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Population dynamics of the two remaining native lake populations of threatened Lahontan cutthroat trout
AuthorSimmons, James Benjamin
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Once widespread across lakes of the western Great Basin and the Sierra Nevada mountains of the United States, native self-sustaining populations of Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) remain in two lake ecosystems: eutrophic desert terminal Summit Lake and mesotrophic montane Independence Lake. Maintenance of the life history and genetic diversity within the Summit Lake and Independence Lake populations has been identified as an important goal of Lahontan cutthroat management and restoration efforts. In addition, the Summit population is of significant cultural importance for the indigenous Summit Lake Paiute people. However, little research has been conducted on the population dynamics of lake-dwelling cutthroat trout, especially in desert terminal lakes. Thus, we performed population dynamics research on the Summit Lake and Independence Lake populations to provide direction for their sustainable management and for range wide conservation strategy. At Summit Lake, we quantified the population dynamics (e.g., number of spawning fishes during a 50 year record from 1968-2017, abundance, population growth rate) and performed a sensitivity analysis to identify the life history transitions with the most influence on the population growth rate. Abundance was estimated from a robust design mark-recapture effort. For the population growth rate and sensitivity analyses, we created a stage-classified (Lefkovitch) matrix population model with skipped spawning and parameterized it with data from the mark-recapture-detection of individuals in the lake and spawning tributary (Mahogany Creek), as well as data or results from an Independence Lake population viability analysis study. Adult abundance declined steadily (2096 to 661 individuals), and the growth rate indicated a declining population (0.52). The Nonspawning to Nonspawning and Spawning stages (0.53 and 0.11, respectively) had the most influence on the population growth rate. The growth rate was driven by low adult survival (0.51) and the high and low probabilities that a nonspawner would remain a nonspawner (0.82) or become spawner (0.18), respectively. These results contradict previous findings that identify juvenile life stages as the most sensitive parameters in cutthroat trout population studies. In addition, low fecundity (0.85) likely decreased recruitment. Then we compared the population dynamics of the Summit Lake and Independence Lake populations. Using the model described above for each population, we parameterized the models with data and parameters from their respective population studies and compared their population growth rates and sensitivity analyses. Both population growth rates indicated decline (0.94 and .52 < 1, respectively), and the populations shared the third most sensitive parameter - low repeat spawning rates (0.44 and 0.36, respectively). But the difference between the growth rates was large, the Independence Lake growth rate overlapped 1, and the top two sensitive parameters at Independence Lake were fry (0.03) and juvenile survival (0.25). Also, the much higher fecundity at Independence Lake (87) likely contributed to their higher growth rate via recruitment. At Summit Lake, sensitive adult parameters may be indicative of life history adaption to the desert habit, a plastic response to drought, or simply the norm for self-sustaining populations not impacted by invasive species or other adverse factors. Our findings suggest managers should focus their efforts on protecting juveniles in Independence Lake and adults and fecundity in Summit Lake, and to guard against assuming that intra-specific populations have the same population drivers, especially populations in disparate habitats.