Relationship Between Steelhead Parr Densities Observed During Snorkel Surveys and Emigration Estimates From Rotary Screw Traps.

Snorkel surveys are widely used for monitoring fish populations because they are a versatile and cost-effective technique. However, snorkel surveys provide instantaneous measurements that might not describe continuous processes, such as the emigration of juvenile anadromous salmonids. A model capable of assessing the level of potential production of emigrating juvenile steelhead (anadromous Oncorhynchus mykiss) from watersheds in Idaho on the basis of snorkel surveys would be useful. Our objective was to determine the relationship between abundance of emigrants estimated from rotary screw traps (RSTs) and parr densities from snorkel surveys. We analyzed over 1,000 snorkel surveys and 70 year-watershed combinations of emigrant estimates. Density was positively related to emigrant abundance. The most reasonable model included a common slope among watersheds, a common year effect, and varying intercepts, supporting a hypothesis that some watersheds produce more emigrating juveniles at similar observed densities. This model explained 82.7% of the variation observed in emigrant abundance. Uncertainty was greater among watersheds than within a watershed. The model can be used to translate mean densities observed in snorkel surveys to abundance of steelhead emigrants and thus to infer the effects of a habitat restoration program or other management actions. This work is important as a step toward understanding juvenile emigrant abundance in watersheds without RSTs and to aid understanding of steelhead emigrant productivity in Idaho watersheds. [ABSTRACT FROM AUTHOR]