Tactile stimulation reduces aggressiveness but does not lower stress in a territorial fish

Abstract

Body tactile stimulation has a positive effect upon highly social animals, such as mammals and cleaner-client coral-reef fish, by relieving stress and improving health. Conversely, some tactile contacts are naturally detrimental, such as those resulted from aggressive interactions. To study whether positive responses from tactile stimulation are generalized among vertebrates, we tested its effect on stress response and aggressive behavior in a territorial fish species, Nile tilapia. We developed an apparatus made of a row of sticks bordered by silicone bristles that was positioned in the middle of the aquarium, and through which fish had to pass to access food, thus receiving tactile stimulation. Isolated fish experienced tactile stimulation for 7 days, and were assigned to 2 types of stressors: non-social (confinement) or social (aggressive interaction). Each of them had a corresponding control treatment without tactile stimulation. Although fish spontaneously crossed the apparatus, we did not observe a decrease in plasma cortisol levels immediately after stressor application as a response to the use of the apparatus, either for social or non-social treatment. However, tactile stimulation reduced aggressive interaction in the social treatment, showing a positive effect on a territorial fish species, and pointing to a way to improve welfare.

Social Behavior and Welfare in Nile Tilapia

“…Tactile stimulation is achieved through a mechanical stimulus, such as touches on the body, when two or more individuals are interacting [80], or artificially, by using some devices [81,82]. Such stimulation has been studied as an alternative to promote animal welfare in several animal groups.…”

“…Soares et al [81] demonstrated that tactile stimulation decreases stress in a coral reef fish, decreasing its cortisol levels after confinement. However, Bolognesi et al [82] demonstrated that tactile stimulation does not decrease the cortisol level in Nile tilapia immediately after the application of a stressor (either social or non-social stressor), but it reduces the aggressiveness in pair staged fights. Such an effect was supposed to be controlled by serotonin (5HT), since this neurotransmitter is released during body tactile stimulation and acts as an inhibitor of aggression in fish [85,86].…”

Preference and Motivation Tests for Body Tactile Stimulation in Fish

“…For data analysis, we considered only the times before and after feeding (10 min) to avoid biases from the presence of food. This protocol had been previously tested and validated by Bolognesi et al [43].…”

“…Furthermore, client-cleaner tactile stimulation is a natural behavior of Ctenochaetus striatus and Labroides dimidiatus [42], while Nile tilapia males establish a rank-based hierarchy by aggressive interaction [44][45][46], a stressful type of body contact. Then, although tactile stimulation can reduce aggressive interaction in Nile tilapia [43], it is not yet clear if tactile stimulation has a positive or negative valence for this species. In this context, we used preference and motivation approaches to solve this problem.…”

“…Studies on fish welfare, for instance, have been increasing considerably [2] where fish are considered sentient beings [3], capable of conscious feelings and sensations [4]. In this the fish has to pass through it when swimming from one side of the aquarium to the other, without any chance of avoiding it [43]. Furthermore, client-cleaner tactile stimulation is a natural behavior of Ctenochaetus striatus and Labroides dimidiatus [42], while Nile tilapia males establish a rank-based hierarchy by aggressive interaction [44][45][46], a stressful type of body contact.…”