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December 29, 2022

Managing the Upper Klamath Basin to meet its many desired uses requires a delicate balance. The shortnose sucker (Koptu) and both lake-spawning and river-spawning Lost River sucker (C'waam) populations, all currently listed as endangered under the Endangered Species Act, are declining, leading to water management strategies to control lake levels in support of fish populations.

In an effort to understand the complex relationships between environmental quality and the sucker populations, researchers examined the connection between fish survival and water levels and water quality of Upper Klamath Lake. The paper, published in the North American Journal of Fisheries Management, found that adult shortnose and Lost River sucker survival were not impacted by historic changes in either water quality or lake surface elevation.  

Scientists evaluated 22 years of survival data to identify water management and environmental factors affecting fish survival. With stakeholder input, USGS scientists developed a model set for evaluation that included several competing hypotheses and management scenarios to better understand their effect on adult sucker survival during the study period. None of the modeled water quality nor lake level explanatory variables were correlated to year-to-year differences in survival. During the 22-year study period, lake levels were part of a water management strategy that recognized specific lake levels for different life-history stages, such as maintaining spawning habitat for lake-spawning suckers. Survival for all three populations had minimal year-to-year variation, with the exception of 2017, which had a known fish kill affecting all populations. The lack of year-to-year variation in adult survival rates makes it more difficult to identify environmental or management factors affecting the populations.   

Once suckers reach maturity, they typically have high annual survival rates and, over time, die of natural causes. From 1999-2021, the number of juvenile suckers in Upper Klamath Lake maturing to adults was negligible in all spawning populations; furthermore, those suckers which did mature into adults were never enough to offset those that died.  Consequently, the populations of suckers in the Upper Klamath Basin have been getting older and are nearing their maximum lifespan.  Without a greater number of juveniles surviving to adulthood, overall sucker populations will continue to decline. Therefore, time is limited for the recovery of suckers because the adult populations are reaching their maximum known ages and young fish are not surviving to maturity and reproducing.  

The successful recovery of endangered suckers in Upper Klamath Lake may rely on shifting research from the causes of adult deaths to the reasons why juvenile suckers do not survive to maturity, including studying the effects of lake levels and water quality on juvenile sucker survival.  This paper does not identify the barriers juvenile suckers face to reaching reproduction/maturation but does recommend that new research on young suckers could benefit the development of management strategies and lake level regimes that accommodate both societal and environmental needs.