FISHBIO, on behalf of the City of Marysville and HDR, performed the removal of invasive common carp (Cyprinus carpio) from Ellis Lake, a reservoir in Marysville, CA. While Ellis Lake provides fishing and other recreational opportunities to the public, it is negatively affected by seasonal algal blooms that decrease both the aesthetic value of the park and water quality for fish species that are popular targets of recreational fishing, such as largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus). The carp removal occurred prior to the annual spawning period, as indicated by the presence of gravid females, and before a planned application of Phoslock®, a water treatment intended to bind phosphorus and make this essential nutrient inaccessible for algal growth. While nutrient-binding treatments can be effective on their own, the removal of common carp from the lake is expected to increase efficacy of this treatment by reducing the amount of nutrients (such as phosphate) that become resuspended in the water during carp foraging activity, which disturbs sediment on the lake bottom.
Over the course of several days in 2022, FISHBIO staff used a combination of electrofishing and gillnetting methods to remove more than 250 carp with a combined weight of approximately 3,500 pounds, which were humanly euthanized and disposed of. Other incidentally captured species were temporarily held in live wells and subsequently released back into Ellis Lake. Sections of the lake were subdivided using block nets, and isolated areas were sampled three times to generate a removal estimate of abundance. The removal estimate suggested a 55% probability of capture during any given electrofishing pass, and an overall carp removal efficiency of 91% after three passes.