Sample Size Estimation for On-Site Creel Surveys

Abstract

Conducting sample size analysis is important to ensure that sample sizes are adequate to meet objectives for precision but not so large that valuable resources are wasted. When simple survey designs are used, sample size analysis is straightforward. However, creel surveys often follow complex designs that can make sample size estimation difficult. The objectives of this study were to provide sample size estimators for commonly used creel survey designs and investigate sample size requirements to achieve varying levels of precision. For estimates of angling effort, the average sample size among fisheries required to achieve relative 95% confidence intervals of 40% (i.e., coefficient of variation of approximately 0.2) was 16 d (range = 7–40 d). To estimate mean catch rate with the same level of precision, 43 survey days (range = 8–95 d) were required when the daily catch rate estimator was used, and 11 d (range = 3–30 d) were required when the multi-day estimator was used. Fifty-five days (range = 14–89 d) and 57 d (range = 19–140 d) were required to estimate total catch with relative 95% confidence intervals of 40% when the daily and multi-day estimators were used, respectively. Although absolute precision will vary among fisheries, our results suggest that approximately 30 d of surveying may serve as a reasonable starting point for planning creel surveys when no prior information is available on expected sample variance, as relatively large increases in sample size beyond 30 d were needed to achieve relatively small increases in precision.

Received February 14, 2017; accepted June 9, 2017 Published online August 4, 2017

ACKNOWLEDGMENTS

We thank Tim Bailey, Greg Huchko, Steve Mazur, Mike Meeuwig, and Tim Porter (Oregon Department of Fish and Wildlife) and Joe Kozfkay, Rob Ryan, and Jon Flinders (Idaho Department of Fish and Game) for sharing data from previously conducted creel surveys, which facilitated our study. We are also grateful to Jeff Dillon, Dan Schill, Mike Gauvin, and three anonymous reviewers for reviewing a draft of the manuscript.