h for broodstock in an attempt to overcome these problems in hatchery stocks merely increases the risks for wild populations.
Large or repeated introductions of hatchery fish (foreign stock, hybrid stock, or local-origin stock altered by artificial selection) pose additional risks for wild fish (Steward and Bjornn 1990, Waples 1991, Hindar et al. 1991). Introduction of hatchery fish can adversely affect wild fish through competition and other processes. Maladaptive phenotypes can be introduced (Nickelson et al. 1986) and may temporarily persist in the population, particularly where wild population densities are low or habit has been altered (Campton 1987).
The failure of hatchery programs to boost or re-establish wild populations of coho salmon has been repeatedly demonstrated. Nickelson et al. (1986) documented the net negative effects of stocking coho pre-smolts in coastal Oregon streams, a practice still widespread in Washington and California. The effects of hatchbox programs and fry releases, widespread in all three states, have never been adequately evaluated. Hallock and Fry (1967) reported that numerous introductions of coho salmon had failed to re-establish a self-sustaining run in the Sacramento River. Hope (1993) documented in detail the many failed efforts to restore northern California coastal coho with artificial propagation, starting in the 1940s and continuing through the present. Many if not most coho streams in the region have a similar, long history of introductions of hatchery-origin coho salmon. In many cases no fish return; in others first-generation returns are followed by failure of the second generation to reproduce successfully. Little monitoring of these programs occurs.
The litany of repeated failure in attempts to rebuild or re-establish self-sustaining wild populations with diverse kinds of artificial propagation argues that conservation of wild, indigenous populations of coho salmon in their native habitat is the only prudent and defensible course of action. The many potential side effects of small-scale and large-scale artificial propagation activities, led the Klamath River Basin Fishery Task Force (1991) to stress the overriding importance of habitat protection and restoration. Although recent successes in Scott Creek have been reported, this emergency propagation was combined with Bacterial Kidney Disease treatment (a disease likely introduced by past hatchery programs). A long term study of the success is necessary to determine the efficacy of this program for other streams (Dave Hope personal communication).
Deterioration of Freshwater Habitat
The long-term decline of coho salmon populations parallels the deterioration of freshwater habitat caused by human disturbances (Lawson 1993). Coho are especially vulnerable to loss or degradation of spawning, summer rearing, and winter rearing habitats (Pearcy et al. 1992). Pearcy et al. (1992) pointed to degradation of freshwater habitat as perhaps the largest contributor to long-term declines of coho productivity and recent shortfalls in escapement. Loss of woody debris and habitat complexity in estuaries may reduce survival of outmigrating smolts and winter migrants (McMahon and Holtby 1992). Coho habitat is lost when large woody debris and the stable, complex channels and wetlands associated with floodplain forests are damaged or destroyed by logging, grazing, channelization, cropland agriculture, or urbanization. Flow diversions for irrigation and hydropower generation poses serious problems for coho salmon throughout California. Sedimentation, debris flows, loss of channel stability and complexity, and increases in turbidity or summer stream temperature often result from disturbance of small headwater slopes and stream channels by logging roads and timber harvest. These impacts alone may be sufficient to damage or destroy coho populations even where buffer zones are left along larger, fish-bearing streams. Clearcutting of entire basins has the effect of increasing ambient air temperature and decreasing humidity. Instream water temperature