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Population genetics and population dynamics of Moapa dace
AuthorHereford, Danielle M.
AdvisorPeacock, Mary M.
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ABSTRACT: The Mojave Desert is an arid environment where precipitation ranges from 3.4 to 31.0 cm of rain per year. Species adapted to live in the Mojave Desert tend to be drought resistant and typically utilize little water. Geothermal springs are scare but provide consistent dependable water resources from large carbonaceous aquifers. Springs in the Mojave Desert were manipulated by human populations for agriculture, recreation, or municipalities. As a result, aquatic ecosystems and aquatic species were disrupted on many levels. For example, habitat fragmentation limited movement and dispersal of organisms; population isolation constrained meta-population dynamics and gene flow, and non-native species disrupted food webs, trophic interactions, and displaced native species. As water demands continue to increase in Southern Nevada, aquatic ecosystems are at greater risk and need to be carefully managed. Endangered species risk losing genetic variation and evolutionary potential when habitat is fragmented, limited, or both. Restricted habitat can also limit survival of individuals, recruitment within a population, and the size of a population. This study quantifies genetic variation, population structure, and population dynamics of Moapa dace Moapa coriacea in its restricted and fragmented habitat. Moapa dace is a thermophillic cyprinid endemic to the Muddy River and its tributaries in Clark County, NV. Historically, Moapa dace occurred in the upper 16 kilometers of the Warm Springs area- the Muddy River and its tributaries. Moapa dace are drift feeders that have unique physiology and biology adapted to live in warm water with low levels of dissolved oxygen. Populations of Moapa dace have experienced substantial population declines since they were first described by Hubbs and Miller in 1948. Humans manipulated dace habitat by diverting spring outflows for regional municipalities, agriculture, or recreation. After substantial population declines, Moapa dace were listed as endangered in 1967 and United States Fish and Wildlife Service began purchasing property at spring sources to create the Moapa Valley National Wildlife Refuge to protect Moapa dace habitat. Moapa dace populations increased, but later declined when a downstream diversion dam was removed and introduced the non-native piscivore blue tilapia. A gabion barrier was installed in 1997 at the confluence of the Apcar tributary and the Muddy River to protect 2.8 kilometers of stream habitat from further tilapia invasion. Moapa dace have been restricted to the Apcar, Pederson, and Plummer tributaries since 1997.The Moapa dace population was around 1000 individuals from 1999 to 2007, then substantially declined to less than 500 in 2008. From October 2009 to September 2012 I conducted a bimonthly mark recapture study and estimated Moapa dace abundance, survival, recruitment, and rate of population growth. DNA was extracted from fin clips and ten polymorphic microsatellite loci were used as tags to identify individuals. I also used genetic samples to quantify genetic diversity and population structure among stream tributaries over the three year period. Moapa dace survival varied by population and over time. Survival was highest in Upper Apcar and lowest in Lower Plummer. Bimonthly Moapa dace survival was highest from February to September and lowest from October to January. Throughout the three year study, annual survival increased by more than 20%. Recruitment was highest in April and June but occurred year round. The rate of population growth declined in Upper Pederson, Upper Plummer, and Lower Plummer in 2011, but increased in Upper Apcar and Lower Apcar. In 2012, the rate of population growth increased in all populations except Upper Plummer where population growth was not detected in 2012. Over the three year period, increases in Moapa dace population growth was more influenced by increases in survival than increases in recruitment. Moapa dace were genetically diverse. Allelic richness and heterozygosity were high (R<sub>S</sub> = 11.05 ± 6.03, H<sub>E</sub> = 0.70 ± 0.23) while the effective population sizes was very low in Apcar (N<sub>e</sub> = 22 - 34) and Pederson (N<sub>e</sub> = 21 - 71) and relatively low in Plummer (N<sub>e</sub> = 82- 104). A population bottleneck was detected using the heterozygosity excess method, supporting snorkel abundance estimates that depicted recent population declines between 2007 and 2008. Bayesian genotype clustering analysis identified three distinct populations that are separated by current or historic barriers. Significantly different F<sub>ST</sub> values support the Bayesian genotype clustering analysis depicting limited gene flow in this fragmented stream habitat. The distinct genetic signature depicted in Pederson has been retained from a translocation that occurred in 1984. High levels of genetic variation suggest that evolutionary potential remains high, but low effective population sizes per tributary and geographically distinct populations also suggest that erosion of existing variation due to drift remains a possibility. Management efforts should be aimed at removing barriers and expanding available habitat for Moapa dace to allow for gene flow and maintenance of genetic variation. This study is the first to quantify the genetic diversity of Moapa dace. It adds to the growing literature of conservation genetics and specifically identifies how habitat fragmentation has resulted in sub-population differentiation. It is also the first robust population dynamics analysis of Moapa dace survival, recruitment, and rate of population growth. In this study, I establish essential baseline information that will be crucial for assessing the effectiveness of current and future conservation efforts.