Effects of cyprinid removal and reintroduction: Diamond Lake, Oregon

Abstract Eilers J, Miller R, Loomis D, Vogel A. 2023. Effects of cyprinid removal and reintroduction: Diamond Lake, Oregon. Lake Reserv Manage. 39:156–173. Diamond Lake, located in the Oregon Cascade Range, was treated with rotenone in 2006 to remove invasive populations of cyprinids, Gila bicolor and Notemigonus crysoleucas. The treatment successfully removed all fish, and the lake was restocked in 2007 with rainbow trout (Oncorhynchus mykiss). The treatment resulted in large increases in transparency, large cladocerans (Daphnia pulicaria), and benthic invertebrates. The previous cyanobacterial blooms were comprised almost exclusively of Anabaena [Dolichospermum], whereas the current populations of cyanophytes include Gloeotrichia. Cyprinids were reintroduced into the lake and documented in 2008 (Notemigonus crysoleucas) and 2015 (Gila bicolor), likely contributing to a decline of several metrics of water quality. Piscivorous trout (Salmo trutta and Salmo trutta x Salvelinus fontinalis) were introduced into the lake starting in 2016 to control the introduced cyprinids. The cyprinid populations have stabilized, and most metrics of lake status (Secchi disk transparency, phytoplankton biovolume, abundance of large cladocerans, zoobenthic biomass, trout condition factor) indicate that the lake has improved substantially since the treatment and introduction of piscivorous trout. It is unclear whether the cyprinid populations are constrained by behavioral mechanisms associated with the introduction of the piscivorous trout or whether other factors currently keep the cyprinids in check. The success of this biomanipulation project requires continued monitoring and use of adaptive management strategies to respond to changes in fish composition.