Fisheries catches worldwide have shown no increase over the last two decades, while aquaculture has been booming. To cover the demand for fish in the growing human population, continued high growth rates in aquaculture are needed. A potential constraint to such growth is infectious diseases, as disease transmission rates are expected to increase with increasing densities of farmed fish. Using an extensive dataset from all farms growing salmonids along the Norwegian coast, we document that densities of farmed salmonids surrounding individual farms have a strong effect on farm levels of parasitic sea lice and efforts to control sea lice infections. Furthermore, increased intervention efforts have been unsuccessful in controlling elevated infection levels in high salmonid density areas in 2009–2010. Our results emphasize host density effects of farmed salmonids on the population dynamics of sea lice and suggest that parasitic sea lice represent a potent negative feedback mechanism that may limit sustainable spatial densities of farmed salmonids.

Infection by parasitic sea lice is a substantial problem in industrial scale salmon farming. To control the problem, Norwegian salmonid farms are not permitted to exceed a threshold level of infection on their fish, and farms are required to monitor and report lice levels on a weekly basis to ensure compliance with the regulation. In the present study, we combine the monitoring data with a deterministic model for salmon lice population dynamics to estimate farm production of infectious lice stages. Furthermore, we use an empirical estimate of the relative risk of salmon lice transmission between farms, that depend on inter-farm distances, to estimate the external infection pressure at a farm site, i.e. the infection pressure from infective salmon lice of neighbouring farm origin. Finally, we test whether our estimates of infection pressure from neighbouring farms as well as internal within farm infection pressure, predicts subsequent development of infection in cohorts of farmed salmonids in their initial phase of marine production. We find that estimated external infection pressure is a main predictor of salmon lice population dynamics in newly stocked cohorts of salmonids. Our results emphasize the importance of keeping the production of infectious lice stages at low levels within local networks of salmon farms. Our model can easily be implemented for real time estimation of infection pressure at the national scale, utilizing the masses of data generated through the compulsory lice monitoring in salmon farms. The implementation of such a system should give the salmon industry greater predictability with respect to salmon lice infection levels, and aid the decision making process when the development of new farm sites are planned.

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The control of the ectoparasite, sea lice (L. salmonis), on farmed Atlantic salmon in Eastern Canada was complicated by the development of resistance to Emamectin Benzoate, the primary in-feed medication used since 2000. Field efficacy and bioassay assessments were initiated to address the emergency authorization of Deltamethrin (Alpha Max ® ) used in limited circumstances in 2009-2010. Under farming conditions present in the Bay of Fundy, Deltamethrin consistently reduced pre-adult (male and female) and male adult lice stages in the range of 88-98% compared to pre-treatment levels. Cage-level reductions for both adult female and chalimus lice stages varied considerably with median reductions of around 50% or less commonly observed for either stage. In vitro bioassays using field collected mobile stages of sea lice generated average effective concentration (EC 50 ) values that were lower for combined stages of pre-adult and adult male lice compared to either pre-adult female or adult female lice stages. Stage (p<0.001) and temporal (p<0.001) differences were observed for EC 50 values. Both field treatment observations and in vitro assessments of sea lice responses reflected greater reductions after Deltamethrin exposure for pre-adult and adult male lice compared to adult female lice stages. Variable response occurring in different lice categories is likely to affect the successful field application of this treatment and is an important factor to consider when deciding how best to report efficacy.