Overfishing is Emptying World's Rivers,
Lakes, Experts Warn
By James Owen
for National Geographic News
Published December 1, 2005
Ocean species such as cod, dolphins, and sea turtles have
been grabbing headlines as victims of unsustainable fishing.
But policymakers and the media are neglecting freshwater
rivers and lakes that are also being emptied of fish,
a new report warns.
Scientists say exploitation of fish stocks is threatening
biodiversity in fresh waters globally while also putting
jobs and food supplies in developing nations at risk.
"Overfishing of inland waters is a neglected crisis,"
said the report's co-author Kirk Winemiller, a fish researcher
at the Texas Agricultural Experiment Station in College
"Most of the focus is on oceans, with inland waters
rarely mentioned," he added. "Yet fish from
inland waters are more threatened than those in oceans."
Rivers and lakes highlighted in the report, published
today in the journal BioScience, include the Mekong River
in Southeast Asia.
The river is home to the Mekong giant catfish—believed
to be the world's largest freshwater fish—and various
other huge but increasingly threatened species.
Other critically endangered fish cited in the study include
the Murray cod of the Murray River Basin in Australia
and the lake sturgeon in the Great Lakes of North America.
The report reveals that humans are fishing their way
through different-size fish, starting with the largest,
then targeting progressively smaller species until there's
nothing left to catch.
"Tens of millions of people in developing countries
fish inland waters for food and to earn a living,"
Winemiller said. "Typically fishing pressure shifts
from species to species as preferred types or those more
easily captured decline in number."
The report stresses the need for government agencies
and fisheries experts to work with local people to manage
"critically harvested" waters.
Two-thirds of the total catch is taken in Asia, with China
alone home to some 12 million fishermen.
Average yearly fish consumption in the Mekong Basin in
Southeast Asia is estimated at 123 pounds (56 kilograms)
per person. Items on the menu include some of world's
largest and rarest river fish, including the Mekong giant
catfish, freshwater whipray, and giant barb.
Zeb Hogan, a fish biologist at the University of Wisconsin
at Madison and a National Geographic Society emerging
explorer, has been collecting data on these fish in Cambodia
Hogan says catches of the legendary Mekong giant catfish,
which can grow to 660 pounds (300 kilograms) and 9 feet
(2.7 meters) in length, have fallen drastically in recent
years—from 60 in 1995 to just 4 in 2005.
"There may be a season soon when no fish are caught,"
The researcher says there is currently no comprehensive
conservation strategy to save these fish from extinction.
He says priorities should include sustainable catch limits
for rare Mekong species and the creation of freshwater
"Environmental education is also very important,"
Hogan added. "Few people know about the endangered
animals of the Mekong or how important fish are to the
livelihoods of people living within the basin.
"Had a coordinated effort been undertaken a decade
ago, the situation would not be so dire."
The report says big fish are particularly vulnerable
to being captured in nets, and the global trend in overfished
waters is towards fish of ever diminishing size.
"Large adult fish are the broodstock that sustains
the population," said David Allan, lead author of
the report and a conservation biology professor at the
University of Michigan in Ann Arbor.
"[Fertility] increases with body size, so large
females are especially important reproductively."
And as populations of bigger fish dwindle, smaller species
take their place. The Queme River in Benin, West Africa,
has seen large predatory fish like the Nile perch replaced
by small species of cichlids and catfish.
Allan says fishermen lower their targets accordingly
to fill their nets, using smaller mesh sizes to catch
the fish that are available.
Depleted fish stocks can have serious repercussions on
freshwater ecosystems. The researchers point to the example
of waters in western Canada and Alaska where the rotting
bodies of Pacific salmon, which die after breeding, provide
"As salmon populations have decreased because of
overfishing and other causes, declines have also occurred
in lake productivity and juvenile salmon recruitment,"
the authors state.
They also warn that overfishing has the potential for
severe impacts on human health, especially in developing
For example, increased incidence of schistosomiasis in
Africa has been linked to declines of fish species that
eat the snails carrying the disease-causing parasites.