In September 2006 Oregon officials carried out a large scale treatment of Diamond Lake after an invasive fish species caused the lake’s trout fishery to collapse and contributed to recurring water quality problems.
The invasive fish, tui chub, is a minnow that is not native to Diamond Lake and was believed to have been illegally introduced as live bait. By 2006, officials estimated the population had reached about 90 million young fish and 8 million adults.
The chub competed with rainbow trout for food and consumed large quantities of zooplankton, disrupting the lake’s food web. Their presence also contributed to conditions that encouraged harmful algae blooms, leading to repeated water-contact advisories during the early 2000s.
Diamond Lake had been a popular trout fishing destination for decades. Rainbow trout had been stocked there since 1910, transforming the naturally fishless lake into a major recreational fishery. Tui chub had also caused a similar collapse in the past, prompting a rotenone treatment in 1954.
Before the 2006 treatment, crews attempted to reduce the invasive population using mechanical and chemical methods. In 2005, around 103,000 pounds of tui chub were removed. However, these measures were not enough to control the population, leading authorities to undertake a full-lake treatment.
Rotenone, a fish toxicant derived from plants, was applied to the lake in September 2006. The treatment was designed to eliminate the fish population, with the chemical expected to break down relatively quickly at the concentrations used.
The project followed years of planning, environmental reviews and coordination between state and federal agencies. After the treatment, officials monitored the lake to ensure the rotenone had dissipated before fish were reintroduced.
The removal of tui chub also helped the lake’s wider ecosystem recover. With the invasive fish gone, zooplankton and other aquatic organisms began returning, water quality improved and the food supply needed by trout recovered.
Rainbow trout were restocked beginning in 2007. Continued monitoring focused on water quality, insects, zooplankton and fish populations to determine whether the lake’s ecosystem was recovering.
By 2011, the Oregon Department of Fish and Wildlife reported nearly 400,000 rainbow trout in Diamond Lake, with roughly half measuring more than 16 inches.
The restoration came at a significant cost. The 2006 project cost nearly $6 million and involved years of monitoring and cooperation among government agencies and local groups. Officials also increased public education and encouraged boaters to clean their equipment to reduce the risk of future invasive species introductions.
Diamond Lake’s experience illustrates that removing an invasive species from a large water body can require extensive intervention, long term monitoring and public cooperation. The case also highlights an important ecological complication: while the tui chub was invasive, the rainbow trout fishery being restored was itself introduced rather than native to the lake.
For the communities around Diamond Lake, the recovery extended beyond ecology. The return of the trout fishery helped restore recreational fishing and supported businesses such as resorts, campgrounds and local outfitters that depend on seasonal visitors.
The Diamond Lake project is therefore regarded as an example of a large scale invasive species intervention in which chemical treatment was followed by sustained ecosystem monitoring, fish restocking and prevention efforts.