In September 2006, Oregon officials took the drastic step of treating Diamond Lake with rotenone after an invasive fish had pushed the lake’s trout fishery into collapse. The fish was tui chub, a minnow not native to Diamond Lake that was believed to have been illegally introduced as live bait. By the time the treatment began, the population had grown dramatically. Oregon Department of Fish and Wildlife estimates put it at about 90 million young fish and 8 million adults. The chub competed with rainbow trout for food, disrupted the lake’s food web and contributed to harmful algae blooms that repeatedly affected the lake’s water quality. The Oregon Department of Fish and Wildlife’s restoration record says the September treatment was carried out to eliminate the tui chub population and restore both water quality and the recreational trout fishery.Diamond Lake had already been known for trout fishing for decades. Rainbow trout had been stocked there since 1910, turning a naturally fishless lake into a popular fishing destination. Then the tui chub appeared again in 1992, and the situation changed quickly. The small fish multiplied, consumed large amounts of zooplankton and other food that supported the lake’s wider ecosystem, and helped create conditions in which algae flourished. Toxic blooms forced parts of the lake to close to water contact during several summers in the early 2000s, while the trout population lost the food supply it depended on. By 2006, officials were facing a problem that went beyond routine fisheries management. They were trying to restore water quality and rebuild the stocked trout fishery that had made the lake a popular destination.How the invasive fish took overThe tui chub population was too large for ordinary fishing methods. In 2005, crews used a combination of mechanical and chemical methods to remove about 103,000 pounds of tui chub from Diamond Lake. The effort was part of the preparation for the full-lake rotenone treatment that followed in 2006, according to the Oregon Department of Environmental Quality’s account of the restoration. The lake was then treated with rotenone in September. Rotenone is a fish toxicant derived from plants, and at the concentrations used in the project it was intended to kill the fish in the lake while breaking down relatively quickly afterward.The treatment followed years of planning, environmental review and discussions among state and federal agencies. This was not the first time Diamond Lake had undergone such a treatment. The lake had also been treated with rotenone in 1954, after tui chub caused an earlier collapse in the trout fisheryThat first effort led to decades of productive trout fishing. When the chub returned in the 1990s, managers chose another full-lake treatment because earlier attempts to keep the population down had not worked. In 2006, the entire fish population was removed, and crews then monitored the lake closely to make sure the chemical had dissipated before fish were put back.The treatment also changed conditions in the broader ecosystem. The invasive chub had eaten large amounts of zooplankton, including organisms that normally fed on algae. Once the fish were gone, those parts of the food web began to recover. Water quality improved, and monitoring found that the lake recovered rapidly in many respects. The ODFW post-treatment monitoring reports that water-quality standards were being met again and that a productive trout fishery was being restored.
Diamond Lake in Oregon. Credits – Wikimedia Commons
Bringing the trout fishery backWith the tui chub gone, the next challenge was rebuilding the fishery that had made Diamond Lake famous. ODFW planned to restock the lake with rainbow trout in spring 2007, including fingerlings and larger catchable fish. The idea was not simply to put trout back into the water and walk away. Agencies continued tracking water quality, insects, zooplankton and fish populations to see whether the lake’s food web was recovering as expected.The results showed clear improvement. The lake’s water quality improved, aquatic organisms began returning and trout once again had access to a much healthier food supply. By 2011, ODFW reported almost 400,000 rainbow trout in Diamond Lake, with about half measuring more than 16 inches. The restoration went well beyond removing an invasive fish. It had rebuilt the conditions needed for the recreational fishery to function again. The project also underscored how costly invasive-species control can be. The 2006 project cost close to $6 million and required years of planning, monitoring and cooperation among several agencies and local groups. Keeping the lake free of another invasive fish became part of the work after the treatment. Officials increased public education, encouraged boaters to clean equipment and monitored the water for signs of new introductions. A later discovery of other non-native fish showed that removing one invasive species does not end the need for vigilance.Diamond Lake’s history is unusual because the same lake had already experienced a similar collapse and recovery more than half a century earlier. The return of tui chub in the 1990s repeated an old problem on a much larger scale, eventually leading officials to treat thousands of acres of water with rotenone. What happened at Diamond Lake was not simply a matter of treating the lake to kill invasive fish. It was a large-scale attempt to reset an ecosystem that had been pushed badly out of balance. The project showed how difficult and expensive it can be to undo an invasive-species problem after it has taken hold, but it also showed that a damaged fishery can recover when the source of the disruption is finally removed.
