Effects of groundwater fluctuations on the distribution and population structure of two cyprinid fishes in a desert spring complex

Relationships between groundwater levels, surface water levels, and the distribution and habitat use of two cyprinid fishes, Least Chub (Iotichthys phlegethontis) and Utah Chub (Gila atraria), were examined at a spring complex in the Snake Valley of the Great Basin, USA to test the hypothesis that the distribution and structure of fish populations in the spring complex is regulated by the influence of shallow groundwater levels on the depth and distribution of surface water and the availability of suitable spawning and juvenile habitat. Groundwater levels explained 97% of the temporal variation in surface water levels measured at 47 monitoring points and exhibited annual cycles linked to evapotranspiration rates. Least Chub and Utah Chub migrated from springs to ponds, which were used as spawning and juvenile habitat, when groundwater and surface water levels were high during the early spring, but became concentrated in deep springs as ponds receded during the late summer and early fall. Populations of both species became increasingly fragmented as groundwater and surface water levels declined. Specific differences in the relative abundances and body sizes of juvenile Least Chub inhabiting different seasonally isolated regions of the spring complex persisted over multiple years, suggesting that juvenile growth rates and survival were influenced by connectivity of core spring habitats to seasonally deep ponds. The nature of the relationship between groundwater and surface water levels indicated that long-term declines in shallow groundwater levels of >= 40 cm would eliminate most of the spawning and juvenile habitat in the spring complex.