Abstract
Biophysical models of parasite dispersal are being increasingly used as a method for screening marine aquaculture developments, whether in establishment of new sites or expansion of existing sites, or planning of farmed fish health management strategies on local or regional spatial scales. How well these models reflect reality, however, is often brought into question, due to the difficulties in validating their outputs. Larval parasitic sea lice can spend up to around 14 d in the water column, as a result potentially travelling several 10s of km between farms. Lice distribution in the water column is typically patchy and low density. Furthermore, infection can occur from lice carried by wild fish. Combined with rapid population turnover and larval exchange between farms, this causes difficulties in attributing links between juvenile lice and their sources. We sought to validate a biophysical model of sea lice dispersal using plankton trawl abundance data and farm site juvenile lice counts. Unusually high farm lice abundances over the study period allowed model predictions of larval density to be compared with trawled samples, in addition to mapping the link between parent and offspring lice counts found on farm sites. We compared the prediction of the larval dispersal model with a site neighbourhood-based metric of infection pressure. Our results validate the ability of the model to predict variation in larval density over time and space and suggest an exponential relationship between estimated infection pressure and observed site juvenile count.
Highlights
Sea lice are generally considered to be one of the principal environmental and fish-health challenges facing the salmonid aquaculture industry (Brooker et al 2018). These parasitic copepods are endemic in wild salmonid populations (Gargan et al 2016), passing through a pelagic larval stage before spending their adult lives attached to a host fish
Understanding the dispersal process and consequent lice population dynamics has become a priority in all regions practicing salmon aquaculture globally, both in terms of managing farmed fish health and welfare, and broader environmental interactions
We have described our attempts to understand the dispersal and population dynamics of a marine parasite that causes a persistent threat to both farmed and wild salmonids, focussing on a relatively selfcontained geographic area during a period in which abundances were challenging for management
Summary
Sea lice are generally considered to be one of the principal environmental and fish-health challenges facing the salmonid aquaculture industry (Brooker et al 2018). These parasitic copepods are endemic in wild salmonid populations (Gargan et al 2016), passing through a pelagic larval stage before spending their adult lives attached to a host fish. Understanding the dispersal process and consequent lice population dynamics has become a priority in all regions practicing salmon aquaculture globally, both in terms of managing farmed fish health and welfare, and broader environmental interactions. This research has been carried out over many years in all major salmon producing areas (Murray & Gillibrand 2006, Amundrud & Murray 2009, Stucchi et al 2010, Salama & Rabe 2013, Johnsen et al 2016, Kragesteen et al 2018, Myksvoll et al 2018, Cantrell et al 2020), and has led to suggestions on approaches to managing aquaculture operations at local and regional scales (Adams et al 2015, 2016, Samsing et al 2019)
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