Abstract
In recent decades, there has been a decline in the marine growth of Atlantic salmon (Salmo salar) over large parts of the distribution range. One hypothesis for this reduced growth is increased interspecific competition with other planktivorous pelagic fish in the ocean. Here, interactions between salmon postsmolts and other pelagic fish (mackerel (Scomber scombrus) and herring (Clupea harengus)) in the Northeast Atlantic were investigated. There was a low diet overlap between postsmolts and the two other planktivorous pelagic species. Both mackerel and herring were feeding predominantly on copepods and other small zooplankton, while salmon were feeding mostly on fish larvae. All three species feed on euphausiids and amphipods. Furthermore, postsmolts geographically overlapped with mackerel but had a low geographic overlap with herring. There was no correlation between the abundance or survival of salmon from key index rivers and the abundance of pelagic fish. This study did not find evidence to support the hypothesis that observed temporal changes in marine growth and survival of salmon can be explained by feeding interactions with pelagic fish.
Highlights
The life cycle of Atlantic salmon (Salmo salar) involves a period with long migrations in the oceanic phase
Postsmolts prey on larvae of herring and blue whiting (Haugland et al 2006; Rikardsen and Dempson 2011), while 1 sea-winter fish (1SW) and older salmon feed partly on both juvenile and adult pelagic fish (Jacobsen and Hansen 2001)
Mackerel predation on salmon postsmolts in the Northeast Atlantic can potentially happen as mackerel partly feed on small fish opportunistically (Iversen 2004)
Summary
The life cycle of Atlantic salmon (Salmo salar) involves a period with long migrations in the oceanic phase. During the past few decades, wild Atlantic salmon populations have declined dramatically over much of the distribution range (ICES 2018a; Parrish et al 1998). The reason for this decline is complex and not fully understood. One key hypothesis is reduced abundance of prey available for postsmolts due to reduced plankton production and bottom-up regulation driven by climate changes (Beaugrand and Reid 2012; Almodóvar et al 2019) This is supported by correlations between warmer ocean temperatures and reduced individual growth (Jonsson et al 2016; Todd et al 2008). For instance Lepeophtheirus salmonis and to a lesser extent Caligus elongatus (Jacobsen and Gaard 1997), can impact
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