Estimating cage-level mortality distributions following different delousing treatments of Atlantic salmon (salmo salar) in Norway.

This retrospective descriptive study estimates cage-level mortality distributions after six immediate delousing methods: thermal, mechanical, hydrogen peroxide, medicinal, freshwater and combination of medicinal treatments. We investigated mortality patterns associated with 4 644 delousing treatment of 1 837 cohorts of farmed Atlantic salmon (Salmo salar) stocked in sea along the Norwegian coast between 2014 and 2017. The mortality is expressed as mortality rates. We found distributions of delta mortality rate within 1, 7 and 14 days after all six delousing treatments, using mortality rate within 7 days before treatments as baseline. The results show that we can expect increased mortality rates after all six delousing methods. The median delta mortality rates after thermal and mechanical delousing are 5.4 and 6.3 times higher than medicinal treatment, respectively, for the 2017 year-class. There is a reduction in the delta median mortality for thermal and freshwater delousing from 2015 to 2019. There is a wide variability in the mortality rates, in particular for thermal delousing. Our results suggest that the variability in delta mortality for thermal delousing has been reduced from the 2014 to 2017 year-class, indicating an improvement of the technique. However, a significant increase in the number of thermal treatments from 14 in 2015 to 738 in 2018 probably contributes to the overall increased mortality in Norwegian salmon farming.

Pathogenicity of Pasteurella sp. in lumpsuckers (Cyclopterus lumpus L.).

The incidence of disease caused by Pasteurella sp. in farmed lumpsuckers in Norway has been steadily increasing in recent years, causing significant economic losses and fish welfare issues. The disease affects all life stages, both in hatcheries and after release into salmon cages. Therefore, it is important to establish robust challenge models, to be used for vaccine development. Exposure experiments via intramuscular and intraperitoneal injection underlined the high virulence of the bacteria, whereas the cohabitation and bath models allowed the chronic symptoms of the disease to be studied more accurately. Skin lesions and haemorrhage at the base of fins were observed in the more acute cases of the disease. Symptoms including white spots over the skin, especially around the eyes, characterized the chronic cases. The latter were most prominent from the bath challenge model. Histopathology indicated a systemic pattern of disease, whereas qPCR analysis from head kidney showed that bacteria may be present in survivor fish at the end of the challenges. In all the challenge models investigated, Pasteurella sp. was re-isolated from the fish, thus fulfilling Koch's postulates. These findings highlight the importance of screening of lumpsuckers prior to transfer to minimize the risks of carrying over asymptomatic carriers.

The economic impact of decreased mortality and increased growth associated with preventing, replacing or improving current methods for delousing farmed Atlantic salmon in Norway.

Impacts of salmon lice is a major concern for a sustainable production of farmed Atlantic salmon in Norway. Most treatment methods for removal of salmon lice have associated increased mortality and decreased growth in a period after delousing, which affects the profitability of the farmer, and causes poor welfare and sustainability. In addition, the variance in mortality and growth, especially after non-medicinal treatment methods, is high, which makes it hard for a farmer to decide which control measure to apply to keep lice levels below the legal limit. In this study, we have applied a stochastic partial budget approach to assess the economic impact of reducing mortality and increasing growth of farmed Atlantic salmon by preventing, replacing and improving current delousing methods in Norway. We have simulated a production cycle of two different smolt-groups to find the outcome (harvested biomass, average end weight of the salmon, number of dead fish and feed consumption) of production cycles without or with two, three or four delousing treatments in the on-growing phase at sea. The results suggest that accounting for the biological losses associated with lice treatments is important when making choices of delousing strategies. The biological costs of increased mortality and decreased growth associated with especially non-medicinal treatments are expected to be high, but varies substantially. Therefore, the economic benefit of preventing or improving can also be high. The calculations imply that salmon producers could invest a considerable amount in measures for prevention or improvement of thermal treatments before break-even. For example could a farmer use on average 535,313 €/cage/ 1-yearling production in measure to prevent four thermal treatments before it is no longer economical beneficial. Depending on the performance of the four thermal treatments a farmer could use from 319,196-737,934 €/cage/ 1-yearling production on measures of improvement. Replacing one thermal treatment with another immediate treatment method has a minor economic benefit. The results further shows that sales value and feed consumption constitutes the largest share of the change in profit between different treatment regimes. The results from this study also show that not taking into account the risk of mortality and reduced growth associated with the different treatment methods of delousing, could lead to underestimating the benefit of improving, preventing and replacing treatments.