FEATURE ARTICLE
The Imperiled Giants of the Mekong
Ecologists struggle to understand—and protect—Southeast Asia's large migratory catfish
M. Jake Vander Zanden, Zeb Hogan, Peter Moyle, Bernie May, Ian Baird
Caveat Emptor
While Hogan was tagging fish in the Tonle Sap River, he was becoming
increasingly concerned about the plight of the giant catfish.
Populations were clearly in a nosedive, yet this species continued
to be caught, and there didn't seem to be any readily available
means of regulating the fishery. Then in 1999 he and Nicolaas van
Zalinge (head of the Mekong River Commission's Freshwater Capture
Fisheries Program in Cambodia) hatched an idea: Why not buy any live
specimens caught and release them? In Cambodia, fishermen capture
giant catfish essentially by accident—as "bycatch"
in the local bagnet fishery. These fish sell for very little: about
fifty cents a kilogram. In Thailand, this species was in greater
demand and thus was more expensive. A large fish there could fetch
as much as $4,000. Although purchasing live Mekong giant catfish
from local fishers clearly wasn't a long-term solution, starting a
buy-and-release program seemed better than doing nothing.
The fishers were happy enough with our scheme, because we reimbursed
them for the fish at market price. This approach was attractive to
us, too, for a reason that went beyond just saving the few
individuals that were caught: By purchasing, tagging and releasing
giant catfish, we had a chance—albeit a very small
one—to document any link that might exist between the
specimens found upstream in Thailand and those found downstream in Cambodia.
Hogan figured that it would be straightforward to mark any live
specimens caught with labeled plastic tags and then release the fish
back into the river. Because he had developed contacts in both
Thailand and Cambodia and was thus able to monitor both fisheries,
he'd soon know when one of these marked fish was recaptured. And,
obviously, if a fish tagged in Cambodia showed itself in Thailand,
or vice versa, he'd have concrete evidence that these fish moved
between the two locations (and past the proposed dam sites).
The study of migratory connectivity between these two populations
was not just of academic interest. Indeed, developments taking place
at the time made it seem especially important to understand what the
catfish were doing: The upstream section of the river posed several
threats to this species, the most obvious being the continued
fishing in Chiang Khong, Thailand, where catches of the giant
catfish were shrinking dramatically. Would a decline in the numbers
of giant catfish upstream carry over to the downstream population?
To address such concerns, we needed to know whether the two stocks
intermingled. But suppose no "northern" fish turned up
down south (or vice versa)—would this finding, or rather lack
of finding, mean that these two populations lived in isolation or
merely that all of the tagged fish had been lucky enough to escape
recapture? Knowing that the results of the tagging program might be
ambiguous, Hogan joined the Genomics Variation Laboratory at the
University of California, Davis, where with the help of another one
of the authors (May) he developed genetic markers to study the
Pangasiidae. Using tissue samples from the upstream and downstream
stocks of the giant catfish, Hogan and May hoped to be able to
determine whether these two populations mix.
In 2000, Hogan traveled to northern Thailand to observe the giant
catfish fishery in Chiang Khong. His intent was to buy, tag and
release the giant catfish captured there, as well as to obtain
tissue samples. It was mid-April, the hottest time of the year. So
Hogan found a small, well-shaded guesthouse and checked himself in
for the month. Fishing records showed that most giant catfish were
caught at about this time—and that the season for them was
getting shorter each year. In 1992, for example, the season began
with a catch on April 26 and lasted until June 9. In 1999, the
season started on May 6 and finished just two weeks later. So for a
month, Hogan waited on the patio of his guesthouse, walked down the
street three times a day for a plate of fried rice, read books and
worked on his laptop. But the locals caught none of the big fish.
As it turned out, 1999 was the last year that the catch of giant
catfish in Chiang Khong could be termed a "fishery." After
failing to locate any of these fish in 2000, Hogan returned there in
2001 and again in 2003, yet he never saw a specimen. During his last
trip, Hogan spent a month interviewing local fishers about their
practices and the catch of giant catfish. Everywhere the story was
grim. In one village, locals said that the giant catfish had
disappeared in 1960. In another community, they reported netting the
last one 20 years ago. In Chiang Khong, the giant catfish held out
only through 1999. Taken together, these accounts all pointed to the
same conclusion—that the Mekong giant catfish was all but gone
from northern Thailand.
Fortunately, downstream in Cambodia at least some giant catfish
remained. And the Cambodian Department of Fisheries was eager to
conserve its catfish stocks. So Hogan, with financing from the
University of California and the National Geographic Conservation
Trust, started a program to buy and release the giant catfish that
survived capture, beginning in 2000. In all, he and colleagues in
the Cambodian Department of Fisheries have purchased 21 adult giant
catfish—about 80 percent of the total reported
catch—letting them slip back into the Tonle Sap River. (They
are confident that they hear about most captures of giant catfish,
both because news of these events travels quickly on the river and
because their project has garnered enough publicity that most
fishers know to contact them.) Hogan and his Cambodian counterparts
do the same with 10 other vulnerable species, including the giant
carp (Catlocarpio siamensis), the giant sting ray
(Himantura chaophraya) and the river catfish. In all,
they have bought, tagged, and released approximately 5,000 fish.
But with no giant catfish to examine from the Thai sections of the
Mekong, Hogan had no way to verify whether the tagged
"Cambodian" fish migrate upstream, and he, Moyle and May
had no way to compare genetic makeup between the two populations, if
indeed there still is an upstream population worth talking about.
Despite this setback, we don't consider the investigation a total
washout—far from it. Our genetics work has proved valuable for
other reasons. For one, our results can be used to study the
genetics of other catfish species. And the genetic markers that we
developed also allowed us to examine the diversity of stocks bred in
captivity and to anticipate the effect of release of hatchery-raised
fish into the wild.
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