  Salmon farming has become a prosperous international industry over the last
decades. Along with growth in the production farmed salmon, however, an
increasing threat by pathogens has emerged. Of special concern is the
propagation and spread of the salmon louse, Lepeophtheirus salmonis. In order
to gain insight into this parasites population dynamics in large scale salmon
farming system, we present a fully mechanistic stage-structured population
model for the salmon louse, also allowing for complexities involved in the
hierarchical structure of full scale salmon farming. The model estimates
parameters controlling a wide range of processes, including temperature
dependent demographic rates, fish size and abundance effects on louse
transmission rates, effects sizes of various salmon louse control measures, and
distance based between farm transmission rates. Model parameters were estimated
from data including 32 salmon farms, except the last production months for five
farms which were used to evaluate model predictions. We used a Bayesian
estimation approach, combining the prior distributions and the data likelihood
into a joint posterior distribution for all model parameters. The model
generated expected values that fitted the observed infection levels of the
chalimus, adult female and other mobile stages of salmon lice, reasonably well.
Predictions for the time periods not used for fitting the model were also
consistent with the observational data. We argue that the present model for the
population dynamics of the salmon louse in aquaculture farm systems may
contribute to resolve the complexity of processes that drive that drive this
host-parasite relationship, and hence may improve strategies to control the
parasite in this production system.
