Ecology of kokanee salmon and rainbow trout in Crater Lake, a deep ultraoligotrophic caldera lake (Oregon)

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

Crater Lake Revealed

Around 500,000 people each year visit Crater Lake National Park in the Cascade Range of southern Oregon. Volcanic peaks, evergreen forests, and Crater Lake’s incredibly blue water are the park’s main attractions. Crater Lake partially fills the caldera that formed approximately 7,700 years ago by the eruption and subsequent collapse of a 12,000-foot volcano called Mount Mazama. The caldera-forming or climactic eruption of Mount Mazama drastically changed the landscape all around the volcano and spread a blanket of volcanic ash at least as far away as southern Canada. Prior to the climactic event, Mount Mazama had a 400,000 year history of cone building activity like that of other Cascade volcanoes such as Mount Shasta. Since the climactic eruption, there have been several less violent, smaller postcaldera eruptions within the caldera itself. However, relatively little was known about the specifics of these eruptions because their products were obscured beneath Crater Lake’s surface. As the Crater Lake region is still potentially volcanically active, understanding past eruptive events is important to understanding future eruptions, which could threaten facilities and people at Crater Lake National Park and the major transportation corridor east of the Cascades. Recently, the lake bottom was mapped with a high-resolution multibeam echo sounder. The new bathymetric survey provides a 2m/pixel view of the lake floor from its deepest basins virtually to the shoreline. Using Geographic Information Systems (GIS) applications, the bathymetry data can be visualized and analyzed to shed light on the geology, geomorphology, and geologic history of Crater Lake

Morphology, volcanism, and mass wasting in Crater Lake, Oregon

Crater Lake was surveyed nearly to its shoreline by high-resolution multibeam echo sounding in order to define its geologic history and provide an accurate base map for research and monitoring surveys. The bathymetry and acoustic backscatter reveal the character of landforms and lead to a chronology for the concurrent filling of the lake and volcanism within the ca. 7700 calibrated yr B.P. caldera. The andesitic Wizard Island and central-platform volcanoes are composed of sequences of lava deltas that record former lake levels and demonstrate simultaneous activity at the two vents. Wizard Island eruptions ceased when the lake was ∼80 m lower than at present. Lava streams from prominent channels on the surface of the central platform descended to feed extensive subaqueous flow fields on the caldera floor. The Wizard Island and central-platform volcanoes, andesitic Merriam Cone, and a newly discovered probable lava flow on the eastern floor of the lake apparently date from within a few hundred years of caldera collapse, whereas a small rhyodacite dome was emplaced on the flank of Wizard Island at ca. 4800 cal. yr B.P. Bedrock outcrops on the submerged caldera walls are shown in detail and, in some cases, can be correlated with exposed geologic units of Mount Mazama. Fragmental debris making up the walls elsewhere consists of narrow talus cones forming a dendritic pattern that leads to fewer, wider ridges downslope. Hummocky topography and scattered blocks up to ∼280 m long below many of the embayments in the caldera wall mark debris-avalanche deposits that probably formed in single events and commonly are affected by secondary failures. The flat-floored, deep basins contain relatively fine-grained sediment transported from the debris aprons by sheet-flow turbidity currents. Crater Lake apparently filled rapidly (ca. 400–750 yr) until reaching a permeable layer above glaciated lava identified by the new survey in the northeast caldera wall at ∼1845 m elevation. Thereafter, a gradual, climatically modulated rise in lake level to the present 1883 m produced a series of beaches culminating in a modern wave-cut platform, commonly ∼40 m wide, where suitable material is present. The new survey reveals landforms that result from intermediate-composition volcanism in rising water, delineates mass wasting and sediment transport into a restricted basin, and yields a more accurate postcaldera history leading to improved assessment of volcanic hazards

High-resolution Multibeam Mapping of Crater Lake, Oregon

The bathymetry of Crater Lake was mapped using a Kongsberg Simrad EM1002 high-resolution, multibeam echo sounder in order to provide an accurate base map for ongoing limnological and biological studies and to improve understanding of geologic features and processes within the lake basin. Crater Lake occupies the caldera that formed by collapse during the climactic eruption of Mount Mazama $\sim$7,700 (calendrical) yr ago. The new survey provides a 2 m/pixel high-resolution view of lake-floor morphology virtually to the shoreline, includes coregistered, calibrated acoustic-backscatter information, and refines the post-caldera-collapse geologic history deciphered from previous echo-sounding bathymetry (1959), dredge samples, and ROV and submersible traverses. The postcaldera volcanic history began with subaerial andesite lava flows vented west of the center of the caldera and flowing north, followed by construction of the central platform above the same vent. This feature has steep (talus) slopes heading at $\sim$290 m depth from a comparatively flat surface, suggesting subaerial lava flows fragmented where they entered the deepening lake, and earlier shorelines at 378, 358, and 329 m. A similar morphology characterizes the andesitic Wizard Island volcano, west of the central platform, which shows four successive shorelines in the same manner at 358, 329, 180, and 84 m depth. These volcanoes apparently ceased to erupt before the lake filled to its present level $\sim$300 yr after the climactic eruption. Merriam Cone is a symmetrical andesite volcano (summit at 150 m depth) north of the central platform that vented first subaerially, then subaqueously, probably also during this period. A small rhyodacite dome, dated at $\sim$5,000 yr BP, overlies the central platform and abuts the Wizard Island pile. Several large landslides form spectacular deposits on the caldera floor marked by irregular topography and isolated blocks up to 200 m long. The largest, the $\sim$0.5 km$^{3}$ Chaski Bay slide, originated in the south caldera wall. The three major basins of the lake are virtually flat-floored because sediment gravity flows from the caldera walls have buried depressions in the caldera fill. When combined with a 10-m-resolution DEM of the surrounding terrain, the 2-m-resolution lake-floor bathymetry clearly shows the sources of landslides now marked by embayments in the caldera wall and steep underwater outcrops below promontories. Because the survey nearly reached the lake shore, some of the inclined benches resolved on underwater outcrops can be correlated with contacts between geologic units mapped on the caldera walls above Crater Lake

Caryn Ackerman (Cramer Fish Sciences): Graphical design and layout Data Sources and Credits

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