

Video: Nuisance fish Video: Nuisance fish

Ichthyocide on a massive scale (Image: M. Spencer Green/AP)

Sometimes the “few” are made to suffer to protect the many. Tens of thousands of fish were poisoned last week in a drastic attempt to keep invasive Asian carp out of the Great Lakes.

Officials poured more than 8000 litres of the fish poison rotenone into a 9-kilometre stretch of the Chicago Sanitary and Ship Canal, which links the Mississippi river and Lake Michigan, during maintenance work on a barrier that normally keeps invaders at bay using electric shocks.

It is feared that if the voracious bighead and silver carp are not kept out, they will out-compete the native species crucial to the lakes’ sports and commercial fishery, worth $7 billion per year.


“There are some stretches on the Mississippi and Illinois rivers where they have taken over the biology entirely,” says Stacey Solano of the Illinois Department of Natural Resources. “We can’t afford not to do everything possible to keep the Asian carp out of the Great Lakes.”

Sinking feeling

Rotenone, derived from plant roots, is highly toxic to fish but only mildly toxic to humans. It is commonly used by US government officials to kill unwanted fish in rivers and lakes. The toxin enters the bloodstream of fish through their gills and inhibits cellular respiration.

So far only one bighead carp has been found among the thousands of dead fish removed from the canal, but authorities say carp tend to sink when poisoned.

DNA material from the carp has been found in water samples collected both above and below the electric barrier. This material could have come from scales, faeces, or ballast water that was carried past the barrier.

There are fears that it may already be too late to keep them out, but none has yet been found on the lake side of the barrier.

Barrier two

Asian carp, which can grow to about 1.5 metres long and 50 kilograms, were introduced to aquaculture ponds in the southern US in the 1970s but escaped and have slowly made their way up the Mississippi.

The US Army Corps of Engineers has begun construction of another high-voltage electric barrier that will obviate the need for the use of poison in the future.

Mike Cox, Michigan’s attorney general, said on Monday that the state was preparing a lawsuit seeking to have the canal closed, at least temporarily, to protect its fishing and tourism industries.