Use of Thermal Otolith Marks to Manage Kokanee in an Oligotrophic Central Washington Lake

Banks Lake is located in central Washington State and was considered a premier kokanee fishery in the 1960’s and 1970’s. By 1981 kokanee had disappeared from the creel and despite annual stocking since 1992, the fishery did not rebound to historic levels. An evaluation program was funded in 2002 to determine if the kokanee population in the lake could be increased by implementing different rearing and stocking methods. Historically, approximately one million kokanee were stocked in the spring as fry. We employed a stocking regime using three treatment groups; spring fry releases, fall fry releases, and net pen reared spring yearling releases. Unique thermally induced otolith marks were applied to each release group with net pen fish receiving an additional fin clip. Thermal marking was first applied to pacific salmon in 1985 and is now widely used by researchers around the Pacific Rim. When salmonids are artificially incubated at differing water temperatures it becomes possible to place recognizable thermal marks in their otoliths that remain visible through the life span of the fish. Kokanee were thermally treated at the eyed-egg life stage to apply group specific otolith codes over four consecutive years (brood years 2002-2005) and collected near release locations as adults using gill nets and boat electrofishing. Results indicated that survival from fry to adult was the highest for kokanee released as fall fry (p < 0.001) with recaptures values far exceeding the expected recapture values in the chi-square analysis. Survival of net pen yearling releases was relatively successful (88% recapture rate), but moderate for spring fry releases (60% recapture rate). Future kokanee releases in Banks Lake should consist of fall fry and be monitored to detect increases in the population and harvest in the creel.