Impact of early infestation with the salmon louse Lepeophtheirus salmonis on the subsequent survival of outwardly migrating Atlantic salmon smolts from a number of rivers on Ireland's south and west coasts

Abstract

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.

Impact of parasites on salmon recruitment in the Northeast Atlantic Ocean

“…This is owing to the very high marine mortality that naturally affects both control and treatment groups. This, perhaps, underlies the conclusions of some studies that the effect is biologically insignificant relative to the overall mortality within a salmon life cycle [40,46]. However, our analyses based on paired sample t-tests and mixed-effects models allowed us to account for the high natural mortality and isolate the estimated loss of recruitment owing to parasitism, revealing a large effect of parasites.…”

“…), those studies with multiple within-season releases that were grouped (*), and the studies that used the proprietary Substance Ex as treatment ( } ) rather than emamectin benzoate. Note that numbers of recaptured salmon for references [40] and [46] are their survival estimates that incorporated raising factors applied to the raw data (tags recovered), and may therefore be inflated relative to the other data.) rspb.royalsocietypublishing.org Proc R Soc B 280: 20122359…”

“…The parasiticides affect crustaceans, which include other native salmon parasites, particularly Caligus elongatus, though L. salmonis remains the focus owing to its prevalence and adverse lethal and sub-lethal effects [27,28]. Some of the field experiments have identified significant impacts on salmon survival [42,45], whereas others have claimed no significant effect at the population level [40,46].…”

“…A primary reason is that many studies are correlative and results depend on modelling assumptions [32,38,39]. By contrast, manipulative field experiments provide an opportunity for strong inferences based on differential survival between paired releases of control and parasiticide-treated groups of emigrant juvenile salmon smolts [40,42,45,46]. The parasiticides affect crustaceans, which include other native salmon parasites, particularly Caligus elongatus, though L. salmonis remains the focus owing to its prevalence and adverse lethal and sub-lethal effects [27,28].…”

“…These studies were conducted in Ireland and Norway, and typically occurred in river systems discharging to near shore marine waters that accommodated salmon farms [40,42,[45][46][47]. Figure 1 shows the location of coastal salmon farm sites adjacent to the named experimental release rivers in Norway and Ireland; however, not all farm sites were in active production in all years of experimental releases.…”