Jumping to treat sea lice: Harnessing salmon behaviour to enable surface-based chemotherapeutant application

Salmon lice removal with chemotherapeutants often occurs via bathing in cages or well-boats. These methods of administering chemotherapeutants require handling of fish and can expose fish to suboptimal environments which lead to poor performance and mortality. Post-treatment, the medicines used disperse into the environment with potentially harmful effects. An alternative method of delousing that requires minimal handling and allows recollection of chemotherapeutants uses a floating chemotherapeutant-infused oil layer and behavioural manipulation of salmon to stimulate jumping through the oil layer. Jumping can be enhanced via a period of forced submergence followed by reinstating surface access prior to applying the chemotherapeutant. We tested if the presence of a floating oil layer affected surface activity after 44 h of submergence, and found that the number of jumps fish⁻¹ was similar regardless of the presence or absence of an oil layer. To determine if treatment efficacy increased with chemotherapeutant concentration, we tested four concentrations of cypermethrin dissolved in an oil layer; high cypermethrin concentrations removed the most lice (38% lice removal after 1 jump). However, the number of repeated jumps within a short period and hence exposure to the oil layer did not alter lice removal efficacy. Overall, we demonstrate that submerging fish to increase their jumping rate can be used in conjunction with a floating oil layer to deliver delousing chemicals, which could then be re-collected immediately after treatment via standard oil boom and skimming equipment. Future research should determine the short and long-term welfare impacts of this new treatment method on salmon, and test different chemotherapeutant and oil combinations that increase treatment efficacy with minimum welfare impacts on salmon.