Reducing sea lice re-infestation risk from harvest water at a salmon farm site in Ireland using a bespoke sieving and filtration system

Water samples from the harvest water outflow of a salmon farm harvest line were sampled at different stages for the presence of sea lice before and after filtration to establish the quantity of sea lice that escaped back into the water column. During the processing of fish through the harvest line the mechanical abrasion experienced by the fish cause sea lice to be knocked off into the harvest outflow water, these lice have the potential to re-infest remaining stock on site. The use of two types of filtration systems at a harvesting site where in situ culling is on-going reduces the risk of re-infestation. In this site the sieve system was particularly effective. The reduction in sea lice numbers achieved by filtering discharge water using sieves was 89.5% using 1 mm screens and was over 99% using 80 μm filters.