A Regional Approach for Monitoring Salmonid Status and Trends: Results from a Pilot Study in Coastal Mendocino County, California

Abstract

In coastal California, many evolutionarily significant units (ESUs) of Chinook salmon Oncorhynchus tshawytscha, coho salmon O. kisutch, and steelhead O. mykiss are listed under the federal Endangered Species Act (ESA). Monitoring species status at the ESU or ESU subdivision scale requires specialized sampling. The purposes of this study were (1) to evaluate abundance estimated from a generalized random tessellation stratified (GRTS) design and compare the results with those from a more intensive stratified random monitoring program and (2) to evaluate the statistical power of the design to detect population trends. This 3-year pilot study considered five Mendocino County streams as an example region of coastal California to evaluate a two-stage sampling approach for monitoring regional escapement. Under this scheme, regional redd surveys (stage 1) were conducted in stream reaches in a GRTS sampling design. Ten percent of anadromous habitat was sampled in year 1 and 10–35% in years 2 and 3. Spawner:redd ratios were derived from smaller-scale census watersheds (stage 2) where “true” escapement was estimated using capture–recapture methods. Regional escapement was then estimated from expanded redd counts, calibrated by spawner:redd ratios. As an alternative, more intensive method for estimating escapement, three survey streams were also sampled in a stratified random design. The results, added to counts from the census basins, produced more rigorous “sum-of-streams” estimates for comparison with the GRTS sampling. Redd counts and the resulting escapement estimates were reliable for regional monitoring. The GRTS and sum-of-streams estimates overlapped, and the variation in the 95% confidence intervals did not change after 15%. Our results suggest that a sample size of 15% or 41 or more reaches (whichever results in fewer survey reaches) should have adequate precision and statistical power to detect regional trends in salmon populations. We recommend that this monitoring approach be applied at regional spatial scales consistent with ESA recovery planning efforts.