Atlantic salmon Salmo salar has shown declines in abundance associated with reduced survival during marine life stages.

Key impacts on survival may include a changing ocean environment and salmon louse Lepeophtheirus salmonis infestation from aquaculture. A 26 yr record from the Erriff River (Western Ireland) was used to evaluate the contribution of sea lice from salmon aquaculture to declining returns of wild 1 sea-winter (1SW) salmon. Statistical models suggested that returns were >50% lower in years following high lice levels on nearby salmon farms during the smolt out-migration.

The long-term impact of salmon lice was explored by applying predicted annual loss rates as a multiplier to observed 1SW salmon returns. This produced a ‘lice-corrected’ return time series, i.e. an estimate of how returns might have looked in the absence of a serious aquaculture lice impact. The corrected time series was adjusted to account for some reduction in recruitment due to lost spawners.

Comparing observed and lice-corrected time series suggested that salmon lice have strongly reduced annual returns of 1SW Erriff salmon, but that the salmon lice impact does not explain a declining trend in this population.

https://www.researchgate.net/publication/316684886_Quantifying_the_contribution_of_sea_lice_from_aquaculture_to_declining_annual_returns_in_a_wild_Atlantic_salmon_population

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 (<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.

https://www.researchgate.net/publication/344297227_Changes_in_growth_and_migration_patterns_of_sea_trout_before_and_after_the_introduction_of_Atlantic_salmon_farming