Statistical modelling of sea lice count data from salmon farms in the Faroe Islands

Fiskaaling regularly counts the number of sea lice in the attached development stages
(chalimus, mobiles and adult) for the salmon farms in the Faroe Islands. A statistical
model of the data is developed. In the model, the sea-lice infection is represented by
the chalimus (or mobile) lice developing into adult lice and is used to simulate past and
current levels of adult lice—including treatments—as well as to predict the adult sea
lice level 1–2 months into the future. Time series of the chalimus and adult lice show
cross-correlations that shift in time and grow in size with temperature. This implies
in situ the temperature-dependent development times of about 56 down to 42 days
and the inverted development times (growth rates) of 0.018 up to 0.024 lice/day at
8–10°C. The temperature dependence D Tð Þ¼ a1ð Þ T þ a2 a3 ¼ 17; 840ðTþ
7:439Þ
2:128 is approximated by D1ð Þ¼ T 105:2 6:578T 49 days at the mean
temperature 8.5°C—similar to Dchað Þ¼ T 100:6 6:507T 45 days from EWOS
data. The observed development times at four sites for a year (2010–11) were 49, 50,
51 and 52 days, respectively. Finally, we estimate the sea lice production from fish
farms to discuss approaches to control the sea lice epidemics—preferably by natural
means. This study is useful for understanding sea lice levels and treatments, and for
in situ analysis of the sea-lice development times and growth rates.