For specific stocks of Chinook and coho salmon, the proportion of adult returns from a particular year class is not often known. This proportion, or escapement, is the number of juvenile salmon that survive to the smolt stage, migrate to the ocean, and return to spawn as adults after several months or years (Healey 1991).
Ordinarily, the proportions of fish that die in freshwater vs. those that die in the ocean can only be estimated. Thus adult return data, such as counts at dams or traps, can be used only as an index or surrogate measure of ocean survival. With these caveats in mind, we present adult data from various sources with which we compare outlooks based on ocean indicators.
Adult data are lagged behind ocean entry by 1 year for coho salmon and 2 years for spring and fall Chinook salmon; therefore, as of 2018, we have up to 21 years of indicator data but only 18 - 20 years of adult return data. We have two less years of data for the Klamath adult returns because those numbers are not available from the Pacific Fishery Management Council until the following February (PFMC 2018b).
Data used in the rank scores above are shown in the chart below. Again, counts of spring and fall Chinook salmon at Bonneville Dam are shown lagged by 2 years. For example, for fish that entered the ocean in 1998, the number listed for spring and fall Chinook salmon indicates adults that returned in 2000. Although we use a 2 year return lag for Chinook salmon, we acknowledge that there are different age classes of returning adults with lags of 2 - 5 years. For example, spring Chinook salmon that entered the ocean in 2000 may return to spawn in 2002 - 2005.
Note also that these estimates were not adjusted for catch in fisheries, which can have a major impact on adult numbers. For example, ocean fisheries for Chinook salmon off California and most of the Oregon coast were closed in 2008 and 2009; these fisheries typically catch hundreds of thousands of Chinook salmon annually (PFMC 2018b). Consequently, adult returns to basins most impacted by this closure (e.g., Klamath River) in those years reflect both substantially reduced harvest rates and the influence of ocean conditions on marine survival. Accordingly, direct comparisons of adult abundances across years should be made with considerable caution due to this high variation in harvest rates.