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Originally barren of fish, Crater Lake was stocked with approximately 1.8 million salmonids from 1888 to 1941. Rainbow trout (Oncorhynchus mykiss) and kokanee salmon (O. nerka) now inhabit the lake. This study was conducted from 1986 to 1991 to document and compare kokanee salmon and rainbow trout ecology in the lake to better evaluate the ecological implications of the presence of these non-native fish. Kokanee salmon exhibited cyclic patterns in population age structure, condition, abundance, and biomass from 1986 to 1991. One dominant year class of relatively low abundance and high condition was present from 1986 to 1987. Multiple year classes with increasing abundance and decreasing condition were present from 1989 through 1991. Rainbow trout maintained a diverse population structure throughout the study with a trend toward a relative increase of older age classes and larger fish. Vertical and horizontal migrations of kokanee salmon occurred within and between the nearshore and offshore zones of the lake. Rainbow trout were located along the edge of the lake. Kokanee salmon fed primarily offshore on zooplankton and small-bodied insects. Kokanee salmon cropped the Daphnia population and altered the zooplankton community structure during the study period. Rainbow trout fed nearshore on large-bodied vertebrates and invertebrates. Introduced fish in Crater Lake exhibited the potential to impact limnetic and benthic community structure and nutrient flux within and between these communities.
The Fish of Crater Lake
By Mark Buktenica
Every summer, scores of visitors head for the blue waters of Crater Lake with fishing poles in hand and dreams of pulling a monster catch from the depths of this mystical lake.
And, even though Crater Lake was originally barren of fish, there are opportunities to catch a fish or two thanks to the 1.8 million trout and salmon stocked in the lake between 1888 and 1941. The species stocked included rainbow trout, brown trout, cutthroat trout, steelhead trout, coho or silver salmon, and kokanee salmon. Today, only a couple species survive – kokanee salmon, the landlocked form of sockeye salmon, and rainbow trout. These populations sustain their numbers through natural reproduction as no stocking has occurred since 1941.
Trout and salmon prefer to reproduce in the running waters of streams and rivers to keep the developing eggs bathed in fresh, well-oxygenated water. Rainbow trout (Oncorhynchus mykiss) and kokanee salmon (Oncorhynchus nerka) have been reproducing successfully in Crater Lake’s still water because it is also very clean, cold, and rich in oxygen.
The great clarity and pristine conditions of Crater Lake have led to the common misbelief that the fish are starving. But, the results of the park’s on-going fisheries research indicate that Crater Lake fish growth rates are comparable to rates of the same species in other high mountain lakes in the southern Cascade Range. Kokanee Salmon is the most abundant species in the lake, recently estimated to have a population well into the hundreds of thousands. Kokanee salmon reach 10-14 inches in length and feed on small-bodied insects and zooplankton. Zooplankton are minute animals (ten fit on the head of a pin) that live in open water. Kokanee do not filter feed on the zooplankton as whales do on krill, but visually seek and pursue their tiny prey. Rainbow trout reach 17 to 24 inches in length and feed primarily on large-bodied insects that live in the lake or fall onto the lake surface. The largest documented rainbow trout from Crater Lake was 6-1/2 pounds and 26 inches long.
The introduction of fish as top predator in the Crater Lake food chain has undoubtedly changed the ecology of the lake from its natural pre-fish condition. Fish can alter the kinds and numbers of other animals and plants that live in the lake through new food web interactions. The magnitude of these changes is affected by the number of fish in the lake. The Crater Lake fish population is considered relatively small and it is probably here to stay. All currently reliable methods of large-scale fish removal would alter Crater Lake’s ecosystem far more than the fish do.
Fish in Park Streams
Although the lake is by far the park’s largest body of water, fish also inhabit many of the small streams within the park’s boundary. These streams are generally not accessible because of the steep canyons in which they are found.
According to stocking records, two species, eastern brook and rainbow trout, were planted in park streams. However, four species total have been identified:
Eastern Brook Trout (Salvelinus fontinalis) have been found in almost every park stream.
Rainbow Trout (Oncorhynchus mykiss) were originally planted in large numbers throughout the park. Today, it appears that their numbers are few and scattered. They have been collected in recent years from Annie, Bybee, Castle, Munson, and Sun creeks.
German Brown Trout (Salmo trutta) had one representative specimen found in Sand Creek above the falls, which appears to be a barrier preventing upstream migration. Researchers believe that this fish may be the remnant of an unrecorded or unauthorized planting.
Bull Trout (Salvelinus confluentus) are understood to be the only native fish species found within the park. These less competitive fish are a candidate species under the endangered species act, and are considered rare in the Southern Cascades. Programs to conserve this species are now being implemented.
Kokanee (Oncorhynchus nerka) Since there are no stocking record of kokanee in Crater Lake, it is unclear if early fisheries investigations collected one or both of the Pacific salmon Species. The other species being coho salmon, which were stocked in the lake. The kokanee were apparently few in number, since none were found until 1947. Even though there was no record of the kokanee being stocked in the lake, it is speculated that this species was reproducing and had become established there.
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