Biological Control of the Parasitic Copepod Salmincola californiensis in a Commercial Trout Hatchery on the Lower Merced River, California

In the early 1990s, a serious infestation of the copepod Salmincola californiensis developed in rainbow trout Oncorhynchus mykiss held in a large commercial trout farm near Merced, California. Laboratory trials demonstrated that brook trout Salvelinus fontinalis held in experimental aquaria upstream of the rainbow trout effectively removed copepodid larvae from the water and substantially reduced parasite infestation in the rainbow trout by more than 89%. These encouraging laboratory findings prompted a large‐scale commercial hatchery application. A brook trout population was established in the incoming water supply and upper concrete production ponds of the hatchery, and all lower production ponds were depopulated for a 2‐week period. Uninfested rainbow trout were then reintroduced into the hatchery production ponds. Monthly pathological examinations of reintroduced rainbow trout and daily observations have not identified copepods on any rainbow trout in the facility following more than 2 years of exposure to lower Merced River water, which is populated with copepod‐infested rainbow trout. We suspect that the lower Merced River contains a relatively low density of copepodid larvae compared with those in the limited water volumes of the laboratory trials, and this low density may not be sufficient to override the capacity of the brook trout to eliminate the parasite. These studies demonstrate one practical method for the successful rearing of rainbow trout in copepod‐infested waters.