Musslewhite, J. and Pflug, D., 2024. Skagit River Chum Salmon Escapement Distribution and Behavioral Attributes During The 2006-07 Return Years. Skagit River System Cooperative, Burlington, WA. pp. 24.

Skagit River Chum Salmon escapement methodologies were originally developed in 1976 and
1977 (Hendrick 2003). The methodology was based on mark-recapture studies to determine
distribution and proportional abundances along the Skagit River that were used to established six
“index” reaches. The six index reaches, two side channels/sloughs and three main-stem reaches
on the Skagit River above the town of Rockport and one side channels/slough along the mainstem Skagit River below the town of Rockport, are used for estimating the Chum Salmon
population for the entire basin. Development of indexes require that managers assume 1) index
reaches are representative of the entire basin and 2) spawning distributions are similar across all
spawning densities and river flow conditions (Conrad 1988).
Over time, Skagit River Chum Salmon spawning densities have decreased, and the Skagit River
is also having a changing hydrograph (Lee et al. 2016). In addition, the original development of
spawning ground escapement methods resulted in two varying approaches. Given the differences
in number of fish returning 1976 and 1977, expansions generated using 1976 as the base year are
approximately double the estimates generated by using 1977 as the base year. In most cases, the
final estimate derived by averaging both base year expansions. More importantly annual data
indicates that changes in river morphology, habitat restoration actions, and improvements in
managed flow regimes (beginning in 1981 and again in 1990) have changed the distribution of
Chum Salmon spawning along the Skagit River (Connor and Pflug 2004). There is broad
concern among State and Tribal co-managers that spawning distributions have changed