Time series covering up to four decades reveals major changes and drivers of marine growth and proportion of repeat spawners in an Atlantic salmon population

• Published in: Ecology and evolution Vol. 12; no. 4; pp. e8780 - n/a
• Main Authors: Harvey, Alison, Skaala, Øystein, Borgstrøm, Reidar, Fjeldheim, Per Tommy, Christine Andersen, Kaja, Rong Utne, Kjell, Askeland Johnsen, Ingrid, Fiske, Peder, Winterthun, Synne, Knutar, Sofie, Sægrov, Harald, Urdal, Kurt, Alan Glover, Kevin
• Format: Journal Article
• Language: English
• Published: England John Wiley & Sons, Inc 01.04.2022; John Wiley and Sons Inc; Wiley
• Subjects: Age; Anthropogenic factors; Atlantic salmon; Biomass; Demographics; Domestication; Evolutionary Ecology; Exploitation; Females; Fish; Fish populations; Growth conditions; Growth rate; Human influences; Infestation; Lice; Life history; marine growth; Population; Population changes; Rivers; Salmo salar; Salmon; salmon lice; Sea surface temperature; sea temperature; Spawning; Time series; veteran spawners; Zooplankton; marine growth; veteran spawners; Atlantic salmon; sea temperature; salmon lice; zooplankton
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.8780

Wild Atlantic salmon populations have declined in many regions and are affected by diverse natural and anthropogenic factors. To facilitate management guidelines, precise knowledge of mechanisms driving population changes in demographics and life history traits is needed. Our analyses were conducted on (a) age and growth data from scales of salmon caught by angling in the river Etneelva, Norway, covering smolt year classes from 1980 to 2018, (b) extensive sampling of the whole spawning run in the fish trap from 2013 onwards, and (c) time series of sea surface temperature, zooplankton biomass, and salmon lice infestation intensity. Marine growth during the first year at sea displayed a distinct stepwise decline across the four decades. Simultaneously, the population shifted from predominantly 1SW to 2SW salmon, and the proportion of repeat spawners increased from 3 to 7%. The latter observation is most evident in females and likely due to decreased marine exploitation. Female repeat spawners tended to be less catchable than males by anglers. Depending on the time period analyzed, marine growth rate during the first year at sea was both positively and negatively associated with sea surface temperature. Zooplankton biomass was positively associated with growth, while salmon lice infestation intensity was negatively associated with growth. Collectively, these results are likely to be linked with both changes in oceanic conditions and harvest regimes. Our conflicting results regarding the influence of sea surface temperature on marine growth are likely to be caused by long‐term increases in temperature, which may have triggered (or coincided with) ecosystem shifts creating generally poorer growth conditions over time, but within shorter datasets warmer years gave generally higher growth. We encourage management authorities to expand the use of permanently monitored reference rivers with complete trapping facilities, like the river Etneelva, generating valuable long‐term data for future analyses. Using a dataset spanning smolt year classes 1980–2018, we observed a clear temporal decline in growth rate during the first year at sea, with a stepwise reduction over the four decades. In the same time period, we also observed a clear switch from a dominance of 1SW fish to a dominance of 2SW fish, and, more than a doubling in the proportion of repeat spawners in the population. The influence of summer SST on marine growth depended on the length of the time series used, with a negative effect over the longer angling time series, and a positive effect over the shorter fish‐trap time series. Zooplankton positively influenced marine growth while sea lice intensity negatively influenced growth