(May 21, 2007)

Invasive fish threaten South Africa's aquatic biodiversity

By Wendell Roelf
From Reuters

CAPE TOWN (Reuters) - Predatory imported fish including trout, bass and carp are crowding out many local species in one of the few places in the world still rich in biodiversity.

First introduced in the 18th century and hugely popular with both local and foreign anglers, these fish form an integral part of a burgeoning recreational fishing industry, estimated to contribute some 18 billion rand ($2.6 billion) to the South African economy.

But they are invasive and have already made several species in the area extinct. One site in the Cape Floristic Region where indigenous species are fighting for survival is the Bot River, a tourist attraction which flows through wetlands housing waterfowl and wild horses in a fertile valley of wine farms.

"It's a sad river at the moment," said Dean Impson, aquatic scientist at Cape Nature Conservation. "These are sad rivers from an ecological point of view because they are like little deserts, the alien fish are in them and they've eliminated most, if not all, of the indigenous fish."

A new plan to rid local rivers of the fish has sparked a fresh environmental debate, and could pit the interests of fishing and tourism against those keen to preserve indigenous species.

Some conservationists are hoping to use a controversial natural poison to eradicate the invaders as part of a global effort to save the freshwater fish, which experts say are among the most threatened group of animals on earth.

Impson is among conservationists who -- with backing from the World Bank -- hope the biodegradable poison Rotenone can help shield the Western Cape from alien fish.

Other ecologists fear Rotenone, an insecticide and piscicide derived from the roots of beans, could do more harm than good in the fragile freshwater ecosystems.

Although it targets a narrow range of species it also kills insects and in rare circumstances, humans; so poses high risks for an area which is also home to a rich diversity of higher plant species, they say.


A U.N. study last year said human activities are causing the biggest wave of extinctions since the dinosaurs were wiped out 65 million years ago.

Global warming, pollution, expanding networks of cities and roads, a growing human population and invasive species -- such as rabbits in Australia -- are putting many native species under pressure.

In North America at least 150 species of fish are critically endangered or extinct, said professor Jenny Day, head of the freshwater research unit at the University of Cape Town.

In East Africa's Lake Victoria, as many as 200 species have been wiped out by just one fish, the Nile Perch.

"Extrapolated on a global scale, thousands of species of fish are likely to become extinct in our lifetime," said Day.

Many types of local fish in rivers running through the Cape Floristic Region have also fallen victim to the alien species, which were still being introduced up to the 1960s as angling and foodstock from Asia, Europe and North America.

Much smaller than the newcomers, and having evolved without naturally occurring predators, nine out of 19 indigenous species in the Western Cape are listed as critically endangered.

These include the Cape Galaxias, a relic of the ancient polar supercontinent of Gondwana which included most of the land of the southern hemisphere and broke up around 65 million years ago. The fish's closest relatives are found in Chile, Australia and New Zealand.

Another four species are listed as vulnerable in a region with an exceptionally high number of fish found nowhere else in the country.


South Africa's Cape Action for People and the Environment group is spearheading the battle against alien fish with support from the World Bank's Global Environment Facility, which helps countries fund projects to protect the environment.

A study set for June will determine Rotenone's impact on aquatic biodiversity and whether it has any adverse social or economic effects. If approved, Rotenone could be used.

If Rotenone is not approved, conservationists may turn to other methods such as electro-fishing, where an electric current is used to stun and catch fish.

Conservationists may in any case soon have another battle on their hands with local fisherman who re-stock rivers cleared of alien fish for their lucrative industry. Eugene Kruger, editor of SA Bass magazine, said the threat from the alien fish was exaggerated.

"They have only impacted on inconsequential little fish in the streams of the Western Cape," Kruger said.

(Additional reporting by Alister Doyle in Oslo)

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