| Summary: | The impacts of the ectoparasite salmon lice (Lepeoptheirus salmonis (Krøyer, 1837)), is a major challenge for sustainable production of farmed Atlantic salmon (Salmo salar) in Norway, and has been so for over 40 years. The challenge with salmon lice, especially the control of it, is the expense and that lice affect different aspects of the economy, environment, and society. Lice have become central to the governmental regulation of the growth of the salmon aquaculture industry, and the most important obstacle to the stated political goal of quintupling aquaculture production in Norway by 2050. The main reason for control is the spillover of lice from farmed to wild salmon. For many years, lice were controlled by use of pesticides. However, they eventually developed resistance to most of the pesticides available. This led to a paradigm shift from treating medicinally to non-medicinally around 2015. Non-medicinal treatments mainly involve the use of heated water baths (thermal treatment) or brushing or flushing (mechanical treatment) of the fish to remove the lice. In 2020, 52 million salmon, 14.8% of the standing stock, died during the on-growing period at sea. It was suspected that a large part of this mortality was caused by non-medicinal treatments. Veterinarians working in the field had, for several years, raised concerns regarding poor fish health and welfare related to especially thermal treatment. Still, at the on-set of this PhD, we did not know how many salmon died because of these treatments, nor how the treatments affected their growth. Securing good health and welfare is part of sustainable animal production and should be a top priority. Yet, when we do not know the extent of mortality and growth loss associated with delousing treatments, it is difficult to assess how much could be spent on measures to reduce mortality and secure good growth. Consequently, it is also difficult to evaluate how highly we should prioritise work related to reducing mortality and securing good growth after treatment. The ...
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