Biological Progress and Water Quality Reversals Following a 2006 Rotenone Treatment of Diamond Lake, Oregon (USA)

Diamond Lake, a 1226 ha lake in the Oregon Cascades was treated with rotenone in 2006 to eradicate the invasive tui chub (Gila bicolor) that had displaced a stocked fishery of rainbow trout (Oncorhynchus mykiss).  The high biomass of tui chub had also increased internal loading of nutrients, resulting in severe blooms of cyanobacteria, largely Anabaena. The treatment successfully eradicated the tui chub and subsequent recovery of the lake was rapid.  The trout stocking program resumed with excellent growth rates and condition factors.  Biomass and diversity of benthic invertebrates increased dramatically, recovery of large cladocerans (Daphnia pulicaria) occurred in less than a year, and transparency quadrupled as cyanobacteria blooms disappeared.  Although benthic biomass and zooplankton density remained high, water quality began to diminish several years following treatment.  Tui chub had been eradicated, but another cyprinid, golden shiner (Notemigonus crysoleucas), increased in abundance.  It is unknown whether the shiners, which were present in the lake before the treatment, had escaped eradication or they were intentionally introduced following treatment.  As a consequence of increasing abundance of shiners, cyanobacteria blooms (now largely Gloeotrichia echinulata) have returned and water quality standards are routinely exceeded.