Connectivity modelling and network analysis of sea lice infection in Loch Fyne, west coast of Scotland

Abstract

Sea lice are a persistent threat in many areas where salmon farming is practised. In common with the management of disease, infection levels are typically controlled by operating sites within distinct geographical areas, allowing for coordinated treatment and fallow cycles. However, the hydrodynamic connectivity and consequent transmission of lice larvae between sites is often not well understood, which limits our ability to optimise the spatial distribution of farms to minimise infection. We used a multistage modelling approach to investigate the transmission of sea lice larvae between salmon aquaculture sites in Loch Fyne, Scotland. A finite element hydrodynamic model was forced using meteorological data collected over the study period. Output from this model was used to drive a particle-tracking model. The latter model implemented the development and mortality of larvae to estimate the probability of successful larval dispersal between sites. In turn, these dispersal probabilities were used to define a network describing the sea lice metapopulation (its habitat defined by the aquaculture sites). Methods from graph theory allow the identification of those sites in the network that are likely to be key for the control of sea lice in the loch population as a whole. Model outputs were compared with data from a campaign of plankton tows and with lice abundance data from aquaculture sites. The general pattern of abundance was reasonably well replicated, albeit with some notable discrepancies. These differences are worth investigating further, as they may be suggestive of sources of infection by wild fish or of inadequacies in the model.

Effect of exposure on salmon lice Lepeophtheirus salmonis population dynamics in Faroese salmon farms

“…The growth of a sea lice population has an exponential nature (Frazer et al 2012) and is generally modelled as such (Costello 2006, Krkošek et al 2010, Frazer et al 2012. Several dispersion models have been developed which assume that the planktonic stages of the sea lice drift freely with prevailing currents close to the surface (Amundrud & Murray 2009, Adams et al 2012, Salama et al 2013, with recent works including vertical positioning as a response to environmental cues (Johnsen et al , 2016. Sea temperature is widely accepted as a basic factor influencing the growth rate of sea lice populations, as the generation time decreases with increasing temperature (Tully 1992, Heuch et al 2000.…”

Identifying salmon lice transmission characteristics between Faroese salmon farms

“…Dispersion of salmon lice larvae has been studied using numerical models in Scotland (Amundrud & Murray 2009, Adams et al 2012, Canada (Stucchi et al 2011), and Norway , Samsing et al 2017. For a more comprehensive overview, see .…”

Managing aquatic parasites for reduced drug resistance: lessons from the land

“…modern aquaculture) and reliance on chemical control of pests are recent developments [11,12]. In aquatic systems, the dispersal, growth and development of many pests are all strongly tied to environmental conditions, particularly hydrodynamic flow and temperature, which vary in time and space on several scales [13,14]. We are unaware of any studies that have examined how typical resistance strategies for terrestrial systems function to control pests and their evolutionary trajectories in marine systems.…”