Long-term body tactile stimulation reduces aggression and improves productive performance in Nile tilapia groups

Abstract

One concern of the Anthropocene is the effects of human activities on animal welfare, revealing the urgency to mitigate impacts of rearing environments. Body tactile stimulation (TS), like massage therapy, has emerged as an enrichment method to counteract stress and anxiety in vertebrates. In the current study, we evaluated the effects of long-term TS on four-member groups of male Nile tilapia, a worldwide reared species whose socially aggressive behavior is an essential source of stress. We placed a rectangular PVC frame fitted with vertical plastic sticks sided with silicone bristles in the center of aquarium to enable the fish to receive body TS when passing through the bristles. A similar apparatus without bristles was used as the control. Fish subjected to TS for 21 days showed a gradual lowering of overt fights over time, but with no reduction in cortisol or androgen levels. Nevertheless, TS improved the specific growth rate, maintained balanced length/weight gain, and increased feed efficiency, probably owing to the lowered energy expenditure during fights. Thus, we show for the first time that long-term TS provided by a simple device can be used as a tool to improve the welfare and productive performance of territorial fish.

Body Tactile Stimulation Reduces the Effects of High Stocking Density on the Welfare of Nile Tilapia (Oreochromis niloticus)

“…Nevertheless, one possible mechanism that explains the effect of TS on productive parameters is the reduction in aggressive interaction. In previous studies with Nile tilapia, we showed that TS decreases aggression in both isolated [25] and grouped animals [26]. Aggressive interactions, in turn, demand high energy expenditure for fish [48], which can interfere with growth in Nile tilapia [49].…”

“…Both apparatuses, each in their respective treatments, were introduced in the middle of the glass tank (Figure 1C); thus, when passing through the center of the glass tank, the fish in the TS treatment had contact with the silicone bristles (Figure 1D). The TS apparatus does not remove the mucus lining the bodies of animals, nor does it cause injury [25,26,31]. Moreover, Nile tilapia individuals spontaneously cross through this apparatus and overcome an aversive stimulus to access it [31], meaning it is not aversive to the fish.…”

“…The 21-day period was chosen as the long-term period, as it corresponds to three times the time used to provide TS to Nile tilapia [25]. Furthermore, it has already been shown by Gauy et al [26] that 21 days is sufficient to observe a cumulative effect of TS on reducing fights in this species.…”

“…Blood was centrifuged at 3000 rpm for 10 min, and the plasma was frozen at −20 • C for later hormone assays. Cortisol assays were performed by enzyme-linked immunosorbent assay (ELISA), using commercial kits (IBL-Immuno Biological Laboratories, Hamburg, Germany), already validated for Nile tilapia [26,43]. The analysis was performed on all fish in the experiment in duplicate and without dilution of the samples.…”

“…In fish, short-term TS (10 days or less) has been reported to decrease stress in reef fish (Ctenochaetus striatus) [24] and reduce aggression in previously isolated male Nile tilapia [25]. In the long term (21 days), TS decreases aggressiveness and increase productive performance in social groups of male Nile tilapia [26]. As high stocking density is associated with aggressive interactions and social stress in territorial fish, we aimed to investigate whether long-term TS alleviates the negative effects of high stocking density on the welfare of Nile tilapia.…”