Grass carp: One invasive species targets another

Just as one needs fire to fight fire, it can take one invasive species to extinguish another.

Meet the grass carp, an Asian native fish that's generally considered an unwelcome interloper in North America. If left to its own devices, it would out-compete native species and dominate any body of water in which it swam.

This afternoon, however, a mess of grass carp will be dumped into a small pond in Henrietta, where they are expected to begin eradicating an undesirable aquatic plant that was found there not long ago.

The invasive fish will eat the invasive plant, or so the plan goes.

But not to worry. While "natural" grass carp are banned in New York state because of their capacity to overwhelm native fish, the specimens being tossed in the pond at Tinker Nature Park on Calkins Road are sterile "triploid" grass carp. The state Department of Environmental Conservation allows them to be used for aquatic vegetation control because, in the end, the carp will die out and the ecosystem in which they were planted should return to normal.

The introduction of grass carp, which is scheduled for about 1 p.m. Wednesday, is necessitated by the surprising discovery three weeks ago that the small pond was infested with hydrilla. The Eurasian import, sometimes called the Godzilla of invasive aquatic species, grows so fast and thick that it can kill off native plants, disrupt the habitat for native fish and birds and foul boat propellers.

It has been found at the southern end of Cayuga Lake, the western end of the Erie Canal and only a few other spots in New York. No one had any idea it was in the Henrietta pond until Hilary Mosher, the coordinator of the Finger Lakes Partnership for Invasive Species Management, stumbled on it while taking a walk with her son.

Mosher said Tuesday that after consultation among various state, county and town officials, it was decided to try the grass carp, whose introduction is being funded by the Monroe County Soil and Water Conservation District. The release will be made during a day-long workshop that Mosher's group pulled together for people interested in aquatic invasive species control. About 30 people have registered for the workshop.

The carp, which can grow to 25 pounds in size, will be left in the pond for quite some time. "It will likely take several years to completely remove the plant," Mosher said. A fence will be installed to prevent the fish from trying to make their way into the pond's small outlet stream.

A second step, planned for later this fall or next spring, will be to place large mats on the floor of the pond to block the growth of new plants. That's being paid for the DEC, Mosher said.

The mats will lessen the likelihood that small plants or plant fragments will float loose in the water, where they could be inadvertently picked up and carried off by geese or other birds, thus spreading the dreaded plant to other bodies of water.

So far, there have been no reports of hydrilla nearby water bodies, which include the canal and the ponds at Mendon Ponds Park. Mosher said a plan is being developed to monitor those areas.