Water Quality and Biological Response Following Rotenone Treatment of Diamond Lake, Oregon

Diamond Lake, OR is a 1226 ha lake in the Cascade Range with a mean depth of 6.9 m.  The lake was historically fishless and was stocked with rainbow trout circa 1910.  The trout fishery was a major success until an invasive cyprinid, tui chub (Gila bicolor) was introduced, probably as discarded bait fish.  The invasive fish was eradicated with a rotenone treatment in 1954 and the lake was restocked with rainbow trout.  This fishery once again flourished, but tui chub were found in the lake in 1992 and plans were initiated in 1994 to conduct a second rotenone treatment.  There was extensive opposition to the proposed treatment, but opposition greatly decreased following the onset of cyanobacteria blooms beginning in 2001.  Plans were made to conduct a whole-lake rotenone treatment which was carried out in September 2006.  The treatment was successful in eradicating all fish.  Short-term responses included near-eradication of remaining invertebrates and development of an intense bloom of Anabaena, presumably fueled by the nutrients made available during decomposition of the tui chub carcasses.  Decomposition continued through the winter under ice and in spring 2007, a pulse of ammonia supported an intense blooms of Synedra.  By July 2007, large cladocerans (Daphnia pulicaria) experienced a major resurgence, reaching almost 80,000 individuals per cubic meter despite having been nearly absent in the lake during the preceding decade.  During this time, Secchi disk transparency reached 12.5 m and cyanobacteria were nearly eliminated.  Benthic invertebrates recovered more slowly, but by July 2008 they had reached densities of about 38 g/m², which represented a 20-fold increase over pre-treatment values.  The benthic invertebrate population before treatment was comprised largely of chironomids; following treatment, the community exhibited extensive diversification to include major increases in amphipods, molluscs, and numerous non-dipteran insect larvae.  Growth of the newly stocked trout was impressive, with June-October rates reaching 1.3 mm/d.  Average condition factor pre-treatment after tui chub detection (1992-2004) was 1.04 and post-treatment  (2007-2010) was 1.5.  Water quality improvements were equally impressive, with declines in average summer epilimnetic pH from 9.5 in 2006 to 8.2 in 2009 and transparency increasing from 1.9 m in 2006 to 7.6 in 2009.  The lake was removed from the state 303(d) list for impaired waterbodies.  Diamond Lake is noteworthy for being the first lake in the United States to meet its TMDL targets exclusively through eradication of an invasive fish species.