New insights into riverine migration by salmonid smolts

The process of smolting is a critical phase in the life cycle of anadromous sal-
monids, and it has been associated with substantial rates of mortality. Survival
during freshwater and marine migration is known to have population-level
effects; thus, an understanding of the patterns of mortality has the potential to
yield important insights into population bottlenecks. Despite important
advancements in tracking techniques, the specifics of mortality events in anad-
romous salmonids during their initial migration to sea remain somewhat elu-
sive. Here, we develop a framework combining spatial and temporal
detections of smolt riverine migration from two tracking techniques, which
enable inferences to be made about mortality locations, causes, and rates. In
this study, we demonstrate that during their initial riverine transitional phase,
smolts were particularly vulnerable to predators. Specifically, avian predation
appeared to be the main cause of mortality (42%), although piscine predation
events were not trivial (14%). Our results suggested some direct and indirect
tagging-induced mortality (e.g., through increased predation vulnerability),
which highlights the importance of determining tagging mortality in a teleme-
try study to ensure adequate interpretation of migration success. Overall, by
estimating migration loss and its variability, our study framework should help
to guide management actions to mitigate the widespread population declines
these species are currently facing.