The Cottonball Marsh pupfish is found only in the 640- acre Cottonball Marsh in the northwest portion of Death Valley National Park. The marsh is a component of the Salt Creek drainage lying within the saltpan on the floor of Death Valley. The marsh is an extreme habitat. Salinity ranges from 14 ppt (parts per thousand) to 160 ppt, about 4.6 times that of seawater. Water temperatures range from near freezing in winter to almost 104° F in summer. In shallower waters, the temperatures may fluctuate daily as much as 59° F. Cottonball Marsh also supports the endemic Badwater snail (Assiminea infima).
The Cottonball Marsh pupfish is a small (less than 1.5 inches), slender pupfish. Breeding males become deep blue on the sides with dark gray lateral bars and have an iridescent purple sheen on the back and silvery sheen on the sides. Females have a more elongate shape with an overall silvery brown coloration and lateral vertical bars. Little is known about either the reproductive biology or food habitats of Cottonball Marsh pupfish.
Cottonball Marsh is part of the Salt Creek system and located in a designated Wilderness Area. The limited habitat and restricted distribution of the pupfish makes it vulnerable to stochastic events such as droughts or earthquakes, which could disrupt the species' sources of groundwater. Threats to its survival include direct and indirect habitat alteration through regional water diversion, as well as from changes to water levels, water quality, and/or chemistry.
To assess possible effects of groundwater withdrawal on the wetland-dependent species, Death Valley National Park began a three-year study in 1998 to quantify the evapotranspiration rate for the Death Valley saltpan/playa. These data will greatly increase current understanding of the water budget for the Death Valley ground water system and allow the NPS to better represent the ground water interests of the park. Results of this study are not yet available.
In the fall of 1993, a GPS was used to accurately map the distribution of the Cottonball Marsh pupfish for the first time. Habitats occupied by the pupfish were found to occur along a linear line which measured approximately two miles in length. This distribution suggests that the spatial extent of pupfish habitat is probably a function of a geological structural control which results from the contact of permeable alluvial material and the denser playa mud. Although pupfish habitats were found to consist of a variety of pool and stream habitats, it is likely that pupfish occupy less than a hundred discrete water bodies during the summer and fall.
The genetic relationships among the Death Valley pupfish populations and their evolutionary histories have been inferred primarily from morphological similarities of the fishes and the hydrological relationships of their habitats. However, the underlying genetic and historical basis for morphological features had remained unknown. A recent study analyzed the genetic structure and divergence within and among populations using mitochondrial and nuclear DNA markers. The findings of these studies illustrated that the divergence of the small, fragmented pupfish populations from remnant aquatic systems in Death Valley was largely by genetic drift.
Genetic diversity within populations from the Salt Creek drainage (Cyprinodon salinus salinus at McLean Spring and in Salt Creek), and the Cottonball Marsh pupfish, was generally low, with most variation distributed among populations. This lack of genetic variation may point to one or more historical bottleneck events and indicate that the McLean Spring/Salt Creek and Cottonball Marsh populations were the same (monomorphic) prior to their separation about 2000 years ago. The findings also confirmed that many of the isolated populations of pupfish are demographically independent and should be managed as separate units to conserve the species.
(Excerpted from DFG publication, Species Accounts - Fish (PDF)