Table 2. Major anthropogenic factors limiting, or potentially limiting viability of coastal cutthroat trout populations in California, where a factor rated high is a major limiting factor, a factor rated intermediate is a factor that has the 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.
Habitat degradation. According to Gregory and Bisson (1997), degraded habitat is associated with more than 90% of documented extinctions or declines of Pacific salmon stocks. Coastal cutthroat trout stocks are no exception to the rule. Major anthropogenic land-use activities, including agriculture, forestry, urban and industrial development, road construction, and mining, have resulted in the alteration and loss of cutthroat trout habitat and a subsequent loss in production (Johnson et al. 1999). Fish passage issues from loss of over-wintering habitat, changes in geomorphic processes and channel geometry, channelization and simplification of habitat in estuaries, the loss of large wood in channels, and road impacts on small headwater streams are all associated with habitat degradation in cutthroat trout range. Logging and associated road cuts have caused tremendous impacts to their habitat with massive landslides and erosion stemming from excessive tree removal on the steep, unstable soils found in the coastal mountains. Small streams (e.g., those favored by cutthroat trout) are inherently more susceptible to such impacts and have therefore been disproportionately damaged by land use practices. Johnson et al. (1999) cite numerous studies showing the importance of riparian vegetation to fish production and notes that in California; approximately 89% of the states riparian forest has been lost with associated declines in aquatic habitat. Heavy erosion results in stream sedimentation and can elevate turbidity to intolerable levels as well as burying spawning gravel, altering rearing habitat and filling pools. Urbanization plays an important role in reducing cutthroat trout habitat in urban streams in the Humboldt Bay region and around Crescent City (T. Weseloh, pers. comm. 2008) but most impacts to stream habitat stem from agriculture, forestry practices, dams, and aggregate mining. Agricultural practices that impact cutthroat trout the most are likely water diversions and the associated dike building, damming, culverts and runoff. These factors result in degraded water quality, increased temperature, loss of in-stream flows, and loss of estuarine rearing areas (Johnson et al. 1999). Unfortunately, there are few studies that document such impacts specifically for coastal cutthroat trout.
Dams and diversions. Dams and diversions have altered flows on a number of coastal rivers, most conspicuously the Klamath and Mad Rivers, within coastal cutthroat trout range. The impact of these dams on cutthroat trout is not known but altered flow regimes are unlikely to have had a positive effect. Likewise, the effects of small diversions, common in coastal streams, are not known.
Overexploitation. Gerstung (1997) indicates that historical runs of coastal cutthroat trout were quite large and that, in some areas, substantial commercial and sport fisheries existed for them. Today, fisheries for coastal cutthroat occur mainly in coastal lagoons, where populations tend to be largest. Fisheries elsewhere are small and largely catch-and-release, although impacts of legal and illegal fishing are in fact unknown.
Interactions with hatchery salmonids. Coastal cutthroat trout are competitively subordinate to all other species of salmonid (Johnson et al. 1999) and hatchery production of steelhead in particular may deeply affect their numbers through predation and competition (Johnson et al. 1999). Some cutthroat trout are raised at the Humboldt State University and Mad River hatcheries and are planted in lagoons the support the fishery. Their interactions with wild fish are not known (but assumed to be minimal).
Hybridization with steelhead: Steelhead and cutthroat trout naturally co-occur and hybrids occur naturally, with no obvious impacts on cutthroat trout populations (Neillands 2001). However, habitat disturbance and other factors may increase rates of hybridization, with unknown consequences, but presumably to the detriment of the rarer cutthroat trout.