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

Abstract

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.

Growth, survival, and cohort formation of juvenile Lost River (Deltistes luxatus) and shortnose suckers (Chasmistes brevirostris) in Upper Klamath Lake, Oregon, and Clear Lake Reservoir, California—2019 Monitoring Report

“…Although survival was intermediate from August-September 2019 compared to survival in previous years, 2019 is far from being considered a successful cohort. High fecundity may be a life history strategy to overcome high mortality for juvenile suckers in the Klamath Basin, but near complete mortality is unsustainable (Rasmussen and Childress, 2018). Given that the adult populations of Lost River and shortnose suckers have decreased by more than 50 percent since the early 2000s (Hewitt and others, 2018), there would have to be a significant recruitment event soon for both species to recover naturally.…”

Juvenile Lost River and shortnose sucker year-class formation, survival, and growth in Upper Klamath Lake, Oregon and Clear Lake Reservoir, California—2017 Monitoring Report

“…While it is typical for survival to be very low in the early life stages of fish (Houde 1989), near complete disappearance of entire cohorts within the first two years is alarming. High fecundity may be a life-history strategy to overcome high mortality for suckers in the Klamath Basin, but near complete mortality is unsustainable (Rasmussen and Childress, 2018). Given the adult population has decreased more than 50 percent since the early 2000s (Hewitt and others, 2018), there would have to be a significant recruitment event soon for both species to recover naturally.…”

“…Stream-resident populations of shortnose suckers may provide resiliency for Clear Lake suckers during dry years. Rasmussen and Childress (2018) estimated some Lost River and shortnose sucker populations may become extirpated within the next 50 years, which is an optimistic estimate. With adult populations diminishing, it has become more important than ever to determine the causes of recruitment failure of Lost River and shortnose suckers.…”

Effect of Temperature on Survival of Lost River Suckers with a Natural Infection of Ichthyobodo spp.

“…These programs were established due to extremely limited survival of juvenile Lost River Suckers in the wild, with the last recruitment of new adults into the spawning population occurring in the late 1990s (Burdick and Martin 2017; Day et al 2017; Hewitt et al 2018). Both the federal and tribal programs capture and raise wild‐produced larvae, with the goal of releasing juveniles that are at least 1 year of age and a minimum of 172 mm SL, which are assumed to have a high probability of survival after reintroduction into Upper Klamath Lake (Day et al 2017; Rasmussen and Childress 2018). It is hypothesized that larger suckers may survive better after reintroduction than smaller suckers because wild Lost River Suckers rarely live past their first year of life and annual survival of adult Lost River Suckers in the lake is typically greater than 90% (Burdick and Martin 2017; Hewitt et al 2018).…”