Ecological Comparison between Three Artificial Refuges and the Natural Habitat for Devils Hole Pupfish

Attempts to maintain refuge populations of Devils Hole pupfish Cyprinodon diabolis in artificial tanks have achieved limited success. Previous studies have documented changes in morphological, behavioral, and genetic characteristics of refuge populations, which suggest that environmental conditions (and thus selective pressures) differ from those found in Devils Hole (DH). Physical, chemical, and biological characteristics were compared among the three Devils Hole pupfish refuges (Hoover Dam, School Springs, and Point of Rocks) and between each refuge and DH. In contrast to the thermally constant environment in DH (∼33°C), mean refuge temperatures were cooler and fluctuated on a weekly and seasonal basis. On two occasions, extreme temperature fluctuations lasting several weeks (due to water supply malfunctions) were recorded at the Hoover Dam (6°C decrease) and School Springs (22°C decrease) refuges. The physical design of the refuges precludes surface runoff from entering them; thick layers of anoxic sediment accumulated, particularly in School Springs (mean depth = 19.1 cm; range = 1–30 cm) and Point of Rocks (mean depth = 8.9 cm; range = 5–39 cm), thus burying the rocky substrate designed to replicate the upper shelf in DH. All three refuges had higher mean oxygen concentrations and lower coefficients of variation (CVs; range = 12.2–16.9%) than the DH upper shelf (CV = 52.5%), which receives limited direct sunlight. Attached algal standing crops differed several-fold among refuges (range = 31.0–231.7 g of ash-free dry mass/m ²) but were not significantly different between seasons. In contrast, benthic biomass values reported from DH were smaller and varied seasonally. Aquatic invertebrate taxa that were abundant in DH were rare or absent in the refuges. These results demonstrate how artificial refuge environments deviate from natural conditions in DH and further illustrate the challenges faced when attempting to establish and maintain long-term refuge populations as a conservation strategy to preserve threatened and endangered fishes.