Variability in upstream extent of fish distribution in headwater streams in southwest Washington, USA.

The article focuses on assessing spatial and temporal variability in the upstream extent of fish distribution (UDF) in headwater streams of southwest Washington, USA, to inform forest management and regulatory practices. Using repeated electrofishing surveys from 2015 to 2020 on 201 streams, the study found that variability in UDF location typically occurred over short distances—most changes were within 152.4 meters (500 feet)—and was more frequent at “terminal” sites (where the UDF occurs along a single stream channel) than at “lateral” sites (non-fish-bearing tributaries intersecting fish-bearing main stems). The probability of upstream shifts in UDF location was positively associated with wider stream channels and negatively associated with steeper gradients, and natural barriers or blockages reduced variability. Coastal Cutthroat Trout (Oncorhynchus clarkii) were the predominant species at terminal UDF points, with taxa remaining consistent across years and seasons at most sites. These findings support current Washington State forestry regulations that provide riparian buffer protections within distances corresponding to observed UDF variability, aiding resource managers in delineating fish-bearing stream segments more accurately.