FREE CHOICE OF BODY TACTILE STIMULATION ENHANCES WELFARE OF NILE TILAPIA

Introduction

Body tactile stimulation (TS), similar to gentle touch or therapeutic massage, has emerged as a promising form of sensory enrichment for fish. Previous studies have shown that artificial TS alleviates stress in reef fish (Soares et al., 2011), reduces fear-related behavior in zebrafish (Schirmer et al., 2013), and mitigates confinement stress in gilthead seabream (Soares et al., 2026). In Nile tilapia, Oreochromis niloticus, TS has been shown to reduce aggressive interactions and enhance growth performance. However, these behavioral and productive benefits are not accompanied by a reduction in cortisol levels (Gauy et al., 2022, 2023). In the present study, we investigated whether allowing Nile tilapia to freely choose to receive TS would result in greater welfare benefits by reducing group aggression and cortisol levels. A preference test was used as it provides an important approach for assessing the affective valence of environmental enrichment, helping to determine whether animals actively seek or avoid a given stimulus and, consequently, whether it is likely to improve Nile tilapia welfare.

Methods

Groups of 4 adult male fish were exposed for 21 days to one of three treatments: "with TS", "no TS", and "choice TS" (Fig.1). A rectangular apparatus featuring plastic rods with silicone bristles on their sides (Fig.1A); without silicone bristles for control (Fig. 1B); and a half-with / half-without silicone bristles to allow fish choice (Fig. 1C). The apparatuses were placed in the middle of the tank (Fig. 1D), and fish received TS when passing through the bristles. Aggressive interactions were video recorded (10 min each) every three days. Biometric measurements and blood samples were collected at the beginning and end of the period to analyze growth and cortisol levels, respectively.

Results and discussion

Fish crossed by the apparatus more frequently in the "no TS" than in the "with TS" treatment. In the choice treatment, fish passed through the bristles side beginning with ~10% and increasing to ~40% over time. One-way ANOVA revealed that group aggression was higher (p < 0.0001) and the specific growth rate was lower in the "no TS" treatment compared to the other two treatments (p < 0.01). These variables were similar between the "with TS" and "choice TS" (p > 0.21). We also found a positive Spearman rank correlation between crossings by the apparatus and aggressiveness in the "no TS" treatment; no significant correlations in "with TS" treatment, whereas a negative correlation was found in the "choice TS" treatment for bristles choices. Repeated measures ANOVA showed that cortisol levels increased in the "no TS" (p = 0.003) and "with TS" (p = 0.03) groups. Nevertheless, cortisol remained unchanged in the choice group (p = 0.38) and was lower than the other two treatments (p < 0.01). We conclude that allowing Nile tilapia free choice of tactile stimulation (TS) yields more beneficial outcomes by reducing aggressiveness and social stress while enhancing growth. Overall, free-choice TS represents a more effective environmental enrichment strategy for this species.

Figure 1 – Tactile stimulation. The TS apparatus consisted of a rectangular PVC frame filled with vertical plastic sticks, with silicone bristles on their sides that promote TS (A), without silicone bristles for control (B), and a half-with / half-without silicone bristles to allow fish choice. The apparatus were placed in the middle of the tank, and fish received TS when passing through the bristles (an example in D). "no TS" (n = 13), "with TS" (n = 15), and "choice TS" (n = 14) groups per treatment.

Acknowledgment

This study was supported by the São Paulo Research Foundation (FAPESP) through Grants to EGF (# 2023/02306-1); a post-doc scholarship for ACSG (#2023/02991-6), and a technical training scholarship for FDG (# 2024/18073-9). MCS is supported by the Foundation for Science and Technology (FCT), Portugal. (CEEC Individual-2021.01458.CEECIND/CP1668/CT0003).

References

Gauy, A. C. S., Bolognesi, M. C., & Gonçalves-de-Freitas, E. (2022). Long-term body tactile stimulation reduces aggression and improves productive performance in Nile tilapia groups. Scientific Reports, 12(1), 20239.

Gauy, A. C. S., Bolognesi, M. C., & Gonçalves-de-Freitas, E. (2023). Body tactile stimulation reduces the effects of high stocking density on the welfare of Nile tilapia (Oreochromis niloticus). Fishes, 8(6), 320.

Schirmer, A., Jesuthasan, S., & Mathuru, A. S. (2013). Tactile stimulation reduces fear in fish. Frontiers in behavioral neuroscience, 7, 167.

Soares, M. C., Oliveira, R. F., Ros, A. F., Grutter, A. S., & Bshary, R. (2011). Tactile stimulation lowers stress in fish. Nature Communications, 2(1), 534.

Soares, M. C., dos Santos Gauy, A. C., Macieira, J., e Silva, B. C., Banha, F., Gesto, M., Gonçalves-de-Freitas, E. & Saavedra, M. (2026). Tactile stimulation enrichment affects behavioural and physiological responses of a marine commercial fish species, the gilthead seabream Sparus aurata. Aquaculture, 744343.