Spawning escapement of Skagit summer and fall Chinook has been fairly stable since escapement estimates were first made in 1952. Escapements were relatively low in the mid-1950’ and the early 1990s, while escapements were higher in the 1970’, and there has been an increasing trend since 1996 (Table 1; Fig. 1). There are, however, differences in trends between populations. Since about 1984, Upper Skagit summer Chinook have made up an increasing percentage of the total escapement. Prior to 1984, approximately 60% of the summer and fall production unit escapement was comprised of Upper Skagit summer Chinook, yet, since that time, Upper Skagit summer Chinook have averaged about 75% of the total summer and fall production unit escapement. Side by side to this change in escapement composition, a complementary decrease in the percentage comprised of Lower Skagit falls and Lower Sauk summers has also been observed (Table 1; Fig. 2). Escapement estimates of Skagit spring Chinook have been generated back to 1952 (Table 1). However, because a change in estimation methods from peak live and dead counts, multiplied by a length factor, to the current redd count method, the numbers are reliably comparable only since 1994. Since 1994, the escapement trend of Skagit springs has been fairly flat with a slight increasing trend (Fig. 3). Each of the three spring populations has contributed approximately equal percentages of the escapement, and since 1994 there has been no noticeable change in the percentage contributed by each population (Fig. 4). By examining correlations in escapement trends between populations, it may be possible to determine which populations are limited by common limiting factors, as well as hypothesize where those constraints might occur. Correlations in spawning escapement can be examined for summer and fall populations back to 1973, for spring populations, and among spring and summer and fall populations, back to 1994. Because redd counts started in 1992 in the Upper Cascade River, correlations between Upper Cascade spring Chinook and the summer and fall Chinook populations can be examined back to 1992. The spawning escapement trends for the Lower Skagit fall Chinook, over the 1973–2004 time period, are significantly correlated to those of both the Lower Sauk and Upper Skagit summer Chinook populations, with a higher correlation to those of the Lower Sauk summer Chinook population; however, there was a non-significant correlation between the Lower Sauk summer Chinook and the Upper Skagit summer Chinook escapements (Table 2). This may indicate that factors that affected the escapement of Lower Skagit fall Chinook may also have affected the escapements of Lower Sauk and Upper Skagit summer Chinook. However, there may also have been additional factors that affected escapement of either Lower Sauk or Upper Skagit summer Chinook that were not shared by the other population. In addition, since 1994, the correlation between Lower Skagit fall Chinook and Lower Sauk summer chinook escapements has been non-significant. This indicates that, whatever factors that caused the correlation between Lower Skagit fall Chinook and Lower Sauk summer Chinook escapements between 1973 and 1993, they have not had the same effect on each population since 1994. In fact, since 1994, escapement trends for Lower Sauk summer Chinook have not been significantly correlated to
Skagit River System Cooperative, 2005. Skagit Chinook Recovery Plan: Appendix A: Trends in Spawning Escapement. Skagit River System Cooperative, La Conner, WA. pp. 5.
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