Lice treatments and salmon mortality in Norwegian salmon aquaculture

Fish farms constantly struggle with parasites. Norway battles its main parasite, the salmon louse, with targeted control and preventative methods within farms. From 2012-2017, there were four main louse removal methods used: chemotherapeutant bathing (azamethiphos, cypermethrin, deltamethrin, hydrogen peroxide), mechanical treatment, thermal treatment, and general bathing (e.g. freshwater bathing). All farms in Norway report to two national-level databases, with one collecting data on registered delousing treatments and the other on monthly salmon mortality. By combining these two databases, we had access to over 40,000 lice removal events across 6 years to map salmon mortality rates to each delousing method.
We detected a rapid and recent paradigm shift in the industry’s approach to lice control, from chemotherapeutants dominating operations from 2012 to 2015 (>81%), to non-chemical mechanical and thermal treatments dominating in 2016 and 2017 (>40% and 74%, respectively). Thermal treatments caused the greatest mortality increases out of all delousing operations used from 2012-2017, with 31% of all treatments causing elevated mortality. This was followed by mechanical (25%), hydrogen peroxide (21%), and azamethiphos, cypermethrin and deltamethrin (<14%). Further, temperature, pre-existing mortality rates and fish size all influenced post-treatment mortality outcomes for all operations. Generally, as temperature increased, salmon mortality also increased across all treatment operations. Fish with high pre-existing mortality experienced increased mortality after treatment, and large fish were more susceptible to increased mortality than small. Our analysis illustrates the importance of national databases in identifying underlying mechanisms that can influence post-treatment salmon mortality.