Abstract
Lepeophtheirus salmonis infestation is a major challenge for the Atlantic salmon (Salmo salar) aquaculture industry. Submerged farming (sea cages below 20 m depth) has emerged as a promising preventive measure against the parasite, being based on that there is a higher abundance of infective stages of L. salmonis in upper water layers. Although infestation levels seem reduced, some infestation still occur in submerged cages, and infestation mechanisms are poorly known. The current study summarises several investigations of L. salmonis biology and behaviour related to submerged farming.
Production data from submerged farms were collected and analysed. Different life stages of L. salmonis, including egg strings, were collected from submerged cages and brought to the laboratory. Survival was investigated, and hatching and development of larvae from the egg strings was monitored. Behavioural responses of nauplii, copepodites, pre-adult and adult stages to stimuli such as light, pressure, and different salinities were quantified.
In the current data set, there was a 90% reduction in delousing treatments in submerged cages compared to conventional surface cages, highlighting the positive effect of submerged farms but showing that infestations still happen. Survival of L. salmonis collected at submerged farms did not differ from that of lice from surface farms, neither did hatching and development of larval stages. This indicates that the depth of submerged cages does not represent a challenge to the basic biology of the parasite. Behavioural experiments with larvae originating from submerged and surface farms show that the larvae are positively phototactic and negatively barotactic, and avoid certain salinities. However, light, pressure, and salinity mediated behaviour differently depending on L. salmonis origin. The results highlight the need for detailed knowledge on biology and behaviour of the parasite to predict the effects of preventive measures, and that this knowledge is needed to optimise the measures.