Validating a biophysical parasite model with fish farm pen and plankton trawl data

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.