Reduction in fish larvae in diet of young Atlantic salmon, growth and condition
Post-smolt diet and stomach fullness of young salmon (post-smolts) in the Northeast Atlantic changed considerably over the 25-year study period. There was a reduction of fish larvae in the salmon stomachs over a large geographic area from the period 1995–2004 to 2008–2019. At the same time, around year 2005, the growth and condition of the salmon post-smolts dropped considerably.
Mapping of geographic distribution of prey organisms dominating in the salmon’s diet
Sandeel west of Scotland and Ireland and in the northern North Sea, herring in the eastern part of the Norwegian Sea and amphipods in the western part of the Norwegian Sea were particularly important prey species for the young Atlantic salmon at sea.
The reduction of fish larvae in the salmon stomachs over a large geographic area around year 2005 may be result of a bottom-up driven process in the ocean ecosystem, as increasing extent of nutrient-rich Arctic water masses and zooplankton abundance were positively correlated with salon stomach fullness in the Norwegian Sea.
The results also indicate that the collapse of several sandeel populations in the Northeast-Atlantic and poor recruitment of Norwegian spring-spawning herring since year 2004 have reduced prey availability for young Atlantic salmon. The study shows how these changes have affected salmon feeding conditions by reducing the availability of important prey for salmon, and is an important step further to understand the mechanisms behind reduced marine growth and survival of Atlantic salmon.
The interspecific competition for fish larvae between young Atlantic salmon and mackerel may have increased with a larger mackerel stock expanding the feeding migrations northwards since 2007. However, competition with mackerel cannot explain the reduction in Atlantic salmon growth and survival during last decades, but the negative correlation between mackerel spawning stock biomass and biomass of fish larvae in salmon stomachs in the present study suggests that competition with mackerel for fish larvae may have a negative effect on feeding conditions for the young salmon.
Utne, K., Skagseth, Ø., Wennevik, V., Broms, C.T., Melle, W. & Thorstad, E.B. 2022. Impacts of a changing ecosystem on the feeding and feeding conditions for Atlantic salmon during the first months at sea. Frontiers in Marine Science 9: 824614.