Atlantic salmon and sea trout display synchronised smolt migration relative to linked environmental cues

Alison C Harvey,Øystein Skaala,Kevin A Glover,Vidar Wennevik

doi:10.1038/s41598-020-60588-0

Alison C Harvey, Øystein Skaala + Show 2 more

Open Access

https://doi.org/10.1038/s41598-020-60588-0

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Abstract

Anadromous salmon and sea trout smolts face challenging migrations from freshwater to the marine environment characterised by high mortality. Therefore, the timing of smolt migration is likely to be critical for survival. Time-series comparing migration of Atlantic salmon and sea trout smolts in the same river, and their response to the same environmental cues, are scarce. Here, we analysed migration timing of ~41 000 Atlantic salmon and sea trout smolts over a 19-year period from the river Guddalselva, western Norway. Trout displayed a longer migration window in earlier years, which decreased over time to become more similar to the salmon migration window. On average, salmon migrated out of the river earlier than trout. Migration of both species was significantly influenced by river water temperature and water discharge, but their relative influence varied across the years. On average, body-length of smolts of both species overlapped, however, size differences were observed within the migration period and among the years. We conclude that salmon and trout smolts in this river are highly synchronised and migrate in response to the same range of linked environmental cues.

Highlights

Anadromous salmon and sea trout smolts face challenging migrations from freshwater to the marine environment characterised by high mortality
As there is often variability of migration timing within a population, it is likely that the genetic differences in this trait between populations are mitigated by environmental cues which act to initiate migration, probably through the control of the developmental processes involved in smoltification[9,15,16]
Throughout the study period (April – June each year), river water temperature ranged from 1.87 °C to 12.43 °C with an average of 5.9 °C, water discharge ranged from 0.16 m3/s to 16.86 m3/s with an average flow of 4.76 m3/s, while sea water temperature ranged from 1.97 °C to 19.41 °C with an average of 9.88 °C

Summary

Introduction

Anadromous salmon and sea trout smolts face challenging migrations from freshwater to the marine environment characterised by high mortality. Atlantic salmon (Salmo salar) and sea trout (anadromous brown trout; Salmo trutta) occur in sympatric anadromous forms throughout most of their range[2] Both species reproduce in freshwater, and juveniles remain in the river for between 1–8 years before migrating into the marine environment to feed[3]. Migration to the ocean is a challenging phase in a salmonid’s life cycle: they must undergo a physical transformation known as smoltification to adapt to increased salinity, switch between different food types, and are exposed to novel predators[5] These challenges interact together, with the result that the migration phase is characterised by high mortality. Several short- and long-term studies have investigated the influence of environmental variables in triggering migration to the marine environment in Atlantic salmon[15,20] and sea trout[21,22]. Most studies have focussed on one species within a river system [ see 2,25], and there is a lack of long-term studies which investigate the influence of environmental variables on the timing of smolt migration in sympatric species simultaneously

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