Geographic redistribution of farmed salmonids reduces salmon lice infestations and treatment frequency in a simulation study

Host density is a key driver in parasite population dynamics, and often the number of parasites increases rapidly with host density. In the context of Norwegian salmonid farming, this mechanism has led to a disparity between the desire to increase cultured salmonid production, and to reduce the negative effects of parasite infestations. Salmon lice infestations are detrimental to animal welfare due to salmon lice treatments and spillover from farms to wild salmonids. Here, we examine how a redistribution of the salmonid farm biomass may hamper exchanges of lice larvae between farms, and consequently reduce the salmon lice burdens and treatment frequencies. More specifically, we use a stochastic simulation model, fitted to empirical data from Norwegian aquaculture, to examine how lice abundances and treatments responded when the biomass in the system was distributed onto fewer, larger farms situated farther apart. To maintain realistic fish growth, seasonality and cohort development, lice population dynamics were simulated on top of historic production data from Norway. We simulated several scenarios, where an increasing number of farms were closed, and their biomass was redistributed to other farms with matching cohorts. The results indicate that fewer and larger farms reduce lice numbers and treatment frequency, and that a strategic removal of farms, based on their importance for connectivity in an oceanographic lice dispersal network, improves this effect. Some core mechanisms are highlighted that should be considered in regional production planning, and in the allocation of production concessions in salmonid farming.