Changes in growth and migration patterns of sea trout before and after the introduction of Atlantic salmon farming

Sindre Håvarstein Eldøy,W K Roche,T F Næsje,P G Gargan,E B Thorstad,J G Davidsen,D Ryan,A D Sjursen,Jonathan Grabowski

doi:10.1093/icesjms/fsaa125

Abstract

Abstract Marine growth has strong implications for reproductive potential and ultimate fitness of sea trout. Hence, the effects of anthropogenic factors on marine growth are important when evaluating population responses and implementing management measures. Temporal changes in growth patterns of sea trout from three Norwegian and two Irish watercourses were examined, covering time spans of 25–65 years. Elemental chemistry Ba:Ca profiles and visual reading of fish scales were used to estimate smolt length and lifetime growth after first sea entry. Reduced growth after the first sea entry coincided with periods of nearby (&lt;14 km) salmon-farming activity in impacted watersheds in both countries. Increased Ba:Ca levels were also recorded during these periods, likely indicating reduced residency in marine habitats caused by premature return to freshwater and estuaries. An increase in estimated length at first sea entry coinciding with salmon-farming activity, for groups of fish sampled after sea migration, suggests a size-selective marine mortality, with the smallest individuals experiencing a larger mortality.

Highlights

Anadromy is a migratory strategy common among many salmonid fish species, which is characterized by spawning in freshwater and marine feeding migrations
In the populations with nearby aquaculture locations (
This suggests that the sea trout had reduced marine growth and spent increased time in estuarine and freshwater habitats in the periods with active nearby salmon farms

Summary

Introduction

Anadromy is a migratory strategy common among many salmonid fish species, which is characterized by spawning in freshwater and marine feeding migrations. Populations in watercourses with access to the sea may consist of both freshwater-resident individuals and individuals migrating to the sea (hereafter termed sea trout) (Jonsson and Jonsson, 1993; Klemetsen et al, 2003). Migratory individuals exploit the richer feeding opportunities at sea to gain a larger body size, higher female fecundity, competitive advantages on the spawning ground, and a potentially greater fitness than their freshwater-resident conspecifics (Thorstad et al, 2016). As anadromy is considered a quantitative threshold trait, the proportion of anadromous individuals in a populations is thought to be partly governed by the relative fitness of resident vs migratory individuals of previous generations (Ferguson, 2006; Ferguson et al, 2019).

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Conclusion