In Atlantic salmon aquaculture, understanding the infection dynamics of sea lice is critical for sustainable management. For salmon lice (Lepeophtheirus salmonis) the most common infection pathway is direct infection through the planktonic copepodite stage . However, the high level of handling of the farmed fish where a portion of the mobile lice fall off might induce an indirect infection pathway where mobile lice survive and successfully find a new host. The other sea lice species that affects farmed salmon in the north Atlantic is the generalist Caligus elongatus which has been found on more than 80 fish species . This species has different behavioural traits than the salmon lice and the relative contribution of direct and indirect infection pathways to the overall infection pressure on farmed fish is not well understood.
In this study indirect and direct infection pathways of salmon lice and C. elongatus were investigated by analysing comprehensive sea lice monitoring data from the Faroese sal mon farming industry, covering more than 1200 cage deployments. Indi rect infection was identified when mobile or adult lice were found on newly deployed fish before development from a copepodite was possible according to temperature-based development models for the two species .
This approach provided a detailed examination of indirect infection dynamics in an operational farming environment. Preliminary observations suggest a notable difference between the two lice spices, with indirect infection substantially more prominent for C. elongatu s than salmon lice.