Table 1. Major anthropogenic factors limiting, or potentially limiting viability of mountain sucker populations in California, where a factor rated high is a major limiting factor, a factor rated intermediate is a factor that has potential to be a major limiting factor but has had only a moderate effect so far on population viability, and a factor rated low has a low or unknown effect on population viability. Certainty of these judgments is moderate. See methods section for explanation of rating procedures.
Major dams. Habitat degradation due to impoundment streams by dams decreases the distribution of mountain suckers. In Sagehen Creek, impoundment first resulted in a decrease (88%) of the historical longitudinal distribution of mountain suckers (Decker 1989) and then their eventual elimination from the creek (Moyle, unpublished data). Impoundments reduce the amount of stream habitat available and reduce connectivity among mountain sucker habitats because the suckers do not use most reservoirs. Hybridization between mountain and Tahoe suckers may result from reduced populations of mountain suckers combined with increased populations of Tahoe suckers (which do well in reservoirs), resulting in introgressive hybridization and loss of the species.
Agriculture. The effects of agriculture have not been documented and would occur only in the lowermost reaches of streams. There are likely effects on suckers from channel alteration, irrigation diversions, polluted return water and similar consequences of farming along streams.
Grazing. Grazing can alter the quality of stream habitats for mountain sucker by increasing turbidity (decreasing the quality of spawning gravel) and decreasing cover, especially undercut banks (Decker 1989, Moyle 2002). Past grazing pressure incised stream reaches in the upper Truckee River resulting in siltation of stream substrates and a loss of riparian vegetation that provided cover (T. Taylor, pers. comm. 2009).
Rural residential and urbanization. The streams in which mountain suckers occur are increasingly flow through urban areas (e.g.,Truckee) or areas with recreational homes and resorts. The effects of increasing development on the suckers has not been documented but negative effects of stream alteration, siltation from run-off, septic pollution, and similar problems is likely affecting mountain sucker habitat.
Mining. The legacy effects of hard rock mining in the region include acid mine drainage and stream alteration but effects on mountain suckers are not well documented.
Transportation. Roads are generally associated with declines in fish abundance and diversity in the Sierra Nevada (Moyle and Randall 1998). In the eastern Sierra, major highways follow the courses of major rivers (e.g. Truckee, Carson rivers) and alter habitat by confining the streams, reducing riparian trees and cover, and increasing development of the region (e.g. summer homes, ski resorts, etc.) which impacts streams through alteration, pollution, and diversion. Logging, mining, and agriculture are also associated with increased densities of secondary roads, which directly impact streams through channel alteration and indirectly alter them through increased siltation, removal of riparian cover, and other environmental changes.
Logging. Logging is pervasive throughout the basin and while practices are much more stream and fish friendly than in the past, logging can still deliver silt to streams. More important are the legacy effects of the heavy logging in the 19th and 20th centuries which hugely altered streams, from which recovery is far from complete. For example, there is little large wood in streams to provide cover and feeding areas for mountain suckers.
Fire. Fire is a continuous presence in the Lahontan region but its effects on mountain suckers is unknown. Because fire has been suppressed for decades, catastrophic fires, with the potential to alter stream habitats, are now more frequent.
Recreation. Heavy recreational use, including ski resorts, has altered some streams, especially through sedimentation and perhaps changed behavior of fish (e.g. through rafting, swimming, angling). Effects on mountain suckers are not known but likely low.
Alien species. The presence of alien species (e.g. brown trout) can push mountain suckers into suboptimal habitat, increase predation and increase physiological costs (Olsen and Belk 2005, Belica and Nibbelink 2006, Giddings et al. 2006). Habitat use shifts of juvenile mountain suckers can reduce growth and decrease energy available for reproduction (Olsen and Belk 2005). Nonlethal effects, due to increased physiological costs, may result in additional population declines. In the Truckee River alone, they face threats from interactions with largemouth bass, bluegill, and brown bullhead, as well as brown, brook and rainbow trout (T. Taylor, pers. comm. 2009).