The effect of light regimen on settlement patterns of sea lice, Lepeophtheirus salmonis , on Atlantic salmon, Salmo salar , post-smolts while taking into account fish size and fin erosion in a static tank system
Abstract Atlantic salmon post-smolts were exposed to copepodids of Lepeophtheirus salmonis maintained under two lighting regimens. Fish were exposed to lice in either a lit (10:14 h daily light:low light) regimen or unlit (24 h low illumination). After 20 to 22 days fish were euthanized and surveyed for the number and location of settled lice (attached chalimus) while taking into account surface area and fin erosion scores. Surface area did not affect total lice counts; however, densities were higher on smaller fish. On the other hand, fin erosion influenced the number of lice settling on fins based on data collected on paired fins which had the highest number of lice and erosion scores. The absence of a diurnal lighting regimen did not affect louse numbers on salmon post-smolts. However, it did influence the distribution of sea lice on body regions with a higher proportion of sea lice on the posterior versus the anterior region of host fish in lit tanks. Our results confirm the fact that low light does not preclude lice infestations but can affect aspects of louse distribution likely due, in this study, to a change in host swimming activity. Statement of relevance There is a need to assess light effect on lice settlement in controlled conditions due to conflicting results. We analyzed lice settlement while taking into account preexisting fin erosion and fish size. Findings confirm that light is not a requirement for lice infestation. Fin erosion influenced the number of lice on paired fins suggesting the importance of considering fin erosion as a potential biophysical parameter.