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
Tag Team
At the time, fisheries biologists in the Mekong region were
suggesting that fish migrate between the Mekong River and Tonle Sap
Lake, the largest inland lake in Southeast Asia, which connects to
the Mekong through a river also named Tonle Sap. In the dry season
(November to February), this remarkable lake covers about 2,500
square kilometers. At the height of the rainy season (August), the
lake area expands fourfold, and the maximum depth increases from 4
meters to 10. Life around the lake, including that of the local
people, is uniquely adapted to this annual cycle. Fish use the
flooded habitat to feed and to grow. The variety of landscapes,
including inundated forests and fields, ephemeral streams and small
satellite lakes, provides habitat for more than 100 kinds of fish
and many more species of birds, reptiles and amphibians.
Every year at the end of the rainy season, the flow of the Tonle Sap
River changes direction from north to south as the water begins to
drain from the flooded forests and plains into the Mekong. With this
outflow come millions of fish. (Residents take advantage this annual
movement by fixing all manner of traps and nets in the lake and
river to snare the migrating fish.) We wanted to determine where
exactly these animals swim: Do they exit the Tonle Sap River and
enter the Mekong? If so, where do they then travel? That is, do they
move upstream or downstream? How far do they go?
Underwater biotelemetry (fitting fish with acoustic or radio
transmitters) seemed a good way to answer these questions.
Biotelemetry systems have often been used to study fish migrations,
to locate spawning and feeding grounds and to describe important
seasonal habitat. But this high-tech strategy had never before been
applied to chart fish migrations within the Mekong River basin,
because most fisheries biologists believed that such tagging would
not be fruitful in a river system so large and complex. Thankfully,
Hogan was able to obtain support from the World Wildlife Fund to try
this approach as well as the more common form of
tagging—attaching plastic markers to fish.
For this study, Hogan and coworkers from the Cambodian Department of
Fisheries collected live fish from a "bagnet" fishery
located in the lower part of the Tonle Sap River near Phnom Penh.
This particular fishery contains about 60 individual nets, each 120
meters long and 25 meters in diameter at the mouth. The first row of
four side-by-side nets is located just outside the city, and the
final phalanx is located some 35 kilometers to the north. This
operation, like many other fisheries in the Tonle Sap River, runs
from October to March, the period when water flows out of the great
lake and into the Mekong and adjacent Bassac River.
Between November 6 and December 1, 2001, Hogan and his Cambodian
colleagues outfitted two Mekong giant catfish and 11 river catfish
with acoustic transmitters and plastic tags labeled "Please
return to the Department of Fisheries." On the evening of
December 9, the hydrophone we were trailing from our survey boat
picked up signals from one of the tagged river catfish. We were
cruising the Mekong, 20 kilometers upstream of its confluence with
the Tonle Sap and Bassac rivers. This acoustic contact indicated
that the fish had moved out of the Tonle Sap River and on up the
Mekong. Although we never actually saw the fish, we were able to
identify it (a 17-kilogram specimen we had tagged on the last day of
November) using the unique pattern of beats programmed into its transmitter.
Two months later, this same fish gobbled up the baited hook of a
local fisher approximately 300 kilometers upstream from Phnom Penh,
which meant that it had traveled nearly 5 kilometers per day.
Fishers have since recaptured several other tagged specimens in this
same area (we learn about such catches promptly, because we provide
a small reward for the return of our tags), suggesting that this
migration route—from the Tonle Sap Lake, down the Tonle Sap
River and on up the Mekong—is typical of river catfish.
Adult river catfish move into deep water areas of the Mekong River
to survive the dry season. They then migrate upstream and spawn with
the onset of the first heavy rains in May and June. Young fish float
downstream with the rising water, eventually finding their way into
inundated areas during the rainy season. These temporary wetlands,
such as the flooded forest of the Tonle Sap Lake, act as rainy
season nurseries for young fish of many other species as well.
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