stem, where they can be found in small numbers well into July. Juvenile coho become smolts and emigrate to the ocean between March and mid-June; peak migration occurs in mid-May (NMFS 2001). In general, juvenile coho can be expected to occupy places where summer temperatures are low (12–14°C appears to be optimal for growth). They are also favored by deep pools with complex cover, especially large woody debris, which is essential for survival over winter (Sandercock 1991). Such conditions exist primarily in tributary streams of the Klamath Basin.
The reduction in stocks of native coho salmon in the Klamath River Basin has been caused by multiple interactive factors. Drastic reduction in spawning and juvenile habitat has occurred through impoundment and physical alteration of tributaries. Also, large numbers of smolts are released annually from the Iron Gate hatchery. Smolts, which are derived from a combination of Klamath Basin and Columbia River coho, likely compete with or have other negative effects on wild native coho at all stages of their life history, including the smoltification-emigration period, the ocean growth period, and spawning (Fleming and Gross 1993, Nielsen 1994, NRC 1996).
Physical habitat in the main stem is a potential concern for the welfare of the coho in several life stages. The spawning run must have adequate flows for passage, which would be impaired by excessively shallow water (e.g., through amplification of predation losses). Access to tributaries is a related consideration for the spawning run, given that little spawning is likely in the main stem. Also, fry that enter the main stem must find cool, well-shaded pools, or return to a suitable tributary. Smolts moving downstream must find suitable temperature, flow, and habitat conditions compatible with their physiological transformation during migration (Wedemeyer et al. 1980).
Habitat is an undeniable requirement for all life stages; however, assessment of habitat suitability is difficult and subject to considerable uncertainty. Numerical methods are now being applied to the estimation of habitat area in relation to flow (INSE 1999). These methods are commonly used in evaluating habitat, but in final form they require extensive field measurements that are not yet available. Initial modeling suggests that even though habitat for salmonids increases with higher flows, the percentage increase of habitat space corresponding to increases in flow during dry years is relatively small (INSE 1999, NMFS 2001).
Water temperature is a major concern for the welfare of the Klamath Basin coho salmon. Summer temperatures appear to be especially critical. In the nearby Matolle River, which contains coho that are part of the SONCC ESU, the juvenile coho reside almost entirely in tributaries but do not persist when summer daily maximum temperatures exceed 18°C for more than a week