Counterintuitive active directional swimming behaviour by Atlantic salmon during seaward migration in the coastal zone

Abstract

Acoustic telemetry was used to track salmon smolts during river migration and into the open marine coastal zone. We compared migration direction and speed with particle tracking simulations to test the hypothesis that marine migration pathways are defined by active swimming current following behaviour. Habitat-specific survival rates, movement speeds, depths and directions in riverine, estuarine, and coastal habitats were also quantified. Salmon post-smolts did not disperse at random as they entered the unrestricted, coastal zone of the North sea; rather they chose a common migration pathway. This was not the most direct route to marine feeding grounds (ca. 44° N); north in the direction of the prevailing currents. Particle modelling showed that the actual post-smolt migration route was best predicted by active swimming at 1.2 body length.sec.−1 at a bearing of 70° from north but not by current following behaviour. Fish migrating in larger groups and earlier in the migration period had increased migration success. We conclude that: post-smolts have preferred migration routes that are not predicted by the shortest direction to their ultimate destination; they do not simply use the current advantage to migrate; and that they actively swim, occasionally directly against the current prevailing at the time.

Evidence of long-distance coastal sea migration of Atlantic salmon, Salmo salar, smolts from northwest England (River Derwent)

“…Two smolts (34,946 and 34,988) were detected on the COMPASS array and another smolt (34,920) was detected on the SeaMonitor array. Unfortunately, the distances between receivers (ca.1 km) exceed the presumptive detection ranges (ca.400 m) [23] for both marine arrays, therefore, it is plausible that more smolts may have passed both arrays undetected.…”

“…The results of the study presented here and those of Barry et al [4] imply that sea migrating post-smolts may well be navigating with the predominant current in this area, however, these movements may well be considerably more complex and much more detailed study is required. In contrast however, Newton et al [23] used particle tracking to show that post-smolt Atlantic salmon were actively migrating in a direction that was not following the prevailing currents. The importance of using marine currents by migrating post-smolt Atlantic salmon thus requires further investigation.…”

A Persistent Sea-Going Platform for Robotic Fish Telemetry Using a Wave-Propelled USV: Technical Solution and Proof-of-Concept

“…The trial provided an opportunity to experience genuine encounters between the vehicle and tagged fish since it was conducted in parallel with an already ongoing acoustic telemetry experiment targeting the seaward migration of Atlantic salmon post-smolts from rivers in the Nordfjord area. The early marine migration represents a critical life stage of salmon with an urgent need of improved scientific understanding (Thorstad et al, 2012), but insight is limited and precluded by the technical and practical difficulties of obtaining relevant observations beyond estuaries and confined coastal areas (Barry et al, 2020;Newton et al, 2021). Having a primary conceptual and technical focus, rigorous biological inference on post-smolt behaviour was not the aim of our study and it should neither be regarded as an integral part of the original biologically focused experiment, which will be reported elsewhere [see Bjerck et al (2021)].…”

Modelling the effects of currents and migratory behaviours on the dispersal of Atlantic salmon (Salmo salar) post-smolts in a coastal embayment

“…However, such computer-intensive models are subject to the quality of data and model inputs, including the behavioural rule(s) assigned to salmon particles, and require validation against empirical data (Byron and Burke 2014). Combining tracking by acoustic telemetry and IBBMs, while testing different salmon behaviours, can be used to: identify behaviours that are or are not feasible for salmon; estimate the usage of different habitats during migration by an entire population of fish; and infer the potential migratory characteristics of post-smolts and their behaviours when migrating in different habitats, such as coastal and offshore areas (Byron and Burke 2014;Brosnan and Welch 2020;Newton et al 2021).…”

“…Atlantic salmon post-smolts are strong swimmers (Peake 2008), and thus likely use active swimming behaviours during their migration rather than dispersing passively (Hedger et al 2008;Mcilvenny et al 2021;Newton et al 2021). Post-smolts may use a range of migratory behaviours as they leave their natal rivers, including current-following, localized cue-based orientation, or directional swimming via compass navigation.…”