The natural history and ecology of the California Crayfish, Pacifastacus leniusculus (Dana}, was investigated in a portion of subalpine Lake Tahoe from March 1974 through February 1975. Three specific areas of the lake were chosen for study because of their variability in bottom contour, substrate type and primary production. Population estimates in these areas ranged from 16,561 to 192,448 crayfish with almost equal distribution of sexes. Mean standing crop and annual production of the population were estimated at 99.77 and 30.56 g/m² respectively.

Both sexes matured in their fourth year, mating took place from late Sep­tember through October, and hatching occurred in July. Reproductive performance was 78% of the potential capacity.

Age structure of the crayfish population varied in the three areas. The avnilable cover in one of these areas was identified as the limiting factor responsible for slower growth rate and smaller mean size at onset of sexual maturity. Mortality rate was also much higher at this location. The population age structure consisted of at least 9 age classes. At recruitment carapace length of the young was 3.25 mm and the largest adult taken was 57.0 mm (C.L.}.

Crayfish occupied shallow water during the summer and fall months. Decreasing water temperatures and sunlight were identified as the major stimuli causing the population to move into deeper water (to 100 m) where they remained until the following June. Frequent storms with high winds in late fall caused high mortality rates among those crayfish-that did not move to deeper water. Nocturnal activity prevailed and nightly they left their crevices and burrows on the slopes and fed on the open sand flats or in the boulder zones near shore. Adult P. leniusculus fed mostly on "aufwuchs", macrophytes, and detritus while the juvenile diet consisted of 47% animal material.

The role of crayfish grazing intensity on periphyton primary productivity was investigated in field and laboratory enclosures where the ratio of crayfish to substrate was varied (in artificial microcosms). Primary productivity was enhanced in a low density crayfish field site and inhibited in a high density site. These results were duplicated in laboratory trials, where grazer biomasses less than 131 g/m² stimulated productivity while it was inhibited by overgrazing when crayfish exceeded 204 g/m². Grazing effects were also estimated on higher aquatic plant beds found in the littoral area of Tahoe by inclusion-exclusion experiments. Nutrients contributed from feces of the crayfish significantly stimulated algal primary productivity.