Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 9282
Title: Estimating observer error and steelhead redd abundance using a modified Gaussian area-under-the-curve framework
Author: A. R. Murdoch, Chad J. Herring, Charles H. Frady, K. E. See, Chris E. Jordan
Publication Year: In press
Journal: Canadian Special Publication of Fisheries and Aquatic Sciences
Keywords: redd, steelhead, observer error, AUC,

This study examined how a suite of habitat and environmental variables relate to the ability of a stream surveyor to identify (observer efficiency) and distinguish (observer accuracy) steelhead redds from other stream features. Two existing spawning survey protocols that included one or two redd observers were used to develop models to estimate redd observer error. In most cases, steelhead redd abundance using raw redd counts were negatively biased. Mean annual rates of observer efficiency ranged from 0.441 to 0.566 and observer accuracy ranged from 0.674 to 0.827.   Regardless of the observer error model used, adjusted annual redd abundance estimates were generally unbiased (range -1.6 ¿ 0.6 redds). A Gaussian area-under-the-curve methodology that incorporates redd count data and observer error rates was used to generate unbiased estimates of steelhead redd abundance in the Wenatchee (N = 170, CV = 44%) and Methow (N = 106, CV = 41%) rivers. Unbiased estimates of redd abundance will help inform new population viability analyses to better prioritize those populations with the greatest conservation need.

Theme: Recovery and rebuilding of marine and coastal species
Foci: Characterize the population biology of species, and develop and improve methods for predicting the status of populations.