Salmon lice detached during aquaculture practices survive and can reinfest other hosts

Conventional net pens used in Atlantic salmon aquaculture are open to the environment, allowing infestation by salmon lice ( Lepeophtheirus salmonis ), and necessitating delousing treatments to keep within regulatory limits. These treatments, involving crowding and handling, can lead to detachment and loss of mobile louse stages to the surrounding water. It is not known to what extent these individuals pose a reinfestation risk for fish in surrounding net pens. We investigated the capacity of detached lice to survive off the host, and attach to a new host. Lice were placed in incubators at different temperatures (4, 8, 12, 16 °C) and at 12 °C, at three salinities (12, 24, 34 ppt). Once ∼50 % mortality was reached, survivors were placed on salmon to quantify rates of reestablishment on a new host. Significantly shorter survival times were observed in warm and in low salinity conditions. Adult females survived longer (median 12 days at 12 °C, 34 ppt) than mixed-sex pre-adults or adult males (8–10 days), and were particularly long-lived in the coldest seawater (median 25 days at 4 °C, 34 ppt). Adult females were also better at reestablishing on a new host, with a 74 % success rate across all treatment combinations versus 52 % and 51 % for pre-adults and adult males, respectively. Together, these findings indicate that detached mobile lice may pose a reinfestation risk to downstream net pens. Management procedures should be developed to minimize infestation of new farmed salmon or wild salmonids in the surrounding environment. • Aquaculture practices can dislodge the parasitic salmon louse from farmed fish. • Survival of dislodged mobile lice is extended and longest for adult females. • Warmer temperatures and lower salinities reduced survival. • Reattachment was significant in all starved lice groups, but highest in adult females.