Detecting persistent change in the habitat of salmon-bearing streams in the Pacific Northwest

In the northwestern United States, there is considerable interest in the recovery of Pacific salmon (Oncorhynchus spp.) populations listed as threatened or endangered. A critical component of any salmon recovery effort is the improvement of stream habitat that supports various life stages. Two factors in concert control our ability to detect consistent change in habitat conditions that could result from significant expenditures on habitat improvement: the magnitude of spatial and temporal variation and the design of the monitoring network. We summarize the important components of variation that affect trend detection and explain how well-designed networks of 30-50 sites monitored consistently over years can detect underlying changes of 1-2% per year in a variety of key habitat characteristics within 10-20 years, or sooner, if such trends are present. We emphasize the importance of the duration of surveys for trend detection sensitivity because the power to detect trends improves substantially with the passage of years.