Problem Solvers : Carp Put to Work Clearing Weeds From Canals

On the banks of a weedy irrigation canal near this remote Imperial Valley community, a team of biologists was at work Tuesday stocking the waterway with thousands of specially cultivated fish.

The sleek bronze bodies hit the water and then dived for cover in the green clumps of weeds. It seemed a perfect home for what scientists call the triploid grass carp, a great eater of aquatic plants.

It is the carp’s voracious appetite--it weighs as much as 50 pounds and eats up to twice its weight in weeds a day--that has brought it to the Imperial Valley, for it is being introduced into the canals here as an underwater lawn mower, an environmentally safe way to rid the waterways of unwanted weeds.

“They eat everything green they can chew,” said Paul Beaty, the research biologist in charge of an ambitious project to rid the Coachella and Imperial valleys of aquatic plants that clog canals and ditches, hampering delivery of water to local farms.

The project started five years ago with the primary goal of eradicating the hydrilla, an exotic plant first discovered in California eight years ago in the All-American Canal just east of Calexico.

Brought to this country for use in home aquariums and then apparently introduced into the wild, hydrilla has cropped up in thick green mats in canals, lakes and ponds from San Diego to Shasta County, posing a threat to both navigation and irrigation.

In recent years, man has attacked the hydrilla with weapons ranging from herbicides to shovels, but these methods are expensive and often prove lethal for other plants and fish.

“This will solve the problem,” Beaty predicted Tuesday, dumping a net full of the carp into the Coachella Canal. “We expect this stocking to give us five years of control.”

State officials hope he is right.

“It is a tool we haven’t had before,” said Les Sonder, senior agriculture biologist for the state Department of Food and Agriculture. “Chemical and mechanical means (of eradication) are almost too expensive to be practical in the long run.”

Moreover, project directors said that their type of carp is sterile and could not multiply to the detriment of sport fish such as catfish and bass. In controlled numbers, the triploid grass carp would simply eat until they die of old age.

Nonetheless, some officials expressed concern that the carp do not yet have a proven track record in California and could pose a threat to naturally occurring fish if they are overstocked.

In May, the state Department of Fish and Game granted the Hydrilla Control Research Project permission to stock all canals of the Coachella and Imperial valleys with 78,000 of the carp. This was done because the canals offer little avenue of escape.

If the plan works, the $3-million project, which has been financed jointly by several county, state and federal agencies including the U.S. Department of Agriculture, may be expanded statewide, said Jim St. Amant, spokesman for the state Department of Fish and Game.

Meanwhile, “We have to watch they don’t overstock because these fish live a long time and could displace indigenous fish and animals,” St. Amant said. “There’s a good chance it will be introduced elsewhere--as long as it doesn’t reproduce.”

According to Beaty, there is no chance of that.

It was Arkansas fishery owner, Jim Malone, who found a way to render grass carp, which are native to the Amur River of China, sterile. Newly fertilized eggs are subjected to a sudden temperature change. The shock treatment results in creation of three sets of chromosomes instead of the normal two. As a result, these carp are not capable of producing viable sperm or eggs, Beaty said.

Anglers beware. The hefty fish has been declared off limits to fishermen by order of the state Department of Fish and Game.