Ecology Of Kokanee Salmon And Rainbow Trout In Crater Lake A Deep Ultraoligotrophic Caldera Lake Oregon PDF Download

Crater Lake, originally barren of fish, was stocked on an irregular basis from 1888 through 1941 with several species of salmonids. Two species occur in the lake today--kokanee salmon (Oncorhynchus nerka) and rainbow trout (Salmo gairdneri). This study was initiated in the summer of 1986 to evaluate the ecology of adult fish in terms of length, weight, age, growth, morphology, food habits, and distribution in Crater Lake relative to the lake's limnological characteristics. Fish were captured with gill nets, by angling, and with a modified downrigger. Age determinations from scale analysis, supported by modal progressions in length frequency histograms indicated that kokanee salmon age composition was heavily dominated, in number, by the 1984 year class. Spawning by members of this cohort was recorded in January 1988. Both species exhibited growth rates comparable to other northwest populations in oligotrophic lakes. Food resources were partitioned in that kokanee salmon generally fed on small-bodied taxa (mean weight 1.2 mg) from the midwater column and from the lake bottom, rainbow trout fed on large-bodied taxa (mean weight 9.8 mg) from the lake surface and the lake bottom. Distribution and diel migrations of fish were assessed with hydroacoustic techniques during the first week in September 1987. Fish underwent diel migrations within and between the nearshore (0 m to 100 m contour) and offshore (100 m to 589 m contour) zones of the lake. Based on capture records, it appeared that kokanee were primarily offshore and in deep water during the day, and then they moved shoreward into shallower water at night. Rainbow trout appeared to remain nearshore, in shallower water during the day than at night. The maximum depth for an acoustic target was 98.5 m. The maximum depth of capture for kokanee in Crater Lake was 86.25 m.