s 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 bare. Stream cleaning continued through the mid-1970s, in an attempt to reduce flooding and improve fish migration.
The National Marine Fisheries Service (NMFS) reviewed the past destruction, modification and curtailment of freshwater habitat in 1996. NMFS found that the factors for decline of habitat on the west coast were due to dams (blocking juvenile and adult passage), water withdrawal (stranding fish entraining juveniles and increasing temperatures), flood control (stream channelization and simplification), logging and agriculture (loss of LWD, sedimentation, loss of riparian vegetation, habitat simplification), mining (gravel removal dredging and pollution) and urbanization (vegetation removal pollution channelization increased runoff, habitat simplification). Lichatowich (1989) confirms that habitat loss is a significant contributor to stock declines in coastal coho salmon streams.
Gregory and Bisson (1997) showed that habitat degradation has been associated with 90% of documented extinctions or declines of pacific salmon stocks. California has reportedly lost 89% of the states riparian woodlands due to various land use practices (Kreissman1991). Fisk et al. (1966) showed a loss of over 1,600-km of stream had been destroyed or damaged by 1966.
Large deep pool habitat, critical for coho salmon survival has declined on the West Coast by 58% on National Forest Lands, and by 80% in surveyed Oregon coastal streams (FEMAT 1993, Murphy 1995). Logging, agriculture, urbanization, grazing, and mining have led to large reductions in essential summer and winter rearing habitat for coho salmon. Habitats essential for coho survival are now rare in many streams i.e. backwater pools, side channels, off-channels, deep lateral sour pools, dam pools, and stream margins where LWD and boulders form deep cold water pools. In California loss of deep water pool habitat leaves coho salmon vulnerable to high instream summer water temperatures, winter flood events and predation by birds and mammals, due to lack of cover.
Habitat pro