Population Viability of Endangered Lost River Sucker and Shortnose Sucker and the Effects of Assisted Rearing

The Lost River Sucker Deltistes luxatus and Shortnose Sucker Chasmistes brevirostris are two narrowly endemic fish species in the upper Klamath Basin of southern Oregon and northern California. Both species have been federally listed as endangered pursuant to the U.S. Endangered Species Act since 1988 because of dramatic declines in abundance and distribution. In Upper Klamath Lake, Oregon, both species have only recruited a single cohort to the adult populations since that time. Most individuals in this population are at or older than the expected life span of the species. Consequently, the U.S. Fish and Wildlife Service and the Klamath Tribes have initiated assisted rearing efforts to stabilize the population. However, it is unclear how quickly these populations might become extirpated and how assisted rearing might alter population trajectories. We modeled the potential for extinction and recovery of the populations of endangered Lost River Sucker and Shortnose Sucker in Upper Klamath Lake. We simulated population trajectories over the next 50 y with a stochastic population viability assessment approach. Projections indicate that if population trajectories do not change, the Shortnose Sucker population may decline by 78% to number < 5,000 in 10 y and become completely extirpated within the next 30 (18.6% probability) to 40 y (99% probability). The two Lost River Sucker populations have a greater likelihood to remain extant after 50 y, with only 1% probability of extinction given our scenarios and assumptions, but the populations are likely to number fewer than 1,000 individuals. Our results also suggest that rearing of Klamath Lake sucker species in a controlled environment for augmenting the natural population will be effective in reducing extirpation probabilities over the next 50 y if survival to recruitment can be achieved, but a long-term effort of at least 40 y will be required. The necessity of long-term augmentation to ensure population persistence in the absence of natural recruitment underscores the urgent need to determine and address the causes of recruitment failure in the wild.