Adult spawning behavior and emigration of larvae of the endangered cui‐ui Chasmistes cujus were studied in a natural side channel of the lower Truckee River. External radio‐tags placed on eight apparently did not affect spawning behavior. Cui‐uis spawned in clusters of two to seven fish; usually a single female was flanked by two males. Each spawning act lasted 3–6 seconds, and individual fish spawned numerous times. The most active tagged male and female spawned at least 294 times and 114 times, respectively. Individual females broadcast eggs over an area of up to 50 m2. Males spawned over a 4–5‐day period, and females over 2.5–4 days. Most spawning occurred at night in water depths ranging from 9 to 43 cm, water velocities ranging from 23 to 87 cm/second, and temperatures of 12–17 C. The preferred spawning substrate was gravel. Peak emergence and out‐migration of cui‐ui larvae occurred 14 days after peak spawning. Received September 7, 1982 Accepted August 5, 1983

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Salinity of Pyramid Lake increased from 3.7 to 5.5%o between 1933 and 1980. Concern over future reductions in overall species richness prompted experiments to assess responses of dominant lake organisms to elevated salinity. Salinity tolerances of three important benthic invertebrates, Hyalella aztecta, Chironomus utahensis, and Heterocypris sp., were tested in controlled laboratory bioassays and also in a semi-natural environment consisting of large (47 m 3 ) mesocosms.Densities of H. azteca in mesocosms were significantly lower at salinities of 8.0 and 11.0%o compared with 5.6%o controls in year one, but not in 8.5%o salinity mesocosms in year two. The 96-h LC 50 for H. azteca was high at 19.5%o. Short-term mortalities of C. utahensis were 100% at salinities of 13.3%o and greater. Fifty-seven percent fewer larvae matured from third to fourth instar at 8.9 than at 5.5%o salinity in 17 day subacute bioassays. Furthermore, larval chironomid densities and emergence of adults from mesocosms were significantly reduced at salinities of 8.0%o and higher compared with controls. Mortality of Heterocypris sp. was 50%o at a salinity of 18.6%o in laboratory bioassays and populations in mesocosms ranged between 40 and 100% lower at salinities of 8.0 and 11.0%o than in controls.Multiple generation mesocosm experiments indicated all three invertebrates were more sensitive to elevated salinity than results of short-term bioassays. Our studies suggest populations of these invertebrates may be reduced from present levels if Pyramid Lake's salinity were to double, although none are expected to be extirpated. Food habit shifts and reduced production of lake fishes are likely consequences of salinity-induced disruption in the benthic invertebrate forage base.

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