A model of the process of spillover and adaption leading to potential emergence of disease in salmon held with cleaner fish used to control lice

Abstract

Sea lice control is a very important issue for sustainable salmon aquaculture, and cleaner fish (largely wrasse and lumpfish) are increasingly used as a key tool for this control. Such fish can carry pathogens, which they can potentially spread to the salmon. A modelling approach is used to help guide practice on cleaner fish use. The model simulates transmission of pathogens within and between cleaner fish and salmon populations. A function modifying the coefficient of infection allows simulation of pathogens input with cleaner fish to adaptation to salmon with passage. Adaption may require different numbers of passage events and may face obstacles that make adaption easier or more difficult, allowing the exploration of a range of potential emergent pathogens with different properties. The model includes the production cycle of salmon, whereby all salmon are periodically removed and replaced with naïve fish, but a proportion of the cleaner fish may be reused in the next production cycle. An assessment of the likely consequence of emergent diseases relative to consequence of sea lice infection is made using an analysis of literature. The modelling concludes that any emergent disease is unlikely to be of as serious consequence as sea lice already are, and therefore the risk from emergent disease is low relative to benefit of controlling lice. This risk can be further reduced by practices that limit input of infection and by restricting reused of cleaner fish in subsequent production cycle. The risk from reusing small numbers is generally substantially lower than reusing larger numbers, but avoiding reuse is more effective, especially if there is limited input of pathogens with new cleaner fish. More data as cleaner fish are increasingly used will allow more accurate assessment of risk, but the model provides a flexible framework for assessing risk of emergence of potential new pathogens and advising on good practice while information is limited and a precautionary approach is inappropriate owing to the clear benefits of effective lice control.