Demography of lake trout in relation to population suppression in Yellowstone Lake, Yellowstone National Park
Syslo, John Michael
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Introduced lake trout Salvelinus namaycush threaten to extirpate native Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri in Yellowstone Lake, Yellowstone National Park. Suppression of the lake trout population is deemed necessary for the conservation of Yellowstone cutthroat trout. A National Park Service gill netting program removed nearly 273,000 lake trout from Yellowstone Lake between 1995 and 2007. Lake trout population size has not been estimated; therefore, it is difficult to determine the efficacy of the program (i.e., proportion of the population that has been removed). My objectives were to (1) examine catch per unit effort (C/f) through time and catch as a function of effort to determine if the suppression program has caused lake trout abundance to decline, (2) determine if length structure, age structure, individual growth, mortality, body condition, length at maturity, age at maturity, and fecundity have changed as a function of harvest, and (3) develop age-structured models to determine the level of mortality required to cause population growth rate to decline below 1.0 (replacement). An increase in lake trout abundance was indicated by increasing C/f over time. Additionally, catch has continued to increase as a function of effort, indicating lake trout abundance is increasing. Population metrics were not clearly indicative of a response to harvest. However, metrics were comparable to North American lake trout populations where harvest has occurred, indicating that lake trout have not reached carrying capacity in Yellowstone Lake. Results from an age-structured matrix model determined the rate of population growth was 1.1 given the current rate of fishing mortality and that population growth rate would be 1.3 in the absence of fishing mortality. The current rate of population growth is positive; however, it is slower than it would be in the absence of lake trout suppression. Fishing mortality needs to increase from the rate of 0.22 estimated in 2007 to at least 0.32 to reduce population growth rate below replacement. Lake trout suppression is becoming an increasingly common management practice throughout the Intermountain West. Thus, Yellowstone Lake provides a case study for evaluating a strategy to remove the apex predator from a large lake.