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Show fisheries found in the state. Game fish in the lake include largemouth and smallmouth bass, walleye, northern pike, catfish, crappie, bluegill, and-- most important from the standpoint of poundage harvested- striped bass. All of these species are non- native fish. Walleye, northern pike, and catfish were present in the river drainage before the reservoir filled and have thrived in their new environment. The other gamefish are introduced species. Striped bass were introduced into Lake Powell during 1974 following the establishment of shad as the foodbase, and have since developed into an excellent fishery. Striped bass normally require large river systems in which to spawn. The river currents suspend the eggs until hatching, keeping them in well- oxygenated water. In reservoirs the eggs would normally sink to anoxic substrates on the bottom and fail. Riverine spawning does occur at Lake Powell in the Colorado River, but an unusual limnological characteristic of the lake also allows inlake spawning of striped bass to be successful in the lower reservoir. Because of the low productivity of phytoplankton in the lake, high oxygen values are usually found near the bottom of Lake Powell. In addition, dissolved oxygen concentrations in the bottom waters are replenished by the winter underflow current, discussed earlier in this plan. This phenomenon allows striped bass eggs to survive in what would normally be a lethal reservoir environment. The large reproductive potential thus afforded striped bass has allowed their numbers to increase dramatically, and thereby has placed a great strain on Lake Powell's limited forage fish population ( threadfin shad). The populations of shad and striped bass appear to increase and decline in response to poorly understood productivity cycles; consequently, the possibility of introducing a second prey- base fish to supplement the shad and help keep striped bass populations high has been discussed. However, data is lacking on the lake's primary productivity, plankton populations, and associated water quality requirements. Such information is critical to gamefish management and is also needed as a baseline to evaluate future nutrient loading and aging of the reservoir. Fluctuating reservoir levels affect fish habitat through the shoreline vegetation. Vegetation growing on shore during low- water periods provides excellent fish habitat when later inundated by high water. Tamarisk, an exotic species, is presently the dominant shoreline shrub ( or tree) at Lake Powell and is on the increase. Sport fisheries on Lake Powell are managed by the Utah Division of Wildlife Resources under a cooperative agreement with the National Park Service and the Arizona Department of Game and Fish. Ill. l. l. b. Sensitive areas with high fishery values- The majority of Lake Powell is nutrient- poor and would be classified as oligotrophic. Some areas, however, are quite productive and approach a eutrophic state. Such areas include mixing zones where the major rivers flow into Lake Powell carrying nutrients and sediments. These areas have an 43 |