Environmental enrichment reduces the effects of husbandry stressors in gilthead seabream broodstock

Husbandry procedures can be stressful for captive animals. Knowledge of the physiological effects of these procedures and the reduction of stress during regular maintenance is of pivotal importance to ensure good welfare. Environmental enrichment (EE) can be an asset to animal keepers as it has many benefits on captive animals, including the reduction of stress indicators in many aquatic species. This is particularly relevant for broodstock animals, who may spend years in captivity and who are essential for successful fish farming. We studied whether structural enrichment had a stress-reducing effect on broodstock of gilthead seabream (Sparus aurata) during standard husbandry procedures, monitoring their heart rate with surgically implanted internal bio-loggers (DST milli HRT, Star-Oddi) in 18 fish. These fish were distributed in six tanks, three of which had an EE structure consisting of a 1 m2 floating structure with nine suspended organic cables, while the other three tanks had no enrichment. We added seven more unmarked fish to each tank to simulate broodstock farming conditions (i.e., ten fish per tank). After five days of post-surgical recovery, we tested feeding, netting, and cleaning every day for three consecutive days, and a formaldehyde bath as a prophylactic procedure on the fourth day on the logger-implanted fish and continued to record their recovery for eight more days. Overall, when subjected to stressful husbandry procedures, fish reared under EE show reduced heart rate and amplitude, and faster recovery to baseline levels than non-enriched animals. Our results show that EE can be used to improve the welfare of farmed fish by reducing their stress and should be employed as a good management practice in finfish production, and is especially relevant for high-value fish that spend long periods in captivity, such as spawners.