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 Both published and new data are presented including a previously unpublished time series.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.

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Background: Passive integrated transponder (PIT) tags are used to study the movement and behaviour in populations of a wide variety of fish species and for a number of different applications from fisheries to aquaculture. Before embarking on long-term studies, it is important to collect information on both short-and medium-term survival and tag retention for the species in question. In this study, 90 juvenile lumpfish (10-20 g, 30 fish per replicate tank) were implanted with 12.5-mm FDX PIT tags. Results: Tag retention, growth rates and survival were compared to those of fish subjected to handling only (90 fish, 30 per replicate tank). Overall survival was 100% during the 28-day monitoring period, and tag retention was 99%. Conclusions: Results indicate that retention rates of 12.5-mm PIT tags in juvenile lumpfish are high, and there is no significant effect on growth rates or survival in a hatchery environment.