Lumpfish are broadly used as biological sea lice removers in Norwegian salmon farming and are mostly produced in hatcheries. To date, there is little systematic documentation on the mortality causes of lumpfish in hatcheries. In the current study, data from 12 selected fish groups from four hatcheries in Norway were collected to understand the challenges and opportunities related to the categorization of mortality causes of lumpfish in hatcheries. This study indicated that a high proportion of lumpfish mortality was categorized by the hatcheries as unspecified. When specified, mortality was primarily assessed as due to fin damage. Our analyses showed that mortality categorized as fin damage correlated with the detection of infectious agents in dead fish, suggesting that mortality was due to infections rather than fin damage. It was not possible to conclude whether infection with pathogens present in the environment caused fin damage or if injuries from aggression were a gateway for pathogens. Furthermore, due to the lack of information regarding production conditions, it was not possible to assess risk factors causing eventual aggression or the presence of pathogens. This study revealed that mortality causality—the chain of events leading to death—is mainly unclear in lumpfish hatcheries because (1) mortality categorization does not focus on underlying causes and (2) there is little documentation of production conditions, making it very difficult to relate mortality rates to external factors. The present findings highlight that there are gaps in the registrations made by hatcheries and that there is an urgent need to implement standardized monitoring protocols for lumpfish hatcheries. This will help identify the causes of mortality and, therefore, allow for the implementation of proper measures that will ensure better survival.

The objective of this study was to evaluate welfare and health effects following single and repeated non‐lethal blood sampling from the caudal vasculature of Atlantic salmon. Two experiments were conducted at three different temperatures: a 6‐week freshwater experiment with fish weighing 50–100 g, undergoing up to four blood withdrawals, and a 4‐week seawater experiment with fish weighing 300–450 g, undergoing up to three blood withdrawals. Observed clinical signs included dark pigmentation of the skin and vascular disturbances posterior to the puncture site. Histological evaluations revealed injuries to the vasculature, vertebral column, nervous tissue and skeletal muscle. At 12°C and 15°C, fish displayed strong capacity for tissue regeneration and haematological recovery, but also an elevated risk of acute mortality. At 5°C, acute mortality was minimal, but recovery time was approximately twice as long compared with higher temperatures. Mortality rates and clinical findings decreased with increasing fish weight. These findings demonstrate that non‐lethal caudal blood sampling poses a welfare risk in salmon weighing 450 g or less. The risk factors identified here may inform future risk assessments and support the development of welfare‐compatible guidelines for non‐lethal blood sampling in Atlantic salmon.

Cardiac morphology in farmed Atlantic salmon is highly plastic and influenced by environmental conditions during smoltification. In this case study, fish from the same roe batch were reared under two different freshwater salinity regimes (0.5 ppt vs.\ 2.5--3.7 ppt) and sampled from vaccination to slaughter. A higher prevalence of bulboventricular deformity (type 8; right-shifted junction) was observed in the higher-salinity group, emerging during the smoltification period. Affected fish showed altered heart proportions, including increased atrium-to-heart weight ratio and differences in cardiac angles and bulbus-to-ventricle proportions. Ultrasound observations indicated reduced heart rate and abnormal ventricular geometry. The higher-salinity group also exhibited elevated plasma chloride and lower calcium during smoltification. Despite these differences, growth and cumulative mortality were similar between groups. These findings suggest that freshwater salinity during smoltification is associated with altered cardiac development and compensatory functional changes, with potential implications for physiological robustness under stress.

The Atlantic salmon aquaculture industry in Norway is frequently confronted by the biological risks associated with the transport, handling, and treatment of live fish in well‐boats. The aim of this study was to collect experience‐based knowledge from the aquaculture industry about well‐boat operations, to make recommendations to lower the biological risk in well‐boat operations. Well‐boat crew, salmon farmers, fish health personnel, and workers in other relevant professions in the industry participated in semi‐structured individual interviews, followed by group interviews and a survey. This study clearly revealed that even though operational/technical factors are critical for lowering the biological risk in well‐boat operations, their successful implementation hinges on three fundamental human factors: (1) experienced personnel in key positions having correct focus, (2) communication, and (3) planning/risk assessment. Those factors deserve greater attention, as the present study highlights that they are not always sufficiently taken into account. An array of recommendations to lower the biological risk for Atlantic salmon in well‐boat operations is identified and summarized based on the findings from the study and literature.