Oct. 2, 2013 — The
second-most-common catch on Costa
Rica's longline fisheries in the last decade
was not a commercial fish species. It was olive ridley sea turtles. These lines
also caught more green turtles than most species of fish.
These findings and more,
reported in a new study in the Journal of Experimental Marine Biology and
Ecology, indicate that the Costa Rican longline fishery represents a major
threat to the survival of eastern Pacific populations of sea turtles as well as
sharks.
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 research was
conducted by a team from Drexel University, the Costa Rican non-profit conservation
organization Pretoma and a U.S.
non-profit working in Costa
Rica, The Leatherback Trust.
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. Those data provided the basis for a mathematical
analysis of the fishery resulting in maps of geographic locations and estimates
of the total number of captures of sea turtles in the entire fishery.
Stark threats to sea
turtles, including nesting populations
The most commonly
targeted fish, mahi mahi, was also the most common species caught in the Costa
Rican longline fishery.
But the researchers were
surprised by their finding that olive ridley turtles, internationally
classified as vulnerable, were the second-most-common species caught.
They estimate that more
than 699,000 olive ridley and 23,000 green turtles were caught during the study
period (1999 to 2010).
"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 its gets sick."
Spotila, a co-author of
the study, has been studying sea turtles on the Pacific coast of Costa Rica with
colleagues and Drexel students, for 23 years.
The researchers also
noted that even a few deaths of reproductive females may have a significant
toll -- particularly when longline operations are held in shallow waters of the
continental shelf 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.
Catching more and smaller
fish signals an unsustainable fishery, sharks at particular risk
In addition to mahi mahi,
other species targeted in the Costa Rican longline fishery were tunas, sharks
and marlins.
The researchers observed
that longlines caught large numbers of mahi mahi, silky sharks, stingrays,
sailfish and yellowfin tuna.
But the fishing patterns
showed that shark populations have declined in numbers and sharks have become
smaller over 11 years. Adult sharks were generally small, and juvenile sharks
alarmingly abundant, suggesting that some shark species were being overfished:
Overall, only 14.6 percent of the abundant silky sharks observed during the study
period were sexually mature. In 2010, the last year of the study, average fork
length of silky sharks was 97 cm, far below the observed 144 cm average for
mature adults. These decreases in size of silky sharks through time indicated a
reduction in relative numbers of adults in the population.
Additionally, 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.
The small size of adult
sharks and large numbers of juveniles captured suggest that species are being
overfished.
In addition to these
indicators of overfishing of sharks, the researchers warned of broader
uncertainty about the health of the fishery. They said that capture of large
numbers of mahi-mahi does not guarantee that that population is sustainable
because the available data can not determine if mahi mahi will remain abundant
or decline.
Based on these findings,
the researchers caution that 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.
How to manage the fishery
and save the turtles
About 80 percent of
captured turtles are released and survive in the short term, but long-term
effects of being caught on fishing hooks are unknown.
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.
They criticize 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 fisheries laws, such as 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, a co-author of the
study, has been studying sea turtles and fisheries in Costa Rica for
more than 30 years. He directed the at sea observer program that collected the
data on longline boats that were the basis for this study.
Aurauz and Spotila argue
for the need to establish well-enforced marine protected areas where both
turtles and fish are safe from longlines. They also recommend 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 further recommend 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 and provide needed data to manage the fishery, they recommend
placing observers on at least half of longline boats, as was done in Chile.
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.
In pursuit of such
action, Pretoma and The Leatherback Trust are providing leadership for a
coalition of environmental groups in Costa Rica who have united for a
special marine conservation initiative called "Front for Our Oceans"
(http://www.salvemosnuestrosmares.com/).
For fish and turtle
populations to recover successfully, Spotila, who is also chairman of the board
of The Leatherback Trust, said, "the challenge is 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."
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