The contribution of host transfer in the infection dynamics of Caligus elongatus and salmon lice (Lepeophtheirus salmonis) in a salmon farming network

The sea lice Lepeophtheirus salmonis and Caligus elongatus have direct lifecycles, with infective copepodids attaching to hosts and developing into adults. However, increased delousing and fish handling have raised concern about whether preadult or adult lice stages that detach during crowding might reattach to other fish. C. elongatus have a significant reservoir of wild hosts, and little is known to which extent salmon in farms are infected directly by copepodids or by adults transferring from other hosts. In the Faroe Islands, third-party sea lice monitoring ensures consistent data collection across farms. The data is stored in a database including additional farm specific information. When fish are transferred to sea, attached copepodids require a temperature-dependent period to develop into pre-adult and adult stages. If these are found prematurely on the fish, they have likely attached as mobiles. Few L. salmonis were found prematurely, revealing that host transfer of preadult and adult L. salmonis between salmon cages was highly unusual with little impact on the overall epidemiology of salmon lice on the farmed fish. In contrast, transmission of adult C. elongatus was common, accounting for ~44% of the total infections. C. elongatus abundances on farmed salmon were seasonal and new infections were dominated by adult host transfer in July when abundances started to increase. These contrasting transmission patterns highlight the need for species-specific management strategies and further research into the ecology and transmission dynamics of C. elongatus , whose epidemiological behavior and impact remain less understood despite its significant contribution to overall infection pressure. • Infection by host transfer of mobile salmon lice is highly unusual. • Host transfer of adult C. elongatus accounts for ~44% of infections in farmed salmon. • Host transfer drives C. elongatus infection in July as infection pressure begins to rise. • C. elongatus seems to actively leave the farmed fish in autumn.