Nature and Degree of Threat: Blue chubs remain abundant but have declined through a combination of factors that combine with drought to stress blue chub populations. Thus, drought in the 1980s and 1990s further stressed the Upper Klamath basin, a system already strained by the other factors, such as water diversion, pollution, alien species, and entrainment in power plants.
Diversions. Water diversions from the rivers and reservoirs have dried up the low elevation habitats once preferred by blue chubs (e.g. Lower Klamath Lake). The resulting reclamation of land in the upper Klamath basin has significantly altered the landscape, reducing the amount of habitat available to aquatic species. Only about 10% of the open water and marsh habitats once available in the Upper (Oregon) and Lower Klamath Lakes exist today (NRC 2004). The maximum area of Lower Klamath Lake is currently about 4,700 acres, as compared to a historical maximum area of 94, 000 acres. The maximum area of Tule Lake has also decreased significantly from 110,000 to approximately 13,000 acres. Efforts to restore wetlands began in the 1980s and continue today. Water manipulation and diking have also changed the manner in which the lakes in the upper basin behave (NRC 2004). Water management in Clear Lake causes its area and depth to vary outside of its natural range. Similarly, the surface area of Tule Lake historically varied from 55,000 to 110,000 acres, its surface area currently fluctuates from 9,450 to 13,000 acres. These changes likely reduce the productivity of these systems (NRC 2004).
Pollution. Organic pollutants from agriculture and grazing flow into Upper and Lower Klamath Lakes and Tule Lake, making them more eutrophic and less suitable for native fishes to inhabit, even though they are tolerant of fairly extreme environmental conditions. Increased temperature and lower dissolved oxygen levels may impair blue chub populations (Castleberry and Cech 1993). In California, poor water quality, from agricultural drainage, in the Lost River and Tule Lake presumably have made them much less suitable for blue chubs than they were historically.
Alien species. The upper Klamath Basin has a number of alien species present and they dominate some habitats, to the probable detriment of native fishes, including blue chub. Thus fathead minnow (Pimephales promelas), ecologically similar to blue chub (Moyle 2002), has proliferated in the lakes and canals in recent years. The effect of increasing fathead minnow numbers on blue chubs and other native fishes is unknown but is thought to decrease blue chub populations (Castleberry and Cech 1993), presumably because they are better able to survive in warmer waters with lower dissolved oxygen levels than blue chubs. In Clear Lake Reservoir, the Lost River, and Tule Lake, the alien Sacramento perch (Archoplites interruptus) has become abundant. It is a predator on fish but its impact on blue chub populations is not known.
Entrainment. Recently, focus has been placed on the impact of turbine entrainment on blue chub abundance. CH2M Hill (2003) estimated that median turbine entrainment at Copco and Iron Gate reservoirs was 115, 979 fish, while the median entrainment at J.C. Boyle was 75,655. If we assume that the 0.2- 3.2% of entrained fish are blue chub, as estimated by Desjardins and Markle (2002), then an estimated 383 to 6131 are likely entrained by both dams each year. Most of these likely would be young-of-year of sizes 50 to 150 mm (CH2M Hill 2003), perhaps decreasing yearly recruitment. Entrainment in Klamath River dams peaks in spring and summer, between April and June, during the time that juvenile fish are moving into rearing habitats (CH2M Hill 2003). In the reports reviewed by CH2M Hill (2003), Cyprinidae (minnows and chubs) were the third family to be most likely entrained. In the Link River dam, blue chub made up 49% (214204) of the entrained fish (CH2M Hill 2003). However, the high rate of entrainment at Link River dam may reflect their relative high abundance in Upper Klamath Lake.