Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 4504
Title: A simple variance formula for spawner abundance estimated using trapezoidal area-under-the-curve
Author: R. B. Millar, Chris E. Jordan
Publication Year: 2013
Journal: Canadian Journal of Fisheries and Aquatic Sciences
Keywords: variance estimator,escapement,area under the curve

The trapezoidal area-under-the-curve (TAUC) method is the simplest and most widely used for estimating spawner abundance from periodic surveys of spawner counts, but it currently lacks an estimate of its precision. For this reason the Gaussian area-under- the-curve (GAUC) method was recently presented as an easily-implemented alternative that has the advantage of a variance estimator. However, under a diverse variety of simulation scenarios, the TAUC estimator was seen to be a slightly better estimator of escapement on the basis of mean-squared error. It is shown here that a variance estimator for the TAUC method can be obtained as an immediate byproduct of the GAUC method. Furthermore, this new approach can easily be extended to accommodate asymmetric and multimodal patterns of spawner abundance by the use of basis splines. Thus, salmon managers can continue to use historical estimates of spawner abundance obtained using the TAUC method, but can now quantify the precision of these point estimates.

Theme: Recovery, Rebuilding and Sustainability of Marine and Anadromous Species
Foci: Characterize vital rates and other demographic parameters for key species, and develop and improve methods for predicting risk and viability/sustainability from population dynamics and demographic information.