The importance of biodiversity
23 July 2010 Science Magazine
Last Stand on the Yangtze
SHISHOU, CHINA—A hundred meters or so off the bow of our motorboat, a
dark shape breaks the surface and quickly vanishes. Was it a trick of the
light? Heavy rains have washed nutrients into the lake, turning the usually
clearer waters emerald green. A few minutes pass, then ecologist Xie Songguang,
perched like a coxswain, points off starboard and cries: "Over there!" The
earlier sighting was no mirage: Two more glistening humps slice through the
Fading pulse. IHB keeps a few finless porpoises in captivity, but a 10-
year ban on Yangtze fishing may be their only hope of survival in the wild,
argues Wang Ding. CREDIT: R. STONE/SCIENCE
The Yangtze finless porpoises in Tian-E-Zhou Oxbow Nature
Reserve, near the town of Shishou, dare not approach; too many of their kind
have perished in encounters with humans. The world's only freshwater porpoise
is down to fewer than 1800 individuals, less than half the estimated population
a decade ago. A few dozen porpoises live under the watchful gaze of rangers in
Tian-E-Zhou, an oxbow lake that once was a 21-kilometer-long section of the
Yangtze River. But time is running out: Unless threats to its survival are met
head on, the porpoise could be gone in 15 years, says Wang Ding, an ecologist
at the Institute of Hydrobiology (IHB) in Wuhan.
The finless porpoise is an icon of an ecosystem under siege.
The Yangtze, the world's third longest river, faces manifold threats, including
overfishing, heavy boat traffic, and habitat fragmentation due to hydropower
projects such as the mammoth Three Gorges Dam (Science, 1 August 2008, p. 628).
Two charismatic animals—the Yangtze river dolphin, or baiji, and the Chinese
paddlefish, a titan that once reached 7 meters in length—are down to a handful
of individuals or have already gone extinct.
The loss of such top carnivores is an early sign of ecosystem
collapse, says David Dudgeon, an aquatic ecologist at the University of Hong
Kong. Adding insult to injury, Yangtze fishing communities have already begun
to forget that these creatures even existed, Samuel Turvey of the Institute of
Zoology in London and colleagues report in the June issue of Conservation
Biology. "I was extremely surprised that local ‘community memory’ of the
Yangtze megafauna is being lost so rapidly," Turvey says.
Researchers hope to make a last-ditch effort to stabilize the
Yangtze ecosystem and prevent further extinctions. IHB is working with China
Three Gorges Dam Corp. to set up a reserve in the dam's reservoir, and Honghu
Xin-Luo National Baiji Reserve plans to launch a finless porpoise ex situ
conservation program. Meanwhile, influential researchers are calling for a
lengthy fishing moratorium for the entire Yangtze. "If we can't control
overfishing, all other approaches will fail," says IHB's Xie.
The IHB researchers know all too well what it's like to watch a species fade
into oblivion. In 1978, IHB ecologist Chen Peixun was tapped to form a baiji
study team. Within months, the team deduced that the species was in
trouble—"and that it would go extinct if we didn't do anything," Chen says. She
puts the blame squarely on human activity. During the 1990s, about 40% of known
baiji deaths were caused by illegal electric fishing gear. Many animals also
died from run-ins with ships, and others were killed when sandbars—a favorite
hangout for cetaceans—were blasted for easier navigation.
IHB researchers gleaned what they could from a captive male
baiji named Qiqi. The aim was to understand the creature well enough to
transfer rescued animals to Tian-E-Zhou, which in 1992 was designated a
national preserve for baiji and finless porpoises. But it was too late for the
baiji. Yangtze surveys charted a decline from 11 sightings in 1997 to two in
1999. Then in 2002, Qiqi died of old age, apparently. That was the last baiji
Chen and other scientists laid eyes on. In 2006, not a single baiji was spotted
in an exhaustive survey, making it the first human-caused extinction of a
cetacean (Science, 22 December 2006, p. 1860).
Several measures may avert a similar fate for the Yangtze
porpoise. More sanctuaries could help, for starters. The porpoises reproduce
readily in Tian-E-Zhou: In 2008, all five mature females there gave birth. But
small reserves can become a death trap. In early 2008, unprecedented in recent
memory, much of Tian-E-Zhou's surface froze over for 2 days. Five porpoises
asphyxiated, including three pregnant females. "If the ice had lasted much
longer, all the porpoises would have died," says IHB's Hao Yujiang.
Food supply is another constant worry. Tian-E-Zhou is 250
kilometers east of Three Gorges Dam. Before 2003, when the reservoir behind the
dam began to fill, the lake's connection to the Yangtze lasted the whole rainy
season, from May to October. Now, that flow occurs only during the rainiest
months of June and July. A too-brief connection suppresses fish spawning, says
Hao, so researchers must stock Tian-E-Zhou with fish for the porpoises to
Elsewhere, the main threats to the porpoise are overfishing,
which reduces food supply, and the illegal use of nets slung across the river,
which ensnare the mammals. Yangtze fish stocks have plunged since the 1950s,
possibly limiting the porpoise's recovery, says Dudgeon. The key to survival
may be a blanket fishing moratorium. At a workshop in Chongqing last September,
IHB's Cao Wenxuan, one of China's most esteemed fish specialists, called for a
10-year fishing ban for the whole Yangtze. That would be feasible, Wang argues,
because the 100,000 tons of fish hauled from the Yangtze each year is less than
1% of China's freshwater production, including aquaculture.
There is no sign that authorities are ready to take such a
step in the near future. That riles Dudgeon, who penned a requiem for Yangtze
ecology in the March/April issue of Aquatic Conservation. "How many icons do we
need to lose?" he asks.