| Summary: | Salmonid aquaculture plays an important role in increasing global fish production and the biggest factor constraining its further growth and development in Norway is salmon lice (Lepeophtheirus salmonis). Previous projects have shown that flushing with freshwater or seawater with large volume and moderate pressure may be used as a delousing method on Atlantic salmon (Salmo salar). It is also known that bath treatments with cold and especially warm water have a delousing effect but can be experienced stressful and painful for the fish if the temperature is too high. The current trial was conducted to examine if waterfall treatment (low pressure flushing) against lice could be made more efficient by using cold or warm water in the waterfall, while simultaneously maintaining animal welfare by avoiding temperatures that is known to cause pain and by reducing exposure time for the fish in the treatment water. The results suggest that the waterfall treatment had no effect on sessile stages, but an overall delousing effect of 40-56 % on preadult lice and 16-35% on adult lice, with no delayed long-term effects. Over 70% of the delousing efficiency seemed to be caused by the waterfall itself, rather than other procedural factors. The temperature of the treatment water had no significant effect, most likely due to short exposure time. Furthermore, the waterfall treatment had a somewhat negative short-term effect on welfare. However, the applicable welfare parameters improved significantly after 14 days, suggesting no long-term consequences. Additionally, there was no considerable negative effect of treatment temperature on overall welfare. Masteroppgave i biologi BIO399 MAMN-BIO MAMN-HAVSJ
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