Freshwater treatment of amoebic gill disease and sea-lice in seawater salmon production: Considerations of water chemistry and fish welfare in Norway

Amoebic gill disease (AGD) and sea lice are two of the most significant disease issues that the Norwegian Atlantic salmon aquaculture industry is facing. Although both diseases respond to various extents, to freshwater treatment, the chemistry, interactions, and efficacy of treatment can be variable. These variations can have significant impacts upon the success and failure of treatment and costs to the production cycle. Although it is known that soft freshwater is most effective in bathing of Atlantic salmon with AGD and that most of the freshwaters in Norway fall into the soft category, the low alkalinity and buffering capacity of such waters may impact on the pH and metal toxicity of the water source in use. Similarly dissolved organic carbon can be beneficial in treatment, although sequestration of metal ions can be reversed as the water pH drops due to high densities of fish and accumulations of carbon dioxide. Alternative treatments such as the use of oxidative disinfectants such as hydrogen peroxide used for AGD and sea lice control may have potential although the interactions in seawater with organic loads and dissolved organic carbon are unclear. Similarly the use of oxidative disinfectants in freshwater will depend upon the water chemistry and interactions with treatment chemicals, fish, and water organic content. The logistics of treating large biomasses of Atlantic salmon on marine farms are challenging. The use of well boats offers potential although maintaining water quality during treatments is essential for both AGD and sea lice treatments to optimize fish welfare and treatment efficacy. •Norwegian freshwaters that are varied in chemical composition appear to be mostly suitable for freshwater treatment of AGD and sea-lice although the effects of metal ions may impose a level of risk in waters of different pH.•Freshwater treatment of AGD is the treatment of choice, with soft, freshwaters being demonstrated as the most effective.•Oxidative chemical treatments for AGD and sea-lice may offer alternatives to the use of freshwater although the interactions with water chemistry are poorly understood and the safety margins are narrow.