Detatchment of mobile salmon lice and Caligus elongatus during crowding of fish was investigated. Both species detach during crowding but it is very rare that mobile salmon lice infect farmed fish in other cages or other fish farms
Sea lice normally spread when nauplii hatch from egg strings on adult sea lice living on salmon. These larvae drift with the currents and develop into copepodids, which can attach to a fish and continue growing. During delousing of farmed salmon, the fish are crowded together before being pumped onto the delousing vessel. While the fish are crowded, adult and mobile sea lice can fall off the fish and may infect other farmed fish.
In this project, Firum, together with Bakkafrost, investigated how many lice fall off the fish during delousing. In addition, sea-lice counts from 2016 to 2024 were used to study how much of the sea-lice infection came from mobile and adult lice that changed hosts.
Main findings
• Detachment rates of adult C. elongatus are considerably higher than for salmon lice
• With detachment rates of 21.6% per hour, large mobile salmon lice are more likely to detach during crowding compared to adult female salmon lice (detachment rate ~10% per hour)
• Indirect infections of salmon lice, whether from one cage to another or between farms, are rare
• Host transfer of adult C. elongatus accounts for ~44% of infections on farmed salmon
