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

We assessed variations in salmon lice Lepeophtheirus salmonis population dynamics
in Faroese salmon farms in relationship to their physical exposure to local circulation patterns and
flushing with adjacent waters. Factors used in this study to quantify physical exposure are esti-
mates of the freshwater exchange rate, the tidal exchange rate and dispersion by tidal currents.
Salmon farms were ranked according to the rate of increase in the average numbers of salmon lice
per fish. In a multiple linear regression, physical exposure together with temperature were shown
to have a significant effect on the rate of lice infection. The sites with low exposure revealed higher
rates of self-infection and internally driven outbreak dynamics, while high-exposure sites showed
lower rates of self-infection, tending towards externally driven outbreak dynamics. The low-
exposure sites also appeared to have a lower threshold of salmon stocking numbers for outbreaks
of infection. The study presents a simple method of characterizing salmon farming fjords in terms
of their different exposure levels and how they relate to potential self-infection at these sites.