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Clear Lake in Lake County, California, has an endemic fish fauna composed of five lake-adapted forms derived from lowland stream-adapted forms present in surrounding drainage basins. Two of the five endemic forms are extinct. The three remaining endemics maintain themselves despite the destruction of sloughs and tule beds surrounding Clear Lake that are used for spawning and nursery areas. Trophic specializations of the endemic fishes indicate past selection for feeding on small benthic and pelagic invertebrates. The presence of fine particles in the substrate and the reduced activity of tributary streams for at least the past 10,000 yr are major hydrographic features contributing to the evolution of these trophic adaptations.

Subfossil scales of the endemic Clear Lake tuleperch, (Hysterocarpus traskii lagunae) present in three U.S. Geological Survey cores (CL-73-7, -6, and -8), removed from the bottom of Clear Lake in 1973 were analyzed by Casteel and others (1975, 1977a, b, 1979) for age and growth rate. Periods of increased scale growth were inferred to represent warming of the lake. Comparison of the Casteel data with pollen data (Adam and others, 1981) indicate that maximum scale growth (core CL-75-8) occurred at about 19 ka (=15 ka, according to Robinson and others, this volume) during a cold interval. Fluctuations in scale density in cores CL-73-4 and CL-73-7, however, seem to follow fluctuations in oak pollen. It is therefore concluded that maximum-scale growth represents cool periods, whereas maximum-scale density represents warm periods in the history of the lake. During the period that maximum-scale growth occurred, Clear Lake basin may have also been closed off from surrounding basins and the lake enriched with nutrients.

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