The most common infection pathway of salmon lice is through the planktonic copepodid stage, but there is an increasing concern that the mobile stages of salmon lice leave/fall off the salmon during crowding operations, potentially re-infecting treated fish or fish in adjacent cages within the same or nearby farms . Knowledge on how many lice fall off during the crowding operation as well as behavioural traits of the various mobile development stages and gender will help to identify the severity of the potential problem.
In this study the number of lice that fell off fish during crowding was quantified at the th ermal delousing of 9 individual net pens. In addition, a large dataset from sea lice monitoring of farmed salmon was analysed for behavioural traits regarding lice that fell off. In the Faroese national sea lice monitoring program registrations of lice that fall off during sedation of the fish is imposed. Data from more than 30000 counted net pens from mid-2016 to February 2024 was statistically analysed.
In the dataset sea lice were registered as species (Lepeophtheirus salmonis or Caligus elongatus ) and for salmon lice the development stage was also registered (adult females, large mobile which include preadult stage 2 and adult males, and preadult 1). Te mperature and date were also registered and used to analyse potential variations in detachment due to temperature and daylength.
C. e longatus were much more prone to detach from the fish than salmon lice and both species showed seasonal variations in the percentage of lice that detached from the fish (Figure 1).