Two similar salmonids, coho and steelhead, cohabit many coastal rivers of British Columbia. Field collections reveal that the distributions of underyearling coho and steelhead are similar along the length of these streams. However, the micro-distribution of the two species is different. In spring and summer, when population densities are high, coho occupy pools, trout occupy riffles. In autumn and winter, when numbers are lower, both species inhabit the pools. Nilsson (1956) stated that segregation (such as that shown by coho and trout in spring and summer) may be indicative of competition resulting from similar ecological demands. To test this hypothesis the distribution and behavior of coho and steelhead were compared in a stream aquarium at different seasons with gradients of light, cover, depth or depth/velocity, and in experimental riffles and pools. Distributions and preferences of the two species in the experimental environments were most similar in spring and summer, the seasons when segregation occurred in nature, and least similar in autumn and winter, the seasons when the two species occurred together in nature. Spring and summer segregation in the streams is probably the result of interaction which occurs because of similarities in the environmental demands of the species and which is accentuated by dense populations and high levels of aggressiveness. The species do not segregate in streams in winter because certain ecological demands are different, numbers are lower, and levels of aggressiveness are low. When the two species were together in the experimental riffle and pool environment, trout were aggressive and defended areas in riffles but not in pools; coho were aggressive in pools but less inclined to defend space in the riffles. These differences in behavior probably account for the distribution of trout and coho in natural riffles and pools.The data support the basic contention of Nilsson (1956) and illustrate the role of behavior in segregation produced by competition for space.

Winter habitat use by juvenile coho salmon (Oncorhynchus kisutch) varied with cover type and flow level in outdoor stream channels. Cover utilization and the number of fish remaining in stream channels increased significantly as cover complexity increased. Most fish emigrated during a simulated freshet unless the most complex cover (low velocity, shade, and wood debris combined) was available. At both high and low flows, emigration occurred primarily during the rapid decline in light levels at twilight. Most coho formed aggregations beneath cover, exhibiting feeding and aggression at temperatures as low as 2.5 °C. We conclude that (1) social interactions, in concert with habitat features, influence the abundance of coho salmon within specific stream habitats in winter, and (2) structural complexity of wood debris is an important consideration for management practices designed to protect or enhance winter habitat for this species.

Numbers of juvenile coho salmon (Oncorhynchus kisutch) in streams are reduced substantially in winter compared to those that occur in summer. Most of this reduction occurs early in autumn with the onset of the first seasonal freshets. Stream sections containing adequate winter habitat in the form of deep pools, log jams, and undercut banks with tree roots and debris lost fewer fish during freshets and maintained higher numbers of coho in winter than sections without these habitat characteristics. These features provide shelter and reduce stream velocities. Microhabitats occupied by coho juveniles in winter after logging were unchanged from those described before logging — all microhabitats were characterized by low water velocities (≤ 0.3 m/s). Up to 48% of the coho population inhabiting stream sections with adequate shelter remained there by midwinter (Jan. 3). This percentage was typical of stream sections where at least some trees remained after logging. Streamside trees stabilized the banks and prevented their collapse. In contrast, two of three study sections that had been clear-cut logged had unstable banks which collapsed during winter freshets. Almost no coho remained in these sections in winter. Many coho emigrate from the main stream to seek the shelter of low-velocity tributaries and valley sloughs concurrent with the decline of coho populations in Carnation Creek during autumn and early winter. This seasonal shift in distribution reverses in the spring when large numbers of coho reenter the main stream. Fish overwintering in these sites have a high apparent survival rate. Before logging a 4-yr mean of 169 ± 44 coho entered one tributary (a slough called 750-m site) in autumn. Of these numbers entering, 72.2% came out in spring. During and after logging, an annual mean of 288 coho entered the same site. The apparent survival rate during and after logging was 67.4%, essentially unchanged from the prelogging value. Logging has neither reduced the numbers of coho juveniles that enter such sites in autumn to overwinter, nor reduced the numbers leaving these sites to reenter Carnation Creek in spring.

No abstract

Ten percent of the juvenile coho salmon Oncorhynchus kisutch rearing in the main channel of Carnation Creek during the summer moved into intermittent tributaries and ephemeral swamps (off-channel winter habitats) during the autumn of 1983. The number of juveniles residing within specific off-channel sites during winter was governed by the magnitude of water levels associated with the first fall storms relative to the flooding levels required for adequate access to these sites (P < 0.05). Off-channel habitats contributed 15.3% of the watershed's coho salmon smolts in 1983 and 23.1% in 1984. A 25-year flood event (65 mVs) occurred in January 1984 and may have reduced the main-channel contribution for that year. The inability of coho salmon smolts to emigrate from off-channel habitats and return to the main channel in spring may have reduced the off-channel contribution in 1983. April-May water levels were 37% below the 13-year mean water level in 1983 and 55% above it in 1984.Fish Commission of Oregon 2:90-95.