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
The King (of Fish) and I
In 1996, one of us (Hogan) received a Fulbright scholarship for
graduate study at Chiang Mai University in Thailand. During his year
in Chiang Mai, he met another of the authors (Baird, a geographer
and fisheries biologist then working in southern Laos with the Lao
Community Fisheries and Dolphin Protection Project), who suggested
to Hogan that he focus his graduate research on the threats to
various fishes of the Mekong ecosystem.
At the time, this river was gaining recognition as the most
important natural resource in the region, because it provides up to
two million tons of food (both animal and plant) for rural people
each year and because only the Amazon and the Congo can boast a
greater diversity of freshwater species. But the Mekong also faced
new threats. Just a year or so earlier, the Mekong River Commission,
a body created by the four countries bordering the lower Mekong
(Vietnam, Cambodia, Laos and Thailand), coordinated a study to
consider building 12 hydroelectric generating stations. According to
plans, the dams would stand, on average, about 35 meters high. The
slack water behind many of these enormous concrete constructions
would stretch for roughly 100 kilometers upstream, representing, in
total, more than half of the length of the Mekong River along the
span of the slated projects. It was obvious that these dams would
have serious environmental consequences. The Commission found, for
example, that
[a]ll of the proposed dams will block fish migration. This
one impact alone may cause the wholesale decline in the fishery
throughout the lower Mekong River. Blocking migration cuts out a
critical link in the biological chain of migrating species. While it
is possible that some species may find alternative spawning and
rearing areas, there is no data to support such a possibility. It is
not known how far certain species migrate [or] whether stocks can
continue … to function between dams, because stocks and their
migration patterns have not been identified.
The urgent need for even this basic knowledge prompted Hogan to
begin searching for ways to chart fish movements through the Mekong
river system, an effort that would end up engaging all of us in one
way or another.
Hogan began by learning the Thai language. Then, with a small grant
from the Wildlife Conservation Society, he traveled to towns along
the Thai section of the river to record the species for sale at
local fish markets. During this time, he narrowed his focus to the
dozen or so Mekong catfish species in the family Pangasiidae, which
were relatively common, important commercially and interesting
ecologically. What is more, the installation of dams was thought to
pose a particular threat to these fish, given their highly migratory
behavior, adaptation to the natural variation in river flow, and
sensitivity to water quality and temperature.
What he found generally supported what was already known about
Asia's pangasiid catfish: They are seasonal spawners, grouping
together in May, June and July to breed at the beginning of the
rainy season. Catches of Mekong catfish peak at this time, when most
of the fish apparently migrate in schools up the Thai-Lao segment of
the river.
Hogan couldn't describe specific migratory patterns just by
inspecting the offerings in fish markets, but these surveys were
nevertheless valuable. While traveling from town to town, he had a
chance to learn about the fisheries firsthand and to chart the
distribution in space and time of various species of Pangasiidae
from the border between Isan, Thailand, and Champasak Pro-vince,
Laos, in the south to the Golden Triangle region in the north.
He noted, for example, that the Mekong giant catfish and the
slightly less gargantuan "dog eating" catfish
(Pangasius sanitwongsei) appeared in the northern
section of the river between Thailand and Laos in April, May and
June. Smaller species, including the mouse-faced catfish
(Helicophagus waandersii), the snail-eating catfish
(Pangasius conchophilus) and the whiskered catfish
(Pangasius macronema), inhabited the middle stretches
of the river and represented the majority of the catch in this area
between April and June. Surprisingly, one species commonly found in
markets, the river catfish (Pangasius hypophthalmus),
turned out to come from fish-farming operations, not (as Hogan had
first been led to believe) from the river. Wild examples of this
fish are, in fact, very rare in Thai portions of the Mekong. Perhaps
most interesting was the presence of large (meter-long) silver-toned
catfish (Pangasius krempfi) in many fishmongers' stalls.
Why were silver-toned catfish a surprise? A few years before Hogan
arrived in Thailand, Baird had reported that this species could be
found in the South China Sea and also in southern Laos. Baird
surmised that this migratory catfish might be anadromous,
traveling from the marine waters of the South China Sea up the
Mekong through Vietnam and Cambodia and into Laos, where they
presumably spawned. His basic theory, along with Hogan's later
observation of this species in Nong Khai, Thailand (about 1,600
kilometers upstream of the Mekong Delta), provided impetus for a
study of the silver-toned catfish that could better document its
travels. We (Hogan and Baird) began by carefully examining, of all
things, small structures in its ears.
Hogan realized that this curious tactic might reveal migratory
patterns after a chance meeting with Robert Kinzie and Richard
Radtke of the University of Hawaii at Manoa. These investigators
studied the migratory behavior of a different kind of fish, gobies,
using a novel technique—analysis of strontium:calcium ratios
in otoliths ("ear stones"). These small, hard
deposits are found in the heads of all bony fish. Otoliths can be
used to tell how old a specimen is, because they are built up of
distinct layers that are deposited annually. Radtke and Kinzie found
that otoliths can also indicate events that take place as the
animals mature. In particular, the ratio of strontium to calcium in
an otolith records whether the fish had been living in salt water or
fresh water, because strontium concentrations in the ocean are one
to two orders of magnitude greater than in rivers or streams.
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