Abstract
Increased swimming speed of Atlantic salmon is generally considered an improvement to welfare under aquaculture settings, as group structure is improved and agonistic behaviour re- duced. As such, establishing fish farms in exposed areas with fast water current velocities should be favourable. However, at some locations, velocities exceed what is known as preferable for salmonids, and this may compromise fish welfare. In this study, behaviour and fin erosion were observed on post-smolt salmon stocked at 39 kg m �3 in raceways at 3 water current velocities: fast (1.5 body lengths (BL) s �1 ), moderate (0.8 BL s �1 ) and slow (0.2 BL s �1 ). Movements that affect group structure and interactions between individuals varied by up to 20-fold between velocities. A behavioural change occurred directly after velocities were set. Severe fin erosion decreased over time in all groups, but new injuries increased almost 3-fold in the faster-velocity group. Our results suggest that moderate velocity is ideal from a welfare perspective. At slow velocity, higher frequency of structural movements and between-individual interactions could be stressful for the fish. At faster velocity, the fish have to focus on swimming, which could increase unintentional collisions with obstacles and other individuals and result in new fin erosion. Our results suggest that management of water currents may be an effective way of controlling behaviour and may thereby improve welfare.
Highlights
Motivational drivers for swimming in wild Atlantic salmon include migration, feeding, predatory avoidance and mating
Increased swimming speed of Atlantic salmon is generally considered an improvement to welfare under aquaculture settings, as group structure is improved and agonistic behaviour reduced
Current velocity had a large influence on structural movements, with the highest frequency of behaviours observed in fish kept at Lowercase letters denote significant differences between current velocities, and numbers denote significant differences between weeks based on the cumulated behaviours (p < 0.05)
Summary
Motivational drivers for swimming in wild Atlantic salmon include migration, feeding, predatory avoidance and mating. For fish in aquaculture systems, few of these drivers are functional. Farmed fish cannot move to another habitat, they do not need to hunt food or avoid predators, and they usually do not mature sexually. In the marine life stages, wild salmonids swim on average at 1 body length (BL) s−1 (Drenner et al 2012). This is similar to observed swimming speeds in salmon farms (Sutterlin et al 1979, Kadri et al 1991, Blyth et al 1993, Juell & Westerberg 1993).
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