Skykomish Draft Final Report - Wild Fish Conservancy

Chinook and chum were 14% (1 of 7 carcasses examined) and 16% (10 of 63 carcasses),
respectively. The single female coho carcass found in the braided reach was also a PSM
candidate, bearing full and intact egg skeins.
The locations of all salmon redds and carcasses observed during the spawning surveys are shown
in Figs 2–7. The general pattern evident from these maps is that the distribution of salmon
spawning activity is species-specific. Chinook redds were built primarily in the mainstem, often
near the channel bank (e.g. units #7–13 [Fig. 2] and unit #45 [Fig. 4]), while chum redds were
most commonly found within the four side channel networks (Figs 3, 5, 6, 7, 24A). Several
exceptions to this pattern of non-overlapping spawning areas are noted: Chinook and chum redds
were found together within mainstem units #18–20 at the mouth of side channel network SCA
(Fig. 3). The only instance of confirmed chum redds outside the immediate vicinity of a side
channel was in mainstem unit #35 (Fig. 3). Chinook and chum carcasses showed a similar
distribution to that of redds (Fig. 24B). Although largely restricted to distinct spawning areas,
carcasses of the two species were found in proximity within side channel SCB, resembling the
adjacent mainstem in its habitat unit diversity (Fig. 3), as well as in a seasonal backwater channel
in the vicinity of mainstem units #42–44 (Fig. 4) and in the lower reaches of side channel SCC
(Figs 5, 7). A large proportion of chum redds were occupied by live spawners (93% of mainstem
redds and 86% of side channel redds), whereas Chinook redds contained live fish less frequently
(22% in mainstem; 57% in side channels) (Fig. 24A).
The habitat preferences of spawning salmon in the braided reach were examined in detail by
analyzing the physical characteristics of redd sites. The majority of Chinook redds were
observed in glides, while a large proportion of chum redds were built in pools; for both species,
riffle habitats were also frequently selected for spawning (Fig. 25A,B). Chinook redds were
constructed almost exclusively in large cobble; chum salmon utilized a variety of substrates,
including cobble, gravel and sand (Fig. 25C,D). The depth of water in which redds were
constructed did not differ significantly between Chinook (mean 13.4 in.) and chum (mean 16.4
in.) (t-test: d.f.=116; P=0.09).
Washington Trout’s spawning surveys were generally more extensive in their geographic
coverage than were snorkel surveys within a given channel system. Accordingly, the spawning
surveys provided information about fish use that helped refine the distributional data obtained
from snorkel surveys earlier in the year. For example, the small left-bank braid of the mainstem
at habitat units #38–39 was dry during summer 2004 and excluded from snorkel surveys. In
November, the same channel was found to contain the highest density of chum salmon redds
within the braided reach (Fig. 4).
Sources of Sampling Bias
Discussion
Fish Abundance and Density
Fish counts obtained by snorkel survey typically underestimate the “true” number of fish present
in a habitat unit as determined by more accurate methods of measuring fish abundance (e.g.
electrofishing/removal, mark–recapture) (Rodgers et al., 1992). The calibration recommended
by Hankin and Reeves (1988) involving adjustment of snorkel counts by the ratio of “true” fish
numbers to snorkel estimates was not performed in this study because exhaustive removal
methods were not feasible in the large habitat units surveyed (see also Thompson and Lee, 2000;
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