Coho salmon spawner abundance data are collected annually by WDFW and tribal co-manager staff as part of baseline monitoring and data collection efforts.

Washington co-manager agreed-to datasets, as provided by WDFW staff including WDFW Fish Program District Biologists

Biologists typically estimate annual spawner abundance by counting the number of redds (gravel nests) in a river each year. Redds are counted by walking the stream or from boats or aircraft. In many populations, some hatchery-origin salmon are present on the spawning grounds along with natural-origin salmon, complicating the estimates of natural-origin abundance. The proportions of spawners that were of natural versus hatchery origin are typically estimated based on the composition of carcasses of each origin. These proportions are applied to the total spawning population to estimate the number of natural-origin versus hatchery-origin spawners.

Coho salmon spawner abundance is reported here as the number of natural-origin fish estimated on the spawning grounds (spawning naturally). We report on five key Puget Sound natural-origin coho salmon management units, for which abundance targets have been defined and for which natural-origin escapement estimates are produced.

Our analysis of abundance change over time answers the question:

Has the spawner abundance of Puget Sound coho salmon management units changed since 2000?

The year 2000 was selected as the baseline reference period because it is close to when Puget Sound Chinook salmon and Hood Canal summer chum salmon were listed under the ESA (1999) and provides a substantial time series for analysis.

In this trend evaluation, we fit a multivariate autoregressive state space random walk with drift (MARSS-RWD) model to the log of management unit spawner abundance data for all management units from 2000 to 2021. The model structure was identical to that used by NOAA Fisheries’ Northwest Fisheries Science Center in its 5-year status review with a few exceptions (Northwest Fisheries Science Center, 2015). The slope of each management unit’s smoothed abundance values was converted to percent change per year. If data were not available during these periods for a given management unit, data from the first or most recent years with available data were used.

To assess each management unit’s spawner abundance levels, we generated a “percent of target” by comparing the recent 5-year geometric mean (“geomean”) from the MARSS-RWD model against the low-normal escapement abundance breakpoints in effect under Pacific Fishery Management Council/Pacific Salmon Commission fisheries management. These breakpoint values determine the allowable harvest in a given year (as measured in pre- and post-season assessments of “exploitation rates”), and they help to establish the framework for sustainable harvest. Puget Sound coho salmon are not managed for recovery like our other salmon Vital Sign indicator species, and the breakpoints were not formulated in the context of Endangered Species Act requirements (i.e., they are harvest levels that do not prevent rebuilding).

For more information about the methods used for this indicator, please see the Status and Trends Analysis of Salmon Abundance Data methods report.

• Natural-origin fish: fish produced by parents spawning in natural environments (not in a hatchery)
• Natural-origin adult fish on the spawning ground: fish produced by parents spawning in natural environments that themselves returned to spawn in a natural environment
• Population spawner abundance: number of natural-origin coho salmon estimated on the spawning grounds in a population
• Hatchery-origin fish: fish produced by parents spawned in a hatchery
• Endangered Species Act (ESA): Puget Sound coho salmon are not listed under the ESA
• MARSS-RWD: multivariate autoregressive state space random walk with drift model, which was the statistical model used in our analyses