Authorities in the US have scooped up poisoned fish floating to the surface of a Chicago-area waterway in an operation designed to keep invasive Asian carp out of the Great Lakes and prevent an ecological disaster.

Illinois officials said a single Bighead carp, one of two prolific species of Asian carp viewed as a threat, had turned up in the huge fish kill that began overnight along 10 kilometres of the Chicago sanitary and ship canal south-west of the city.

Poison was dumped into the waterway so maintenance could be performed on an electrical barrier designed to keep the carp out of the Great Lakes.

The Asian carp was found 64 kilometres from Lake Michigan, the closest to the Great Lakes the species has been found, authorities said.

About 90 tonnes of dead fish are expected to be collected, weighed, inventoried, and dumped in a landfill over the next few days. Most of the dead fish scooped up so far have been native carp and shad.

Silver carp and the Asian Bighead, which can grow to 1.5 metres and weigh more than 45 kilograms, have come to dominate sections of the Mississippi River and its tributaries.

Authorities fear that if the carp reach the Great Lakes, the largest fresh-water resource in the world, they could create an "ecological disaster" by consuming the bottom of the food chain and ruining the lakes' $US7 billion ($7.6 billion) fishery.

Since the 1990s, floods allowed the carp to escape into rivers from research facilities and commercial fish ponds in the South, where they were introduced to clean away weeds and other detritus.

The carp have multiplied and become a "nuisance species", according to the US Fish and Wildlife Service.

Along some stretches of the Illinois River the carp make up 95 per cent of the biomass and they are considered poor for eating or as a game fish.

Silver carp, which leap into the air when disturbed by passing motorboats, have injured boaters.

Two electrical barriers in the canal were erected in 2002 and 2006 to shock any fish, particularly carp, that try to swim up the canal to Lake Michigan.

To allow time to complete the maintenance, more than 900 kilograms of the natural poison rotenone, that prevents fish gills from absorbing oxygen, was dumped into the canal.

The toxin, which is used as a broad-spectrum insecticide and pesticide, kills fish and freshwater snails but does not harm other animals.

It dissipates within two days, though authorities planned to introduce a neutralising agent to speed up the process.

- Reuters