Skykomish Draft Final Report - Wild Fish Conservancy

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Introduction Snohomish County Surface Water Management and partners have proposed to identify and prioritize opportunities to restore reach-level channel processes within the braided section of the Skykomish River extending from the cities of Gold Bar to Sultan, WA. The braided reach (“study reach”) includes over ten miles of mainstem channel between “Big Eddy” in Gold Bar (rm 43.3) and Sultan (rm 33), and nearly seven miles of interconnected side channels (Figs 1–7). In 2004 Washington Trout (WT) surveyed the braided reach in support of the following project goals: • to describe patterns of current fish use within the study reach to define a baseline against which future conditions can be compared; • to identify critical fish habitats so that restoration actions with high likelihood of creating and maintaining these habitats can be prioritized; • to evaluate the biological cost and benefit of proposed restoration actions. Washington Trout’s 2004 field work served two general tasks designed to evaluate fish use within the braided reach. First, fish species composition and relative abundance were surveyed seasonally within a subsample of habitat units chosen to represent the overall habitat-type distribution in the study reach. The specific objectives of these surveys were (i) to compare species diversity and relative density of adult and juvenile fishes in the study reach’s mainstem and in associated side channel networks; (ii) to identify physical habitat features significantly related to fish relative density, including habitat unit type and size, and within-unit survey location (e.g. channel center versus edge habitat; unit middle versus upstream–downstream unit interface; Fig. 8); and (iii) to examine seasonal and diel variation in species composition, distribution, and relative abundance. The second task related to fish-use assessment was characterization of salmon spawning activity in the study reach. Redd and carcass surveys were performed in order to (i) quantify the incidence and timing of spawning activity throughout the braided reach; (ii) to document species-specific patterns in the spatial distribution of spawning; and (iii) to examine the relationship between spawning activity and physical habitat characteristics. Patterns of fish use in the Skykomish River braided reach provided guidance for developing both a general strategy and specific projects for restoring and protecting habitat. Recommendations for designing and prioritizing such projects are presented in Chapter X. Methods Reach Nomenclature The braided reach was partitioned into distinct habitat units (pools, riffles, glides) according to the Snohomish County Preliminary Habitat Analysis of July 2004 (Figs 1–7). To facilitate comparison among units, unique identifier numbers were assigned to each mainstem and side channel habitat unit surveyed. Side channel networks were named from upstream to downstream as follows: SCA (right-bank channel near the city of Gold Bar) (Fig. 3), SCB (left-bank network near Gold Bar) (Fig. 3), SCC (left-bank network between the cities of Startup and Sultan) (Figs 5, 7), SCD (right-bank network near Sultan) (Fig. 6). Preliminary habitat-type designations, based on inspection of channel morphology in aerial photographs taken during summer low-flow conditions, were ground-truthed by Washington 1
Trout in August and September 2004. During these summer surveys, WT crews qualitatively assessed each mapped unit’s physical characteristics including water depth, velocity, and surface turbulence. These observations were measured against the geomorphic definitions of each unit type following Cramer (2001): Pool: a unit with residual depth and no surface turbulence except at its inflow. Glide: a unit with no residual depth, little surface turbulence, and relatively uniform flow velocity. Riffle: a unit with discernable gradient and surface turbulence. Several of the preliminary habitat type designations in the mainstem channel were modified on the basis of WT’s on-site surveys. When correction of the original habitat unit type resulted in two consecutive units of the same type, the two units were consolidated into a single larger unit and renamed accordingly (e.g. mainstem unit “70+71”). In side channels, where most mapped habitat units were pools, WT crews identified and named a number of additional units not appearing on the preliminary maps. Note that the fish use surveys, which relied on these corrected preliminary maps, preceded the final habitat unit delineation based on quantitative, onsite physical surveys (cf. Appendix X). Fish survey areas and final habitat unit boundaries were largely congruent; in cases where areal discrepancies were large, associated fish use data were excluded from analysis. Species Composition and Relative Abundance Snorkel surveys were performed seasonally (Table 1) to provide estimates of fish species diversity and relative abundance within the braided reach. Survey teams were comprised of three snorkelers who floated abreast in a downstream direction during daylight hours (summer, fall and winter 2004), and at night (winter 2004), recording in notebooks the species, life stage (young-of-year and yearling juvenile versus adult) and number of all fishes observed within each habitat unit. Waterproof habitat unit maps and aerial photographs (Figs 2–6) were used to assist in identifying habitat unit boundaries in the field. Surveyors occupied three longitudinal snorkel lanes, one located in the center of the channel and one at each channel edge (Fig. 8) (cf. Fig. 3 in Pess et al., 2002; Cramer, 2001 p. 42). Snorkelers changed lanes frequently between habitat units to minimize potential spatial bias in fish counts. The width of each snorkel lane was defined by visibility on the day of the survey. Although adult fish could be discerned underwater at distances up to 25 feet in the summer and up to 15 feet in the fall (Table 1), the maximum distance at which the smallest juvenile fish could be confidently identified was approximately 5 feet (cf. Hillman and Chapman, 1989). Each snorkel lane, therefore, was conservatively estimated as 10 feet wide. Within each habitat unit polygon (Figs 2–6) the ten-foot wide curvilinear corridor along the unit’s longitudinal midline was used to calculate snorkel lane area. To approximate relative fish densities for each habitat unit, total fish counts were divided by the estimated total wetted area surveyed (i.e. three-times snorkel lane area). As emphasized previously (Cramer, 2001), fish relative density calculated from a large-river census of this type must be considered representative only of the surveyed fraction of the total habitat area. To examine patterns of within-unit spatial variation in the relative density of fish species observed, each mainstem habitat unit was partitioned into nine survey subareas. The three longitudinal snorkel lanes defined left-bank, center, and right-bank areas; units were further 2
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Page 39: Figure 25. Salmon redd counts by ha

Skykomish Draft Final Report - Wild Fish Conservancy

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