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 remains 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. We embed this framework into a fine-scale behaviour study of migration and social structure. 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. There is evidence that predation induced selection on smolt morphology. Unsuccessful river salmon migrants with a phenotype comprising a shorter head and jaw and smaller eye had a higher probability of mortality in the later parts of riverine migration where avian and aquatic predation mortality dominated. In contrast, mortality earlier in river migration was independent of phenotype, most likely a result of tagging effects. Successfully river migrants were found to be interacting with each other, often in a pair or as a trio, indicating that a few individuals of Atlantic salmon and sea trout formed notable intra- and interspecific social associations. However, the heterogeneity of the social associations of successful migrants did not support the assumption that social network features attributed any clear specific benefits of reduced predation risk. 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.

Investigating the behaviour of Atlantic salmon (Salmo salar L.) post‐smolts during their early marine migration through the Clyde Marine Region

“…Nonetheless, the detection efficiency of acoustic telemetry is not always 100%, and therefore it must be assessed when providing estimates of survival (Halfyard et al ., 2013 ). Receiver efficiency in the freshwater zone was assessed for the final River Endrick receiver (Figure 1 , R17) as well as for the receivers deployed in the River Leven (Figure 1 , R18–R19) and Gryffe (Figure 1 , R1–R2; excluding the final freshwater receivers) by calculating the proportion of smolts detected at a downstream receiver that were not detected at the prior upstream receiver (Chavarie et al ., 2021 ). Lastly, receiver efficiency in the estuarine zone was calculated using a mark‐recapture model as discussed next (Halfyard et al ., 2013 ).…”