Network Analysis of Salmon Lice Infestations and Connectivity Between Farms: Exploring Cluster Isolation in Norwegian Salmon Farming

Abstract

Salmon lice (Lepeophtheirus salmonis) are a major constraint to the sustainable growth of the Norwegian salmon farming industry. Salmon farming in open net pens leads to a proliferation and transmission of salmon lice which increases the infestation pressure on wild salmonids. Frequent delousing treatments are being performed as lice levels reaches the regulatory limit, leading to increased production related mortality and economic losses for the farmers. Lice larvae are released from salmon farms and transported by ocean currents which result in connectivity infestation networks of lice dispersal between farms. A biophysical dispersal model was in this thesis used to simulate lice dispersal between farms. The modelled lice dispersal was simulated over a four-year period, governed by ocean currents and historical production data from farms across three production zones in Norway (PZ4, PZ5 and PZ6). A network analysis was performed to analyse lice infestation networks, connectivity between farms, to calculate isolation, and explore the possibility of further isolating clusters of farms. There were spatial variations in connectivity between farms and infestation pressure. Stronger connections and higher infestation pressure were observed at farms located in the fjords compared to open coastal areas in PZ4 and PZ5. The farms partitioned into clusters within fjords and along open coastlines, where cluster size varied based on the fjord size, the number of farms, and the length of coastline covered by the farms. The clusters were highly isolated, with high internal infestations and low external infestations. Highly isolated clusters may indicate that coordinated management practises within clusters, aimed at reducing lice infestations, may be beneficial. The individual farms had in general a lower isolation than the clusters. The farm isolation was higher in the fjords than in open coastal areas. Strategic removal of external farms within the infestation networks increased the clusters´ isolations, while simultaneously decreasing the clusters´ lice infestation pressures. Isolation may be a useful term to describe the efficacy farmers can have by coordinating their management, which can be a functional preventative strategy against salmon lice in the future.