Sea lice infestation as a source of marine mortality of outwardly migrating Atlantic salmon smolts has been investigated by treating groups of ranched salmon, prior to release, with a prophylactic sea lice treatment conferring protection from sea lice infestation. A number of studies have been carried out in Ireland using both established ranched populations and groups of hatchery reared fish imprinted for 5–8 weeks in the sites of experimental releases. In this study, data on 352 142 migrating salmon from twenty-eight releases, at eight locations along Ireland's South and West coasts covering a 9-year period (2001 to 2009) are reviewed. Both published and new data are presented including a previously unpublished time series. The results of a meta-analysis of the combined data suggest that while sea lice-induced mortality on outwardly migrating smolts can be significant, it is a minor and irregular component of marine mortality in the stocks studied and is unlikely to be a significant factor influencing conservation status of salmon stocks.

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The potential impact of sea lice infestation on outwardly migrating Atlantic salmon smolts has been investigated by treating populations of ranched salmon, prior to release, with a prophylactic sea lice treatment conferring protection from sea lice infestation, for up to 9 weeks. Established populations of ranched Atlantic salmon with well described rates of return were chosen to investigate the potential contribution of early infestation with the salmon louse, Lepeophtheirus salmonis to mortality in Atlantic salmon. Results of five releases from four locations are presented and compared with a time series of releases from the Lough Furnace in Newport, County Mayo. The results of this study would suggest that infestation of outwardly migrating salmon smolts with the salmon louse (L. salmonis) was a minor component of the overall marine mortality in the stocks studied.

Spares, A.D., Reader, J.M., Stokesbury, M.J.W., McDermott, T., Zikovsky, L., Avery, T.S., and Dadswell, M.J. 2007. Inferring marine distribution of Canadian and Irish Atlantic salmon (Salmo salar L.) in the North Atlantic from tissue concentrations of bio-accumulated caesium 137. – ICES Journal of Marine Science, 64: 394–404. Atlantic salmon returning from marine migrations to eastern Canada and western Ireland during 2002 and 2003 were analysed for tissue concentrations of bio-accumulated caesium 137 (137Cs). Salmon from Canadian and Irish waters demonstrated concentrations (0.20 ± 0.14 Bq kg−1 and 0.19 ± 0.09 Bq kg−1, mean ± s.d., respectively) suggesting similar oceanic feeding distributions during migration. Canadian aquaculture escapees had a similar mean tissue concentration (0.28 ± 0.22 Bq kg−1), suggesting migration with wild salmon. However, significantly higher concentrations in 1-sea-winter (1SW) escapees (0.43 ± 0.25 Bq kg−1) may alternatively suggest feeding within local estuaries. High concentrations in some Canadian 1SW salmon indicated trans-Atlantic migration. Low concentrations of Canadian multi-sea-winter (MSW) salmon suggested a feeding distribution in the Labrador and Irminger Seas before homeward migration, because those regions have the lowest surface water 137Cs levels. Estimates of wild Canadian and Irish salmon feeding east of the Faroes (∼8°W) were 14.2% and 10.0% (1SW, 24.7% and 11.5%; MSW, 2.9% and 0.0%), respectively. We propose that most anadromous North Atlantic salmon utilize the North Atlantic Gyre for marine migration and should be classified as a single trans-Atlantic straddling stock.