Blue substrate modifies the time course of stress response in gilthead seabream Sparus aurata

• Sparus aurata reared with or without blue substrate was subjected to confinement.
• Blue substrate modified the time course of cortisol and brain monoamine responses.
• Brain serotonergic activity failed to recover in fish without substrate.
• Fish with blue substrate are in better fitness to cope with an acute stressor.
• Blue substrate as environmental enrichment enhances S. aurata welfare.

Environmental enrichment may attenuate stress response of several vertebrates. In the case of fish, although their stress response is well investigated, the effect of structural complexity as a means of environmental enrichment on stress response is not widely studied. The present study aims to investigate the possible effect of structural enrichment (blue gravel substrate) on gilthead seabream Sparus aurata primary and secondary stress responses during confinement and after recovery. Fish (initial mass 20.3 ± 0.22 g; age 0 +) were reared in tanks with blue substrate (BS) or no-substrate (NS) for 75 days. Fish (triplicated groups) were subjected to 90 min of confinement stress and sampled at 30, 60 and 90 min of confinement, as well as at 2, 6 and 24 h after stress (recovery). Triplicate groups of BS and NS fish were left undisturbed (unstressed fish). During rearing BS fish had better growth performance and less aggressive behavior than NS fish. Plasma cortisol levels and brain neurotransmitters of BS and NS fish were increased during confinement. Cortisol reduction (after stress-induced increase) occurred earlier in NS fish (60 min) than BS fish (90 min), while cortisol levels recovered in 2 h in both treatments. Serotonin metabolite 5-HIAA levels and serotonergic activity (5-HIAA/5-HT) were recovered in 6 h in BS fish, while they remained elevated for NS fish. Dopaminergic system response to confinement showed a transient increase in BS fish (60 min), while no effect was observed in NS fish. Concerning secondary stress response, glucose levels of BS fish decreased after 6 h of recovery while NS fish were not affected. On the other hand, NS fish presented a reduction of triacylglycerides (60 and 90 min) during confinement, while BS fish response was not affected. Osmolality and plasma total protein levels increased at 30 min of confinement and returned to unstressed levels in 24 h in both treatments. Present results indicate that the presence of blue substrate did modify the time course of gilthead seabream stress response to confinement. The overall performance of fish reared with blue substrate suggests that they are in better fitness than fish reared in plain tanks. The efficacy of the blue substrate as environmental enrichment enhancing gilthead seabream welfare is further reinforced and strongly encourages for additional investigations.