South Twin Lake will soon become the latest in a series of Central Oregon water bodies to be treated for invasive fish species by poisoning the entire lake.

In early September, the Oregon Department of Fish and Wildlife suspended all daily catch limits, possession limits and minimum length requirements at South Twin Lake, allowing anglers to keep as many fish as they want before the lake closes to fishing on Oct. 24, one week earlier than usual.

That week, the ODFW will treat the lake with the plant-derived poison rotenone, effectively killing all fish in the popular angling lake. South Twin will be restocked with catchable rainbow trout as soon as it is accessible in spring 2012, according to Mike Harrington, a Bend-based fish biologist for ODFW.

“I was up there right after Labor Day, and that was a very high-pressure weekend,” Harrington said of South Twin. “(Anglers) really hit it hard. People were pretty successful. We still want people to go out and fish hard. We want them to get full opportunity for the resource.”

The department’s goal is to remove illegally introduced bullhead catfish and three-spine stickleback, thereby restoring the rainbow trout fishery in the 120-acre lake, located 44 miles southwest of Bend and just north of Wickiup Reservoir.

Since the unsanctioned introduction of bullhead catfish at South Twin in the early 2000s, trout fishing has declined as the population of bullhead has increased, according to the ODFW. The bullhead compete for food with the stocked rainbow trout, and they eat small trout.

By early summer 2012, Harrington said, the trout fishing should once again be excellent at the lake, a favorite of fishing families.

At the concentrations used to kill fish, rotenone is not toxic to humans, mammals or birds, according to the ODFW. It breaks down completely in the environment and is undetectable within weeks of treatment.

Rotenone was used by the ODFW to restore trout populations in Central Oregon’s Walton Lake and Antelope Flat Reservoir in 2009, and, on a much larger scale, in Diamond Lake in Southern Oregon in 2006.

Anglers on those lakes reported excellent fishing after the lakes were restocked with rainbow trout.

Joann Frazee, owner of Twin Lakes Resort, said that South Twin has not had much pressure from fishermen during the past couple of weeks. She is confident the rotenone process will improve fishing in the lake.

“There’s species in there that shouldn’t be there,” Frazee said. “I think it’ll improve the conditions of the lake for the trout that are reintroduced. The trout will have more food, and they will be healthier and produce better, I think.”

Harrington said that North Twin Lake, less than a mile north of South Twin, actually contains a larger population of bullhead. But because South Twin hosts more anglers, the ODFW figured treating South Twin would provide a greater economic benefit.

“We have a higher probability of success on South Twin,” Harrington explained. “With the lower numbers of bullhead, they can’t metabolize the chemical as well. With a lot of fish, the chemical loses its potency. With South Twin, we can treat it once and most likely get a complete kill.

“We feel we have the ability to head it off before it does become a problem. It’s not a huge problem yet in South Twin.”

Harrington said the reintroduced rainbow trout will have a higher growth rate, allowing for bigger fish by next summer. The ODFW will also return to its fingerling stocking program for South Twin, which it had to abandon because the bullhead preyed on the fingerlings.

South Twin Lake is popular with fly anglers, trollers and bank anglers, who like to simply fish with worms and relax. Bank access is available around most of the lake.

South Twin has been treated before with rotenone, Harrington noted. The chemical has been used on five prior occasions (1941, 1957, 1965, 1972 and 1987) to rid the lake of tui chub, another illegally introduced invasive species.

Harrington said rotenone, which comes from stems and roots in the jewel vine or flame tree of South America, has been used for thousands of years.

“Indigenous tribes in South America discovered it,” Harrington said, “and used it to poison fish so they could harvest the fish and eat them.”

For more information, visit www.dfw.state.or.us or call 541-388-6363.