Thermal habitat of adult Atlantic salmon Salmo salar in a warming ocean.

John Fredrik Strøm,Audun Håvard Rikardsen,Eva Bonsak Thorstad

doi:10.1111/jfb.14187

John Fredrik Strøm, Audun Håvard Rikardsen + Show 1 more

Open Access

https://doi.org/10.1111/jfb.14187

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Abstract

The year-round thermal habitat at sea for adult Atlantic salmon Salmo salar (n = 49) from northern Norway was investigated using archival tags over a 10 year study period. During their ocean feeding migration, the fish spent 90% of the time in waters with temperatures from 1.6-8.4°C. Daily mean temperatures ranged from -0.5 to 12.9°C, with daily temperature variation up to 9.6°C. Fish experienced the coldest water during winter (November-March) and the greatest thermal range during the first summer at sea (July-August). Trends in sea-surface temperatures influenced the thermal habitat of salmon during late summer and autumn (August-October), with fish experiencing warmer temperatures in warmer years. This pattern was absent during winter (November-March), when daily mean temperatures ranged from 3.4-5.0°C, in both colder and warmer years. The observations of a constant thermal habitat during winter in both warmer and colder years, may suggest that the ocean distribution of salmon is flexible and that individual migration routes could shift as a response to spatiotemporal alterations of favourable prey fields and ocean temperatures.

Highlights

Temperature has a strong influence on ectothermic fishes, as it affects their physiological performance, behaviour and spatial distribution (Fossheim et al, 2015; Graham & Harrod, 2009)
The ocean temperatures experienced by Atlantic salmon in this study, with most temperature recordings ranging from 1.6–8.4C, documented residency in colder waters than indicated in previous studies, where salmon have primarily been recorded in waters with temperatures between 5 and 15C (Table 3)
While there was no effect of body size on temperatures experienced by individual fish in the present study, it is possible that the overall large size of the repeat ocean migrants may partially explain the lower temperatures observed here compared with previous studies of salmon post-smolts (Table 3), indicating some differences in ocean distribution among life stages

Summary

| INTRODUCTION

Temperature has a strong influence on ectothermic fishes, as it affects their physiological performance, behaviour and spatial distribution (Fossheim et al, 2015; Graham & Harrod, 2009). In studies using archival tags, maiden salmon from Iceland spent most of the time in waters ranging from 7–11C (Guðjónsson et al, 2015), partly contrasting the thermal habitat of adults from Atlantic Canada that primarily inhabited waters with temperatures from 2–15C during summer and autumn (Reddin et al, 2004; Strøm et al, 2017). As sampling at sea only provides snapshots of the thermal habitat and the data available from archival tags. Studies of the thermal habitat in different geographic areas and across multiple years would add to the overall understanding of the marine phase of Atlantic salmon. Adult Atlantic salmon were sampled and tagged in the Alta River, northern Norway (70 N, 23.4 E), in May 2006–2015 after spawning the previous autumn. Tags deployed in 2006 and 2007 failed to log data during the last months at sea, due limited battery capacity (Table 1)

| MATERIALS AND METHODS

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| RESULTS

| DISCUSSION

Findings

Method