Abstract
During the last decades, many wild Atlantic salmon populations have declined dramatically. One hypothesis for an observed reduction in salmon marine growth and survival is reduced abundance of prey. However, the effect of spatial and temporal variation in marine prey abundance on post-smolt feeding conditions is poorly understood. Here we use stomach content data from 2572 salmon post-smolts sampled during 25 years in the Northeast Atlantic Ocean to examine spatial and temporal changes in diet and stomach fullness. Sandeel larvae west of Scotland and Ireland and in the northern North Sea, herring larvae in the eastern part of the Norwegian Sea, and amphipods in the western part of the Norwegian Sea were particularly important prey species. There was a reduction of fish larvae in the post-smolt stomachs over a large geographic area when comparing the period 1995–2004 to 2008–2019. This may be result of a bottom-up driven process, as increasing extent of Arctic Water masses and zooplankton abundance were positively correlated with post-smolt stomach fullness in the Norwegian Sea. Furthermore, the interspecific competition for fish larvae between post-smolts and mackerel may have increased with a larger mackerel stock expanding the feeding migrations northwards since 2007.
Highlights
Since the 1960s, many wild Atlantic salmon populations have declined dramatically, both because of human impacts in rivers and coastal areas, and ecosystem effects in the ocean causing increased mortality in the marine phase (Parrish et al, 1998; Otero et al, 2011; Forseth et al, 2017; ICES, 2021a)
The proportion of amphipods in the post-smolt diet increased further north and west, and this prey was important for postsmolts in the Norwegian Sea
In contrast to previous years, gadoids and sandeel were in general absent from the diet these 2 years, even though the small proportion of unidentified fish larvae could belong to these groups
Summary
Since the 1960s, many wild Atlantic salmon populations have declined dramatically, both because of human impacts in rivers and coastal areas, and ecosystem effects in the ocean causing increased mortality in the marine phase (Parrish et al, 1998; Otero et al, 2011; Forseth et al, 2017; ICES, 2021a). Individual growth during the marine phase has been reduced in many regions, concurring with lower marine survival (Friedland et al, 2000; Jonsson and Jonsson, 2004; Todd et al, 2008; Jonsson et al, 2016). The vast ocean areas used by Atlantic salmon as marine feeding grounds, and the lack of annual ocean surveys targeting Atlantic salmon, have limited the possibilities of detailed studies addressing the different hypotheses on why marine growth and survival have declined. There is large regional and temporal variation in marine growth and survival of Atlantic salmon (ICES, 2021a; Vollset et al, 2022), further complicating such studies.
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