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

1. 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. 2. 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. 3. 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 3-7%. The latter observation most evident in females, and
likely due to decreased marine exploitation. Female repeat spawners tended
to be less catchable than males by anglers. 4. Depending on the
time-period analysed, 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 is 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 data sets 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.