Dynamite and one form of chemical treatment are out as options for ridding Diamond Lake of unwanted tui chubs and the unhealthy water they help create.

The Umpqua National Forest has narrowed to four the number of alternatives now being studied for addressing the chub and water-quality problems plaguing the lake.

Using dynamite to shock the fish to death appeared to be too logistically difficult and joined the scrap heap of about two dozen other alternatives, said Sherri Chambers, who is heading a multi- agency group studying the alternatives.

Using the chemical antimycin, which was thought to kill the fish while harming fewer frogs, insects and other "non-target" species, proved ineffective and expensive, said Scott Lightcap, a Bureau of Land Management fisheries biologist working on the project.

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Diamond Lake’s high pH levels and relatively cool fall temperatures required such high doses of antimycin that more non-target species would die, Lightcap said.

"The low temperatures combined with the high pH in the lake was a 1-2 punch against antimycin."

The knockout blow was that it would have cost about $4.2 million for enough antimycin, which is sold under the brand name Fintrol, to treat the 48,000 acre-feet of water in the eastern Douglas County lake, Lightcap said.

That leaves the more lethal rotenone as the only chemical option now under consideration in the Forest Service study addressing the environmental impacts of the chubs in the lake.

The Forest Service’s preferred option is to draw the lake down 8 feet, then mechanically remove as many of the estimated 30 million chub in the lake and then kill and remove the remaining fish with rotenone.

A second option is to rotenone the lake, then develop different fish-stocking strategies that would improve the lake’s famous trout fishery as well as protect water quality.

A strategy pressed by some environmental groups calls for mechanical and biological strategies for reducing chub numbers without chemical treatment. Estimates suggest this option would not rid the lake of chubs, but remove enough over a six-year period so predatory fish could keep the population in check.

A fourth option calls for no action, a consideration required under federal law.

The draft study is due out in February.

Discovered in 1992 and likely introduced illegally by an angler using them as bait, the chubs have overrun the lake and now represent 92 percent of the lake’s biomass.

The chubs out-compete trout for food and space, and they eat so much zooplankton that they trigger toxic algae blooms that have led to swimming closures each of the past three summers because of health risks.

Reach reporter Mark Freeman at 776-4470, or e-mail mfreeman@mailtribune.com