Geographical and temporal variation in abundance of salmon lice (Lepeophtheirus salmonis) on salmon (Salmo salar L.)

Surveys in the area around Austevoll Aquaculture Research station have failed to reveal the presence of a large number of the free-swimming stages of salmon lice in the water column. However, lice were abundant on salmon held in cages in this area. Lice-free salmon were used as collectors of lice at three sites chosen at varying distances from other known sources of lice (salmonid fish farms). The abundance of lice on salmon at particular sites was found to vary during the year and also between years. During the study period, variations were correlated with temperature, increased abundance occurring at higher temperatures (>6 degrees C). Comparisons between sites during the period with the highest temperatures showed that lice abundance was negatively correlated with distance to other salmon farms in the area. During periods of colder temperatures (<6 degrees C) lice abundance was positively correlated with distance to salmonid fish farms. In January 1995, it was possible to predict that 1995 would be a year with heavy lice burdens. The basis of the prediction was the accumulated difference between average temperature during the period 1978-1996 and the sea temperature for the 4-month period preceding the settling period. (C) 1997 International Council for the Exploration of the Sea.