Welfare and Physiological Consequences of Non‐Lethal Blood Sampling From the Caudal Vasculature of Atlantic Salmon ( <i>Salmo salar</i> L.)

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.