Introduction
The breeding of animals for research purposes must meet high standards of animal welfare to ensure that experimental outcomes accurately reflect their biology. In fish, enriched environments (EE) support the expression of natural behaviours and promote positive animal welfare (PAW) (Jones et al., 2021). This is essential not only for obtaining valid and reproducible results but also for developing improved husbandry practices.
In Atlantic salmon (Salmo salar), early-life exposure to EE has been associated with increased brain plasticity and enhanced cognitive (Salvanes et al., 2013) function. In contrast, barren environments often lead to maladaptive behaviours and cognitive impairments, negatively impacting welfare and potentially compromising experimental data.
Previous studies have shown that EE improves the ability of fish to cope with stress and reduces the activation of the endocrine stress response. These benefits are linked to increased neuronal proliferation and behavioural flexibility, both key components of welfare. Serotonin (5HT) plays a central role in regulating anxiety, aggression, depression, and the integrated stress response. It also regulates neurogenesis and synaptic plasticity, both essential for learning, memory, and emotional regulation. Dysfunction in this system can quickly impair these functions (Backström & Winberg, 2017).
This study tested the hypothesis that early-life exposure to EE enhances stress resilience in juvenile Atlantic salmon by modulating serotonergic activity.
Methods
Atlantic salmon fry was randomly allocated into eight 700 litre tanks (n = 375 per tank, initial weight: 1 g), with a continuous water flow (900 l/h), 24-hour light regime, and temperature maintained at 12 °C. Four tanks were provided with structural EE consisting of a circular PVC grid (70 cm diameter) covered with artificial plastic plants, while the remaining four tanks served as non-enriched (NE) controls. Fish were kept under these conditions for 12 weeks.
Throughout the experiment, fish behaviour was monitored and novel object tests were conducted to assess emotional state. Three days before and 30 minutes after an acute stressor (5-minute chasing), 10 fish from each tank were euthanized using an MS222 overdose. Brain and blood samples were collected to analyse cortisol levels, brain serotonin content and absolute gene expression for markers related to stress, cognition, and serotonergic function.
Results and discussion
Fish reared in enriched conditions exhibited greater group cohesion, reduced neophobia, and more stable serotonin levels compared to NE fish. These results suggest that EE helps mitigate the effects of stress and promotes more positive emotional states.
The highest levels of serotonin (5HT) and its primary oxidative metabolite 5HIAA were found in the EE group. Following the acute stressor, significant increases were observed in 5HIAA levels and the 5HIAA/5HT ratio, indicating elevated serotonergic turnover. However, only NE fish showed a significant drop in 5HT levels post-stress, suggesting that EE fish maintained more stable serotonergic tone and were better equipped to cope with the stressor.
Gene expression analysis revealed increased levels of markers associated with neuronal development and activity (bdnf, ndf1), cellular stress (hsp90, hsp70), and serotonin synthesis (tph2) in enriched fish after the stress event. Strong correlations between bdnf and both hsp90 and tph2 further support a link between neural plasticity and the stress response.
Overall, our findings demonstrate that early-life EE enhances cognitive function, stress resilience, and emotional state in juvenile Atlantic salmon. These outcomes highlight the importance of environmental enrichment in improving PAW in captive settings, ensuring that animals are free from negative experiences and have access to positive ones (Prentice et al., 2025).
References
Backström, T. & Winberg, S. (2017) Serotonin coordinates responses to social stress-What we can learn from fish. Front. Neurosci. 11, 294719. https://doi.org/10.3389/FNINS.2017.00595/BIBTEX
Prentice, P.M., Chivite Alcalde, M., Císař , P. Rey-Planellas, S. (2025). Early-life environmental enrichment promotes positive animal welfare for juvenile Atlantic salmon (Salmo salar) in aquaculture research. Sci Rep 15, 5828 (2025). https://doi.org/10.1038/s41598-025-88780-0
Salvanes AG, Moberg O, Ebbesson LO, Nilsen TO, Jensen KH, Braithwaite VA (2013). Environmental enrichment promotes neural plasticity and cognitive ability in fish. Proc. R. Soc. B Biol. Sci. 280, 20131331. https://doi.org/10.1098/RSPB.2013.1331 (2013)
Jones, N. A. R., Webster, M. M. & Salvanes, A. G. V. Physical enrichment research for captive fish: Time to focus on the DETAILS. J. Fish Biol. 99, 704–725. https://doi.org/10.1111/JFB.14773 (2021).
Funded by the AquaExcel 3.0 project, WP6 Joint Research Action (ID 871108) awarded to Rey Planellas, S., University of Stirling. Chivite, M. is supported by a postdoctoral fellowship from the Xunta de Galicia (ED481B 2022 082)