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 are useful to 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 based on snorkel surveys would be useful. Our objective was to determine the relationship between abundance of emigrants estimated from rotary screw traps and parr densities from snorkel surveys. Over 1,000 snorkel surveys and 70 year-watershed combinations of emigrant estimates were analyzed. 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 are more productive even at similar 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 towards understanding juvenile emigrant abundance in watersheds without rotary screw traps and to aid understanding of steelhead emigrant productivity in Idaho watersheds.