Identifying sources of error in adult surveys of Devils Hole p upfish Cyprinodon diabolis

Authors: Maria C. Dzul, Michael C. Quist, Stephen J. Dinsmore

Year: 2009 (xli)

Abstract

The Devils Hole pupfish, Cyprinodon diabolis, is an endangered fish that is endemic to Devils Hole, Nye County, NV. Adult surveys for Devils Hole pupfish began in 1972 and continue to present day. Adult surveys estimate population size from SCUBA diver visual assessments because Devils Hole pupfish cannot be handled. Variability associated with adult surveys makes it difficult for managers to differentiate population trends from sampling error. We tested the effects of four potential sources of error (fish movement, time of day, observer, and diver order) on Devils Hole pupfish counts in July 2009 by implementing a set of twelve experimental dives in Devils Hole. The dive route was divided into four levels designated by depth boundaries. Fish abundance estimates were recorded at each level. The experimental dive design included four days of dives with three dives per day. Dives were conducted in morning, midday, and afternoon to test the effect of time of day. On two days of dives, a mesh block net was placed as a barrier between habitats to test the effect of fish movement on population estimates. To test the effects of observer and diver order, divers switched orders during the course of the dive. Pupfish counts were analyzed using a split-split plot ANOVA, with level added as an additional factor to account for differences in pupfish densities at different depths of Devils Hole. Our results indicate that there was no effect (P > 0.05) of observer and diver order. However, we found strong effects of fish movement (P < 0.01) and time of day (P < 0.01) by level, which indicates that (1) density was greater at shallower depths on dives conducted with a block net compared to dives without a block net, and (2) density was greater at shallower depths during morning and midday dives than during afternoon dives. Our results suggest that fish may swim away from divers during the course of the dive, thus implying that historic Devils Hole pupfish surveys may underestimate abundance. The interpretation of these results highlights the advantages and disadvantages of using a block net on subsequent adult surveys of Devils Hole pupfish.