The U.S. Fish and Wildlife Service has an ongoing program to assess the feasibility of reestablishing naturally spawning populations of Lahontan cutthroat trout in the Truckee River-Pyramid Lake system in Nevada. Previous in situ egg-survival studies have documented a 100 percent mortality of cutthroat trout eggs artificially planted in potential spawning gravels in the Truckee River downstream from Reno. The relation between ambient river-quality conditions and the observed mortality of eggs, however, has not been adequately documented. This study was designed to monitor the quality of surface and intragravel water during a trout-egg incubation period that began March 10, 1980. Five sites were monitored: two upstream from Reno (background sites), one near Reno, and two downstream from Wadsworth.
After an incubation period of about 30 days, poor egg survival was recorded at all sites, including an unexpected high mortality at the upstream background sites. Analyses of the data indicated that the principal cause of egg mortality at the two downstream sites was low concentrations (less than 5 milligrams per liter) of intragravel dissolved oxygen. Low water temperatures, rather than degraded water-quality conditions, largely contributed to the poor survival at the upstream sites.
Based on the results of this study, the following were considered unlikely to be mortality factors during the incubation period: (1) high water temperatures; (2) toxicity due to ammonia, nitrite, nitrate, arsenic, cadmium, copper, iron, lead, manganese, mercury, and zinc; and (3) decreasing intragravel dissolved oxygen caused by inflow of oxygen-poor ground water.