Quantifying the contribution of sea lice from aquaculture to declining annual returns in a wild Atlantic salmon population

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.


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

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.


System-specific salmon louse infestation thresholds for salmon farms to minimize impacts on wild sea trout populations

Samuel Shephard Paddy Gargan

Salmon lice from aquaculture can cause negative impacts on sea trout Salmo trutta and other wild salmonids.

Long-term records from 5 Irish rivers were used to explore relationships between annual sea trout runs and the estimated total number of lice on nearby salmon farms. It was hypothesised that local environmental conditions may result in system-specific differences in realised louse pressure on sea trout. Louse count was thus tested as an absolute number and as a relative pressure, i.e. standardised by farm.

When the standardised total number of mobile lice on a given salmon farm in April was above ‘baseline’ level (50th percentile of observed annual values on that farm), there was a high probability of a below average sea trout run in the local river. Absolute louse counts did not show an important effect on runs.

This finding suggests that salmon farm louse production in spring can have a strong system-specific regulating effect on wild sea trout populations.

Total number of lice on a farm was most strongly driven by changes in individual infestation rate, with a lesser effect of stocking density.

Thresholds for number of mobile lice per farmed salmon required to maintain total louse count below the baseline varied with stocking density and among systems; greater density required lower infestation rate.

Regulations relying on a generic louse threshold to trigger treatment are not sufficient to protect sea trout populations – stocking density and site characteristics must be considered to evaluate system-specific infestation pressure and impacts on wild salmonids.