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 temperatures show direct response to this increase in ambient air temperature and decrease in humidity. The potential environmental effects of the myriad of chemicals applied or discharged into watersheds supporting coho salmon and the complex breakdown products of those chemicals, have been insufficiently investigated and regulated.
Coho salmon spend their first 15-20 months in streams and rivers and are therefore particularly vulnerable to adverse impacts of past and current land use practices. Reeves et al. (1989) defined physical habitat requirements for coho salmon at each freshwater life history stage. Spawning habitat is best carried out in small streams with stabile gravels, summer and winter freshwater habitats most preferred by coho salmon consist of quiet areas with low flow, such as backwater pools, dam pools and side channels. Habitats used during the winter generally have greater water depth, and have large amounts of large woody debris (LWD). Studies by Nickelson et al. (1992), show coho salmon smolt production is probably limited by the lack of adequate winter cover. Nutrients and food sources do limit production of juvenile coho salmon but procedures for identifying these limitations is not well developed at this time.
The role that LWD plays in creating and maintaining coho salmon spawning and rearing habitat is critical in all sizes of streams, but has only been recognized in the last two decades.
Descriptions of the pre-development conditions of river with abundant salmon suggest that all these rivers had large amounts of LWD, which in some cases blocked navagation, but in all cases stored sediment and nutrients, impounding and slowing waters, and creating many side channels and sloughs. (Sedell and Frogatt 1984, Sedell and Luchessa 1982) Many streams were so filled with logs and snags they could not be ascended by the early explorers. In California many smaller streams were splash dammed to facilitate log drives. These practices scoured, widened, and flushed gravels and essentially cleaned the streams b