Heart rates, activity, and welfare of Atlantic salmon reared to harvest size in a semi-closed prototype sea-cage or ambient conditions

The salmon aquaculture industry is developing technologies to solve challenges with sustainability and poor animal welfare. A potential solution is to use semi-closed systems to mitigate lice infestations and provide more controlled rearing environments. Here, we tested Atlantic salmon in a novel semi-closed prototype cage and compared growth and welfare with a standard open cage to harvest sizes of ~5 kg. Additionally, data storage tags were used to record heart rate and activity in fish from both cage types. The semi-closed cage was subjected to a continuous photoperiod, constant moderate currents, and water supply from greater depths, while the open cage experienced a natural ambient surface environment. Growth trajectories differed throughout the trial, but final harvest sizes were similar in the two groups. Mortality was lower in the semi-closed cage, although welfare scores were worse, likely related to sub-optimal sampling methods. In both groups, heart rates were generally between 40 and 60 beats min −1 and were influenced by seasonal temperatures. Owing to the continuous photoperiod, closed-cage fish showed weaker diurnal variations in heart rate and activity, while open-cage fish were calmer at night. Additionally, closed-cage fish had higher activity overall, indicative of higher swimming speeds. Finally, by using a refined tagging protocol, tag-related mortalities were prevented and tagged fish showed excellent growth following 7-months in sea cages. While data quality indexes did decline towards the end, this shows that bio-loggers can be used over long periods of fish production to record core welfare parameters for evaluating novel rearing systems. • Atlantic salmon in a semi-closed cage had similar harvest size as in an open cage. • Mortality was lower, but welfare scores were worse owing to rougher sampling methods. • Data storage tags were used to record heart rate and activity over a 7-month period. • Heart rate and activity differed diurnally between cage type, temperature, and size. • A refined protocol prevented mortalities and improved growth in tagged fish.