Increased zooplankton biomass was positive for marine growth, and salmon lice negative. Data from the River Etne fish trap are published in a new study by Alison Harvey, Øystein Skaala and colleagues
The River Etne fish trap operated by the Institute of Marine Research. Long-term recordings of downstream and upstream migrating Atlantic salmon provide valuable data. Photo Eva B. Thorstad
Long-term changes in marine growth and life history characteristics of the Atlantic salmon in River Etne
Marine growth of Atlantic salmon from the River Etne in south-eastern Norway showed a stepwise decline across the last four decades. At the same time, the population shifted from being predominantly one year at sea before returning to the river to spending two years at sea.
The proportion of repeat spawners increased from 3 to 7% over the last decades. This was most evident in females and likely due to decreased marine exploitation. Female repeat spawners tended to be less catchable than males by anglers.
Sea temperatures, zooplankton biomass and salmon lice influenced marine growth
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.
The 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.
Important long-term data from the River Etne
The analyses were conducted on age and growth data from scales of salmon caught by angling in the River Etne, Norway, covering smolt year classes from 1980 to 2018, sampling of the whole spawning run in the fish trap from 2013 onwards, and time series of sea surface temperature, zooplankton biomass, and salmon lice infestation intensity.
Harvey, A., Skaala, Ø., Borgstrøm, R., Fjeldheim, P. T., Christine Andersen, K., Rong Utne, K., Askeland Johnsen, I., Fiske, P., Winterthun, S., Knutar, S., Sægrov, H., Urdal, K. & Alan Glover, K. 2022. 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. Ecology and Evolution, 12, e8780