Identifying Sources of Error in Surveys of Devils Hole Pupfish (Cyprinodon diabolis)

We assessed four potential sources of error in estimating size of the population of Devils Hole pupfish (Cyprinodon diabolis): net, time of day, diver, and order of diver. Experimental dives (3/day) were conducted during 4 days in July 2009. Effects of the four sources of error on estimates from dive surveys were analyzed using a split-split plot ANOVA. Diver and order of diver had no significant influence on estimates, whereas the effect of presence or absence of a net was significant. Effects of time of day and presence or absence of a net showed a significant interaction with depth of water. Results indicated that pupfish may move upward during the dive, and as a result, the standard methods of dive surveys may underestimate abundance. RESUMEN—Evaluamos cuatro posibles fuentes de error al estimar el tamaño poblacional del cachorrito del agujero del diablo (Cyprinodon diabolis): la red, la hora del dı́a, el buzo, y el orden de los buzos. Buceos experimentales (3/dı́a) se hicieron durante cuatro dı́as en julio del 2009. Los efectos de estas cuatro fuentes de error sobre las estimaciones provenientes de los muestreos del buceo fueron analizados usando un ANOVA de split-split plot. El buzo y el orden de los buzos no tuvieron ninguna influencia significativa en las estimaciones, mientras que la presencia o ausencia de la red fue significativa. Los efectos de la hora del dı́a y presencia o ausencia de la red demostraron una interacción significativa con la profundidad del agua. Los resultados indicaron que los cachorritos puedan subir durante el buceo, y como consecuencia, los métodos estándares de muestreos de buceo pueden subestimar la abundancia. According to a recent compilation of the conservation status of fishes developed by the American Fisheries Society, ca. 39% of taxa of freshwater and diadromous fishes in North America are imperiled, with 230 vulnerable, 190 threatened, 280 endangered, and 61 presumed to be extinct in the wild (Jelks et al., 2008). Rate of extinction in freshwater ecosystems is estimated to be five times greater than in terrestrial ecosystems (Ricciardi and Rasmussen, 1999). In assessing loss of diversity of native fishes in California, Moyle and Williams (1990) determined that some leading factors characterizing imperiled species were endemicity, small area of habitat, habitat in only one drainage basin, low diversity of the native assemblage, occurrence in isolated springs, or a combination of these. Many of these characteristics are exhibited by aquatic environments in deserts. Although aquatic habitats in deserts may appear depauperate of aquatic fauna when viewed individually, they exhibit much greater diversity when viewed collectively because geographic isolation of habitats has resulted in speciation and endemicity. The Devils Hole pupfish (Cyprinodon diabolis) is the poster child for preservation of aquatic ecosystems in deserts because it has been pivotal to legislation to conserve freshwater fishes and endangered species (Deacon and Williams, 1991). Despite extensive research, population dynamics are poorly understood because researchers cannot physically handle pupfish due to the risk of mortality to the fish. As such, estimates of size of the population of adult Devils Hole pupfish must be obtained using SCUBA-diver, visual-survey methods. Routine monitoring of abundance of adult Devils Hole pupfish started in 1972 and continues to present. During surveys, habitat in Devils Hole is divided into strata: the shallow shelf, a boulder ca. 2 by 5 m in area submersed under 0.2–0.7 m of water, and the deep pool or deeper waters of the cavern. Surveys include dive and shelf surveys; two SCUBA divers count pupfish in the deep-pool habitat (i.e., dive survey) while observers count fish from a platform overlooking the shallow shelf (i.e., shelf survey). Counts from the dive and shelf surveys are summed to estimate size of the population of adult pupfish. Surveys are conducted in April and September when the population is at its annual minimum and maximum, respectively. Potential sources of error associated with visual assessments make attaining replicate counts challenging and may limit detection of trends in populations. Among potential sources of error are effects of movement of fish, time of day, order of diver, and diver. The effect of movement of fish is likely a source of error (Watson et al., 1995) due to tendency of fish to move as a school and linger at the boundary between the shelf and deep pool. Fish at the boundary can be double-counted or missed completely by both dive and shelf surveys. Managers have proposed placing a block net at the shelf-deep-pool boundary during the survey to impede movement of fish between the shelf and the deep pool and improve accuracy of counts. Time of day when the survey is conducted also may influence estimates. While surveys of adults that are conducted at different times of day during the same sampling date are meant to serve as replicated estimates, James (1969) noted that Devils Hole pupfish avoid sunlight during midday. Likewise, order of diver may have an effect on estimates, as the first diver may influence the count of the second diver by disturbing fish. Finally, variability in estimates also may be attributed to differences between divers. In ecological monitoring, multiple observers can produce dissimilar results based on differing abilities to detect organisms (Thompson and Mapstone, 1997; Bue et al., 1998; Nichols et al., 2000; Diefenbach et al., 2003). For example, Thompson and Mapstone (1997) reported that bias of observers in dive surveys in the Great Barrier Reef of Australia varied with species of fish, and that while biases for some species could be corrected by calibration-training the diver, biases for other species persisted despite calibration-training efforts. The extent to which these four potential sources of error affect replicated samples needs to be either corrected, quantified, or have its presence acknowledged. Thus, the objective of this study was to identify and minimize (if possible) sources of error in surveys of adult Devils Hole pupfish. MATERIALS AND METHODS—Devils Hole, a fissure formed by tectonic activity, is adjacent to Ash Meadows National Wildlife Refuge, an oasis in the Amargosa Desert in Nye County, Nevada. Devils Hole is the hydrologic head of a deep and extensive