Monitoring Devils Hole pupfish Cyprinodon diabolis larvae: improving survey design at multiple scales

Authors: Maria C. Dzul, Michael C. Quist, Steve Dinsmore

Year: 2009 (xli)

Abstract

Devils Hole pupfish Cyprinodon diabolis is one of the most endangered vertebrates in North America. The entire population occurs in Devils Hole, a ~20m2 underwater limestone cavern in southwestern Nevada. Since the early 2000s, the threat of extinction has loomed over the Devils Hole pupfish population due to observed declines in adult abundance. In 2005, larval surveys were initiated to better understand patterns in larval abundance and factors influencing recruitment. There are multiple levels in the current sampling design structure: surveys (2 samples per month), events (3 samples per survey), and plots (9 samples per event). The goal of this study was to determine how changing the sample size at each level (survey, event, and plot) affects the ability of scientists to detect a defined change in abundance at some level of statistical power. Using a linear mixed model, variance components were estimated for all three sampling levels (i.e., survey, event, and plot). Next, sample sizes across all levels were allowed to vary, as to represent different combinations of surveys, events, and plots. Variance of the mean was recalculated and used to calculate statistical power. Increasing sample size at the level of survey had the greatest influence on statistical power, followed by plot and event. However, since surveys are more difficult to implement, the optimal sampling design results from increasing the number of plots per event. Higher statistical power will improve the ability to detect trends in pupfish recruitment.