Introduction
Environmental enrichment is considered as a tool to guarantee or improve the welfare of captive fish. The deliberate addition of physical complexity to captive conditions allows the animals to have a greater control over their environment, and provides the opportunity to experience new situations while performing behaviours typical of their species in the wild. Well-designed structural EE may provide sensorial and motor stimulation that meet the animals’ behavioural and psychological needs, while increasing the behavioural options and putatively reducing the stressors. We demonstrated experimentally that simple structural enrichment can be applied at different production stages of gilthead seabream (Sparus aurata) aquaculture, from broodstocks to on-growing sea-cages, not only improving the welfare conditions of captive seabream, but also providing benefits to the farmers. Here, we review the experimental and structures designs, results and conclusions from previous studies carried out on captive gilthead seabream at different life-stages.
Material and Methods
Four different experiments were carried out exposing gilthead seabream juveniles, adults and broodstocks, assessing the effects of structural enrichment of fish welfare with different tools and from different perspectives. However, structural enrichment was similar among experiments, which consisted on plant-fibre ropes, vertically positioned and equally distant, attached to the bottom or to the top of the rearing system and adapted to the appropriate size. First, we studied the effects of structural enrichment on 80 juvenile seabream (weight: 3.8 ± 0.1 g), randomly distributed in eight experimental floating cages within a larger tank. Half of the fish (four tanks) were exposed for 35 days to enrichment. Body condition, growth parameters, fins erosion and brain monoamines were assessed, together with the spatial distribution of the fish during the experiment (Arechavala-Lopez et al. 2019). Second, we explored the effects of structural enrichment on cognition, exploratory behaviour and brain physiological functions of 90 seabream juveniles (weight: 21.9 ± 0.8 g). Fish were distributed in six experimental tanks, half of them (three tanks) reared with the structures for 60 days. During last days, fish were moved into a new-designed experimental maze and video-recorded from above for 1 h, during four different days each group. Fish behaviour, brain monoamines and oxidative stress were assessed (Arechavala-Lopez et al. 2020). A third study was carried out on bigger seabream (weight: 217.58-55.96 g) in a floating sea-cage. Ten fish were tagged with “accel-tag” acoustic transmitters, and reared for one month together with 300 seabream in the same cage. Structural enrichment were deployed during the last two weeks of the experiment. Fine-scale daily activity and spatiotemporal distribution of tagged fish were assessed (Muñoz et al. 2020). A fourth experiment is being carried out on seabream broodstocks (weight ~1 kg), reared in six tanks of 2000 L at commercial densities. Three tanks were enriched with vertical structures (hanging ropes). The aim of this experiment is assessing the physiological and behavioural stress response of broodstock to common aquaculture procedures, such as handling, netting or crowding, as well as the potential effects on spawning and off-spring production (Oliveira, in process).
Results
Results from the first experiment showed that EE modified distribution of seabream inside the experimental cage, entailing a higher use of the inner area of the cage. In addition, a lower interaction with the net and lower aggressiveness among individuals was shown, which consequently, improved pectoral and caudal fins conditions. No effects of growth or brain monoamine levels were observed (Arechavala-Lopez et al. 2019). In the second experiment, we evidenced that deliberate addition of simple enrichment structures enhances welfare status of captive seabream juveniles, influencing positively on cognitive processes, behavioural responses such as exploratory behaviour, and brain-physiological functions. Improvements on spatial cognition and exploratory behaviour may be associated with changes in monoaminergic activity in telencephalon, and serotonergic activity in cerebellum, and are well reflected on antioxidant enzyme activities (Arechavala-Lopez et al. 2020). The third experiment exhibited differences in day vs. night patterns both on swimming activity and vertical distribution throughout the experiment mostly due to the presence of enrichment structures, enhancing the spatial use of fish in the sea-cage (Muñoz et al. 2020). The last experiment is in process, but we hypothesised that the presence of structural enrichment may help seabream broodstocks to cope with common aquaculture procedures, such as handling, netting or crowding, enhancing their physiological and behavioural stress response. It is also expected some positive effects of structural enrichment on spawning and off-spring production, due to the reduction of overall stress (Oliveira, in process).
Discussion
We experimentally demonstrated that simple structures improve the welfare conditions of farmed seabream, and therefore, can be suggested as potential tools to be applied in seabream aquaculture. Moreover, results not only showed that structural enrichment promote positive welfare of seabream, but also that are not detrimental to the growth of the fish and might be used safely by the aquaculture industry. Together with environmental enrichment, innovative technological tools (e.g. acoustic telemetry and underwater cameras), may represent an advancement to monitor fish farming procedures and conditions, helping to promote fish welfare and product quality. Nevertheless, before implementing any environmental enrichment at commercial scale, structures must be appropriated designed and validated, to demonstrate on-farm the applicability and feasibility of these kind of strategies (Arechavala-Lopez et al. 2021).
References
Arechavala-Lopez, P., Diaz-Gil, C., Saraiva, J. L., Moranta, D., Castanheira, M. F., Nuñez-Velázquez, S., ... & Grau, A. (2019). Effects of structural environmental enrichment on welfare of juvenile seabream (Sparus aurata). Aquaculture Reports, 15, 100224. https://doi.org/10.1016/j.aqrep.2019.100224
Arechavala-Lopez, P., Caballero-Froilán, J. C., Jiménez-García, M., Capó, X., Tejada, S., Saraiva, J. L., ... & Moranta, D. (2020). Enriched environments enhance cognition, exploratory behaviour and brain physiological functions of Sparus aurata. Scientific Reports, 10, 11252. https://doi.org/10.1038/s41598-020-68306-6
Arechavala-Lopez, P., Cabrera-Alvarez, M.J., Maia, C.M., Saraiva, J.L. (2021). Environmental enrichment in fish aquaculture: a review of fundamental and practical aspects. Reviews in Aquaculture (submitted).
Muñoz, L., Aspillaga, E., Palmer, M., Saraiva, J. L., & Arechavala-Lopez, P. (2020). Acoustic telemetry: a tool to monitor fish swimming behavior in sea-cage aquaculture. Frontiers in Marine Science, 7, 645. doi: 10.3389/fmars.2020.00645
Oliveira, A.R. Assessing, monitoring and improving welfare of gilthead seabream throughout intensive aquaculture processes. PhD Thesis, in process.