Introduction
European seabass is one of the most commercially important species in the Mediterranean aquaculture sector. Fish reared in captivity are exposed to various factors that may compromise their welfare, such as chronic stressors associated with routine operations and environmental conditions (Rey Planellas et al., 2026). Environmental enrichment (EE) represents a well-established strategy to promote animal welfare by providing novel stimuli that meet behavioral, physiological, and psychological needs (Arechavala���Lopez et al., 2022; Rey Planellas et al., 2026). In aquatic species, structural environmental enrichment, understood as the introduction of physical complexity through objects or environmental modifications, has been shown to enhance cognitive abilities, brain plasticity, and nervous system function. It can also modulate the stress response, reduce stereotypic behaviors, and, in some cases, improve growth and survival (Arechavala���Lopez et al., 2022; Brunet et al., 2022). The effects of environmental enrichment vary depending on the species and its life stage. Despite growing interest in this strategy, there remains a significant lack of knowledge for some of the most relevant species in the industry, such as European seabass (Dicentrarchus labrax L.). In this context, our objective is to evaluate the effects of environmental stimulation through physical complexity on the overall welfare of seabass, considering its different components: functional, natural, and emotional welfare.
Materials and Methods
The study was conducted at the experimental facilities located in Port d'Andratx (Balearic Islands, Spain), belonging to the Laboratory of Marine Research and Aquaculture (LIMIA-IRFAP). A total of 4,000 juvenile European seabass (Dicentrarchus labrax) with an average body weight of approximately 30 g were randomly distributed among eight experimental marine cages. Four of these cages were structurally enriched, while the remaining four were maintained without enrichment and served as controls. The applied structural EE consisted of a floating rectangular cork from which six knotted ropes with a weight at the end were suspended, occupying more than half of the vertical space within the sea-cage (1.5 m). Juvenile individuals were maintained under these experimental conditions for approximately nine months (11/03/2024–18/12/2024; 282 days). Throughout the experiment, four sampling events were conducted at three-month intervals (t0, t3, t6, and t9). At each sampling point, the same parameters were systematically recorded, including biometric measurements (total length and body weight), individual photographs for morphometric analysis (based on distances between 24 anatomically relevant landmarks), and telencephalon and muscle tissue samples. These samples were subsequently used to assess oxidative status through the activity of antioxidant enzymes, as well as to quantify the expression of genes of interest related to stress, neuronal activation, neurogenesis, and neuroplasticity. In addition, feeding response was recorded daily for each cage using a consumption index throughout the entire experimental period. Finally, six months after the start of the experiment, individuals were implanted with acoustic transmitters, allowing their activity to be monitored via acoustic telemetry within the rearing cages.
Results and discussion
The results showed no significant differences between treatments in the biometric or morphometric parameters evaluated, indicating that structural environmental enrichment (EE) does not affect growth, body condition, or morphology in juvenile seabass. Likewise, no effects of enrichment were detected on oxidative stress markers in either muscle or telencephalon, suggesting the absence of a physiological stress response associated with the experimental conditions. In contrast, gene expression analysis in the telencephalon generally revealed higher expression levels in enriched individuals for genes associated with neurogenesis, neuronal activation, and synaptic plasticity. Moreover, a time-dependent expression pattern was observed exclusively in enriched individuals, characterised by a progressive increase in gene expression that peaked during the intermediate phase of the experiment (t3–t6), followed by a decline towards the end of the experimental period (t9). This pattern suggests that structural EE induces an initial activation of neural processes related to memory, neuronal activity, and neuroplasticity, which subsequently decreases after prolonged exposure, likely as a consequence of habituation to a stable and unchanging enriched environment. In addition, structural EE had a positive effect on feeding response, which was higher than in the control group during the last three months of the experiment in enriched individuals. Overall, these results indicate that structural EE transiently modulates neural activity without compromising the general physiological status of individuals, evidencing a temporal window of response to environmental stimulation under static conditions. This suggests that stimulus renewal may be necessary to sustain this positive response over time.
Acknowledgment
The authors acknowledge the funding provided by the Envirobass, Modifish, and Aquareach projects, which made this study possible. They also thank the technical and research staff at LIMIA-IRFAP for their support during the experimental work and for the use of the facilities. Finally, they are grateful to ABSA (Grupo Culmarex) for the donation of the fish and their assistance throughout the experiment.
References
Arechavala���Lopez, P., Cabrera�����lvarez, M. J., Maia, C. M., & Saraiva, J. L. (2022). Environmental enrichment in fish aquaculture: A review of fundamental and practical aspects. Reviews in Aquaculture, 14(2), 704-728.
Brunet, V., Kleiber, A., Patinote, A., Sudan, P.-L., Duret, C., Gourmelen, G., Moreau, E., Fournel, C., Pineau, L., Calvez, S., Milla, S., & Colson, V. (2022). Positive welfare effects of physical enrichments from the nature-, functions- and feeling- based approaches in farmed rainbow trout (Oncorhynchus mykiss). Aquaculture, 550, 737825.
Rey Planellas, S., Saraiva, J. L., Gon��alves���de���Freitas, E., Arechavala���Lopez, P., Bovenkerk, B., Breen, M., Cooke, S. J., F��re, M., Northwood, L., Stien, L. H., Kadri, S., Noble, C., Nilsson, J., Rodriguez, F., Salas, C., Sand��e, P., & Van De Vis, H. (2026). Fish welfare in a changing world: New developments and current challenges. Journal of Fish Biology, jfb.70423.