Upper Columbia River Steelhead Capture-Recapture-Recovery data (2008-2018)

In the Columbia River basin, USA, predation by Caspian terns (Hydroprogne
caspia) on U.S. Endangered Species Act (ESA)-listed juvenile salmonids
(smolts; Oncorhynchus spp.) has led to predator management actions to
reduce predation; however, the assumption that reduced predation
translates into greater salmonid survival, either within the life stage
where predation occurs or across their lifetime, has remained untested. To
address this critical uncertainty, we analyzed a long-term (2008-2018)
mark-recapture-recovery dataset of ESA-listed steelhead trout (O. mykiss)
that were tagged (n = 78,409) and subsequently exposed to predation during
smolt out-migration through multiple river reaches (spatial-scales),
jointly estimating weekly probabilities of steelhead survival, mortality
due to bird predation, and mortality due to other causes. This concurrent
estimation across time-stratified cohorts allowed for the direct
measurement of the strength, magnitude, and direction of relationships
between survival and Caspian tern predation. Results of our analysis
provided novel evidence that predation by Caspian terns may have been a
super additive source of mortality during the smolt life-stage and a
partially additive source of mortality to the adult life-stage. The
estimated levels of compensation have important implications for predator
management actions aimed at increasing the survival of endangered
salmonids, and the modelling approach developed herein provides a
framework to directly quantify the impacts of source-specific mortality
factors on prey populations.