Mechanical Harvest to Reduce Fish Biomass and Improve Water Quality in Lemolo Lake, Oregon

Lemolo Lake is a 182 ha hydropower impoundment on the North Umpqua River, located 14 km downstream from Diamond Lake.  In 2006, Lemolo Lake began to experience intense cyanobacteria (Anabaena) blooms.  One hypothesis was that the onset of the blooms could have been caused by an abrupt increase in biomass of tui chub entering Lemolo Lake during the drawdown of Diamond Lake.  An experimental program was initiated in 2008 to try and reduce the biomass of tui chub through a trap-netting program in an effort to reduce internal loading of nutrients from fish excretion and improve water quality. The netting program removed 29,670 kg of tui chub (163 kg/ha) and water quality improved.  However, water quality began to deteriorate again, coinciding with a large increase in kokanee.  The removal of much of the tui chub population apparently reduced competition for the kokanee, thus allowing a competitor to benefit.  The project illustrated that water quality in the lake was sensitive to changes in fish biomass and modeling showed that biomass of tui chub and kokanee through entrainment during deep winter drawdown could be used as a tool to suppress fish biomass and maintain water quality goals.