Abstract
The response of cultured rainbow trout to their mirrored image was investigated. Thirty fish were placed individually in two novel aquariums consecutively for 10 min each. Walls in one aquarium were covered with mirrors on all four sides, whereas the walls of the other aquarium were non-transparent black. Because all four walls were covered with mirrors, the mirrored image of the fish was reproduced multiple times such that ‘a group’ of fish was created surrounding the individual. Half of the fish started in the aquarium with the mirrors, whereas the other half started in the mirrorless aquarium. Fish swim faster in the aquarium with mirrors than in the mirrorless aquarium (2.95 vs. 2.40 cm/s; p < 0.01), indicating a positive behavioural response towards their mirrored images. Fish did not show aggressive interactions towards their mirrored images. Being confronted with ‘a group’ of fish and not just one ‘opponent’ may have inhibited aggressive behavior, or individuals may not have considered the images to be fellow individuals. Fish that swam faster in the mirrorless aquarium also did so in the aquarium with mirrors (r = 0.73; p < 0.0001), indicating a persistent behavioural coping response (boldness) in response to the two novel environments. Mirrors may be used to influence social behaviour of fish in aquaculture; further research is needed to investigate the influence of mirror placement in tanks of group housed trout on growth and behaviour.
Highlights
In aquaculture, fish to fish interaction is influenced by stocking density, the densities at which welfare becomes compromised remains ambiguous (Laursen et al, 2015)
Half of the fish started in the aquarium with the mirrors, whereas the other half started in the mirrorless aquarium
Not investigated to date is the effect of optically changing group size without affecting the tank size, stocking density or water quality, by using fish tank walls that are covered with mirrors
Summary
Fish to fish interaction is influenced by stocking density, the densities at which welfare becomes compromised remains ambiguous (Laursen et al, 2015). The response of cultured rainbow trout to their mirrored image was investigated.
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