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
Listening to the Stones
With Radtke's offer of help, Hogan and Baird decided to use otoliths
to test whether silver-toned catfish caught far inland had migrated
up from the sea. The base of operation for this study was Hang
Khone, a small village of about 45 families where Baird had been
conducting community-based research on Mekong fisheries since 1991.
This tiny enclave is located in the southernmost province of Laos,
at the edge of Khone Falls, the Mekong's only mainstream waterfall,
and a stone's throw from Cambodia. There, Hogan collected 36
specimens of silver-toned catfish for otolith analysis.
Hogan, Radtke and Baird found that the otoliths contained
significant amounts of strontium—clear evidence that these
fish had lived in salt water. Conversely, the analyses did not turn
up elevated strontium concentrations in related species. These
results helped bring the migratory pattern of this catfish into
clearer focus. Baird had already documented silver-toned catfish
living in the ocean from January through April. And Sophie
Lenormand, a French graduate student working with the Asian Catfish
Project in Vietnam, had determined that adults of this species move
upstream of the estuarine zone in February or March. Higher yet on
the river, in southern Laos, Baird had seen just adults weighing
more than a kilogram or so—and only from May to October. It
thus seems likely that in February and March the silver-toned
catfish move from the sea into the river to spawn, reaching the
Khone Falls, 719 kilometers upstream, in May and June, which is when
the residents of Ban Hang Khone net 98 percent of their yearly haul
of this fish.
This investigation kept Hogan well occupied through his year as a
Fulbright student, but his interest in Mekong catfish did not end
there. Hogan moved back to the United States in 1997 to begin study
for a Ph.D. at the University of California, Davis, under the
direction of another one of us (Moyle). A few years into Hogan's
studies at Davis, Jake Vander Zanden joined Moyle's research group
on a postdoctoral fellowship sponsored by The Nature Conservancy.
Vander Zanden's specialty was stable isotope analysis, specifically
the measurement of carbon and nitrogen isotopes, which can help to
delineate food webs and energy flows in aquatic systems.
So it was quite natural that three of us (Hogan, Moyle and Vander
Zanden) decided to use stable isotopes to fill out the story pieced
together from the earlier otolith study of silver-toned catfish. We
figured that such an analysis could readily tell us whether this big
fish fattens up while at sea. And indeed, our results indicated that
the flesh of this fish has an isotopic signature that reflects
growth in a marine environment, something not seen in other related
species of catfish.
Taken together, our analysis of catch data, strontium in otoliths
and stable isotopes in muscle tissues provided ample evidence that
the silver-toned catfish migrates long distances between fresh and
salt water—the first documented case of anadromy in a Mekong
River species. That is, we had fully confirmed the notion that this
species was a Mekong "salmon," as Baird and Tyson Roberts
of the Smithsonian Tropical Research Institute had dubbed it in
1995. Despite this success, it was clear early on that these
chemical and isotopic methods wouldn't work to investigate the
migratory habits of other species of Mekong catfish, which, as far
as we knew, remain in fresh water throughout their lives. The
inability of these techniques to chart such movements prompted Hogan
to explore an entirely different avenue of investigation, one that
he had earlier rejected as being too expensive and
difficult—following some fish around.
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