnerable -- were the second-most-commonly caught species. In fact, they estimated that between 1999 and 2010, more than 699,000 olive ridley and 23,000 green turtles were caught.

Approximately 80 percent of the captured turtles are released from longlines and survive, in the short term. According to the researchers, long-term impacts are not yet adequately measured.

“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,” said Dr. James Spotila, the Betz chair professor of environmental science in the College of Arts and Sciences at Drexel. “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.”

Spotila, along with Drexler colleagues and students, has been studying sea turtles on the Pacific coast of Costa Rica for 23 years.

The research team notes even a few deaths of reproductive female turtles could have a significant toll on the turtle populations -- especially when longline operations are held in the shallow waters of the continental shelf close to nesting beaches. In Ostional, where massive synchronous nesting occurs, the declines in olive ridley nesting populations were associated with these captures.


Longline fisheries in Costa Rica also target tuna, shark and marlin species. The scientific observers noticed that the longline boats caught large numbers of mahi mahi, silky sharks, stingrays, sailfish and yellowfin tuna.

Fishing patterns have revealed how shark populations have declined in numbers, however, and that sharks have become smaller over 11 years. During the study, adult sharks were generally small, and an alarming amount of juvenile sharks were observed. This suggests some shark species were being overfished. During the study period, only 14.6 percent of the abundant silky sharks were sexually mature. In 2010, researchers observed the average fork length of silky sharks was approximately 3 feet, far below the previously observed 5 feet for mature adults -- indicating a reduction in relative numbers of adults in the population.

The researchers also observed many small blacktip sharks being captured in an area near the Osa Peninsula, suggesting the fishing was occurring at a nursery ground for the shark species.

The researchers warn that beyond the overfishing of sharks and the danger to sea turtles, the study points to a broader uncertainty about the health of the fishery. The capture of large numbers of mahi mahi, the authors said, does not guarantee that population is sustainable because the available data cannot determine if mahi mahi will remain abundant or decline.

The research team cautions that fish populations affected by the Costa Rican longline fishery may be in danger of collapse and that the data is insufficient to predict whether and when such a collapse will occur, or in what species.


Time and area closures for longline fishing must be enforced to better manage the fishery and protect the threatened and endangered species, the researchers argue. The team criticizes both the fishing industry and INCOPESCA, the fisheries management agency of the government, for failing to recognize that the fishery is unsustainable and failing to enforce existing fishery laws -- particularly those against landing of shark fins and harming of sea turtles.

“INCOPESCA has failed to adequately study and regulate the fishery in Costa Rica for many years. It does not even enforce national laws. Board members have serious conflicts of interest because they are commercial fishermen,” said Randall Arauz, president of Pretoma and a world recognized leader in marine conservation. “Until INCOPESCA is reformed in such a way that the Board of Directors is eliminated and its mission is to defend the public interest, neither the fish nor the turtles will be safe.”

Arauz directed the sea observer program that collected the data on longline boats that were the basis for this study, and has been studying sea turtles and fisheries in Costa Rica for more than 30 years.

The need to establish well-enforced marine protected areas where both turtles and fish are safe from longlines is a critical need, according to Arauz and Spotila, who also recommended targeted seasonal closures to longline fishing in coastal waters close to the main turtle nesting beaches when and where sea turtle interactions with the fishery are highest.

They also suggest a general seasonal longline fishery closure for 5 months, from June to November, which can shift, according to the seasonal abundance of mahi mahi.

To enforce these recommendations, as well as provide needed data to manage the fishery, the research team recommends placing observers on at least half of longline boats, as was done in Chile. They also suggest that education of local artisanal fishermen would improve their fishing techniques and encourage them to release sea turtles unharmed.

“There is still time to save both the fishery and the turtles if action is taken soon,” Arauz said.

Pretoma and the Leatherback Trust are providing leadership for a coalition of environmental groups in Costa Rica to pursue such actions. This coalition has united for a special marine conservation initiative called "Front for Our Oceans."

Spotila, who is chairman of the Leatherback Trust, says that for fish and turtle populations to recover successfully, it is necessary to "collect good data on the fishery, establish protected areas of refuge for the animals and to encourage or force INCOPESCA to enforce the laws that have been already passed by the national legislature."

"What is being done up until now obviously is not working," Spotila concluded.