Not too slow, not too fast: water currents affect group structure, aggression and welfare in post-smolt Atlantic salmon Salmo salar

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 reduced. 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 ). 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.

Scale cortisol is positively correlated to fin injuries in rainbow trout (Oncorhynchus mykiss) reared in commercial flow through systems

“…In salmonids, aggressive attacks, demonstrated to mainly aim for the dorsal and/or caudal fins (Abbott and Dill, 1985;Bosakowski and Wagner, 1994a;Moutou et al, 1998;Maclean et al, 2000), could be reduced by increased swimming activity due to an increased flow velocity (Jobling et al, 1993;Davison, 1997). From a welfare perspective, studies on salmonids indicate that a velocity is considered ideal when balanced between territorial behaviour and individual interactions at too low velocities, and collisions with tank walls or other individuals at too high velocities (Solstorm et al, 2016). Velocity was not measured in this study however a comparably higher velocity is indicated by a high water supply and shorter total water exchange duration.…”

Habituation and conditioning in gilthead sea bream (Sparus aurata ): Effects of aversive stimuli, reward and social hierarchies

“…Farmed fish are reared in high densities and agonistic behaviour has been observed in several species (Barreto, Boscolo & Gonçalves‐de‐Freitas, ; Fernö & Holm, ; Jobling, ; Solstorm et al., ). Aggressive individuals should be expected to influence the baseline behaviour and stress level of subordinate fish and could thereby interfere with the processes of habituation and conditioning.…”

Comparison between Atlantic salmon Salmo salar post‐smolts reared in open sea cages and in the Preline raceway semi‐closed containment aquaculture system

“…Swimming behaviour of fish in the Preline was quite different than in the open pen (Supporting Information Video S1). In Preline, fish swam against the current, showing fewer interactions and less aggressive behaviour, as it has been previously observed in fish exposed to constant currents (Christiansen et al ., ; Jobling et al ., ; Solstorm et al ., ). In calm waters, salmonids tend to form dominant hierarchies, inducing differences in size within the fish population (Adams et al ., ; Brännäs, ; Winberg et al ., ).…”

“…Aggressive behaviour can produce lesions, which can in turn facilitate infections. Furthermore, high aggression can lead to increased spontaneous activity with higher energetic costs and increment of the feed conversion ratio of the population (Christiansen et al, 1991;Solstorm et al, 2016).…”

“…Another important advantage is reduced aggressive behaviour. Fish swimming towards a constant current tend to form schools and present fewer interactions, reducing hierarchies and resulting in more food available for subordinate fish (Adams et al, 1995;Brännäs, 2009;Solstorm et al, 2016).…”