The benefits of merging passive and active tracking approaches: New insights into riverine migration by salmonid smolts

Abstract

The process of smolting is a critical phase in the life cycle of anadromous salmonids, 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 anadromous salmonids during their initial migration to sea remain somewhat elusive. 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 telemetry 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.

Potential Impact of Climate Change on Salmonid Smolt Ecology

“…Survival during migration and patterns of mortality have the potential to yield important insights into population bottlenecks [154]. Smolts are vulnerable to predators during their downstream migration in the surface layer.…”

“…Smolts are vulnerable to predators during their downstream migration in the surface layer. In the southern River Minho (Spain/Portugal) and in the River Endrick (Scotland), the mortality of Atlantic salmon smolts by avian and piscine predators was high, demonstrating that the number of smolts lost in the river is likely to constrain population abundance in these rivers [154,155]. High inriver mortalities during downstream migration have also been found in Pacific salmon [156].…”

Measuring Detection Efficiency of High-Residency Acoustic Signals for Estimating Probability of Fish–Turbine Encounter in a Fast-Flowing Tidal Passage

“…Acoustic telemetry tags have been used in many ways to study the ecology and behavior of fish. Strategically placed arrays of acoustic receivers can be used to observe and quantify migration [1][2][3] or demonstrate seasonal presence [4] and indicate species' residency patterns [5,6]. With a sufficient density of acoustic receivers, localization can be achieved so that fish can be tracked with high resolution and their behavior studied within a small area [7][8][9].…”

Turn left at the next rocky atoll: small fish fare well with new satellite tags

“…Telemetry can be further split into active telemetry (whereby data is relayed actively back to a receiver using an internal battery) or passive telemetry (whereby data is relayed back to stationary listening stations with no internal power) (Chapman et al, 2012). Passive sampling has often been adopted in the past, but it requires extensive infrastructure in the form of receiver arrays, and so is limited to migrations that occur in a constrained area (Chavarie et al, 2022). This is not an issue with satellite tagging, which is one of the most versatile types of active telemetry, and is ideal for answering ecological question relating to movement corridors or resource selection patterns; not only this, but satellite tags can even provide a cheaper alternative to passive acoustic tags in some scenarios (Hardin, 2022).…”