Reduced exposure of farmed salmon to salmon louse (Lepeophtheirus salmonis L.) infestation by use of plankton nets: Estimating the shielding effect

Salmon louse infestation is arguably the most limiting factor for further growth in the production of farmed salmon in Norway. Since the use of pesticides has become less effective in controlling infestations due to evolving resistance in the salmon louse, the development of novel non-medical control methods is in focus. A variety of new methods are being launched, but limited effort is spent on documenting control effects. Here we present an analysis that quantifies the effect of shielding farmed salmon from exposure to infestive salmon lice by using plankton nets surrounding the upper parts of cages. The temporal development of salmon louse infestations was compared for production cages shielded with 10m and/or 6m deep plankton nets, or no nets (controls), in five full scale salmon farms. Only weeks where lice counts were available from both control cages and plankton net shielded cages were included in the data. Varying external and internal infestation pressure exposing the test farms was controlled for in the analysis by quantifying the reproduction of infestive lice from external farms and from the test farm. We found that use of plankton nets significantly reduced the rate of new salmon louse infestation. The shielding effect amounted to an estimated 30% weekly reduction (95% CI: 18-41%) in the rate of new infestations of pre-adult and adult male lice (PAAM) one week ahead in large scale production cages. We conclude that the use of plankton nets represents a non-medical complementary method in the pest management of salmon lice that may reduce the need for treatments, either with pesticides or other nonmedical measures, and further contribute to conserve effects of pesticides used to control salmon lice infestations in salmon farming.