A simple modelling tool for assessing interaction with host and local infestation of sea lice from salmonid farms on wild salmonids based on processes operating at multiple scales in space and time

Abstract Sea lice are marine ectoparasitic crustacea which limit the potential for sustainable salmon aquaculture due to risks of impacts on iconic wild salmon and sea trout. Control of the parasite on farms costs an estimated 9% of farm revenue. Sea lice develop through planktonic nauplii to copepodid stages which attach to, and mature on, salmonid hosts until females become ovigerous (egg laying) Impacts of lice on wild fish depend on their exposure to planktonic larval lice transported from salmon farms, which consists of (A) production of larval lice from ovigerous female lice on salmon farms, (B) local concentrations of planktonic larval lice infectious copepodids in adjacent waters, (C) rates of infestation of wild fish given these concentrations, and (D) impact on fish given this level of interaction. A model of this local exposure around salmon farms was developed. Production rates for nauplii as a function of the numbers of adult lice on salmon farms and maturation to copepodids (A) is well studied, as are impacts of infestation (D), with >0.75 lice per gram of host fish considered to present a high risk of mortality. Using existing assessments of infectious copepodid production (A) we develop a model of copepodid concentration (B) based on a simple kernel of copepodid distribution around farms; within this kernel the copepodids are assumed either to disperse evenly or to be transported in a concentrated plume, allowing comparison of the range of different concentration distributions. These distributions combine with a model of infestation (C) based on small-scale movements of copepodids in the immediate vicinity of a swimming fish. Fish swimming at intermediate velocities are most susceptible to infestation, as slow fish exhaust lice in their immediate vicinity while fast fish move on before lice copepodids can approach. These models are combined to create an assessment of the risk that concentrations can result in infestation of fish at levels considered to cause mortalities (D). The results can be used in combination with empirical assessments as a tool to link potential impacts on wild salmonids to aquaculture biomass and on-farm lice management in different environments in support of strategic aquaculture planning. Our modelling demonstrates the, often neglected, importance of fine-scale processes in sea lice infestation of salmonids.