The present study aimed to compare effects of increasing chronic stress load on the stress response of European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) to identify neuroendocrine functions that regulate this response. Fish were left undisturbed (controls) or exposed to three levels of chronic stress for three weeks and then subjected to an acute stress test (ACT). Chronic stress impeded growth and decreased feed consumption in seabass, not in seabream. In seabass basal cortisol levels are high and increase with stress load; the response to a subsequent ACT decreases with increasing (earlier) load. Basal cortisol levels in seabream increase with the stress load, whereas the ACT induced a similar response in all groups. In seabass and seabream plasma α-MSH levels and brain stem serotonergic activity and turnover were similar and not affected by chronic stress. Species-specific molecular neuro-regional differences were seen. In-situ hybridisation analysis of the early immediate gene cfos in the preoptic area showed ACT-activation in seabream; in seabass the expression level was not affected by ACT and seems constitutively high. In seabream, expression levels of telencephalic crf, crfbp, gr1, and mr were downregulated; the seabass hypothalamic preoptic area showed increased expression of crf and gr1, and decreased expression of mr, and this increased the gr1/mr ratio considerably. We substantiate species-specific physiological differences to stress coping between seabream and seabass at an endocrine and neuroendocrine molecular level. Seabass appear less resilient to stress, which we conclude from high basal activities of stress-related parameters and poor, or absent, responses to ACT. This comparative study reveals important aquaculture, husbandry, and welfare implications for the rearing of these species.

Aerobic swimming exercise in fish has been shown to improve robustness of some species. However, the optimal conditions to be applied and the mechanisms underlying remain unknown. We investigated the effects of 6 h of induced swimming on the immune response of gilthead seabream (Sparus aurata), by analysing markers related to immune status in plasma, skin mucus, gills, heart and head-kidney. Forty fish were individually exercised in swim tunnels by applying different water currents: steady low (SL, 0.8 body lengths (BL) s^-1), steady high (SH, 2.3 BL s^-1), oscillating low (OL, 0.2/0.8 BL s^-1) and oscillating high (OH, 0.8/2.3 BL s^-1) velocities, including a non-exercised group with minimal water flow (MF, <0.1 BL s^-1). Swimming conditions did not trigger a stress response or anaerobic metabolism, suggested by similar levels of cortisol, lactate, and glucose in plasma among groups. Blood haemoglobin and innate immune parameters in plasma and skin mucus also remained unaltered. However, decreased blood haematocrit was observed in fish swimming on the OL condition. Interestingly, gene expression analysis revealed that the OL condition led to the up-regulation of pro-inflammatory mediators (nfκb1 and mapk3) and cytokines (tnfα, il1β and il6) in gills. A similar response occurred in heart, with an up-regulation of nfκb1, tnfα, il6 and cox2 in the OL condition. Gene expression of these cytokines was unaltered in the head-kidney. The inflammatory response in gills and heart of gilthead seabream triggered by the OL condition highlights the importance of establishing suitable rearing conditions to improve welfare of cultured fish.

Swimming has relevant physiological changes in farmed fish, although the potential link between swimming and oxidative stress remains poorly studied. We investigated the effects of different medium-term moderate swimming conditions for 6 h on the antioxidant status of gilthead seabream (Sparus aurata), analyzing the activity of enzymes related to oxidative stress in the liver and skeletal red and white muscle. Forty fish were induced to swim individually with the following conditions: steady low (SL, 0.8 body length (BL)·s^-1), steady high (SH, 2.3 BL·s^-1), oscillating low (OL, 0.2-0.8 BL·s^-1) and oscillating high (OH, 0.8-2.3 BL·s^-1) velocities, and a non-exercised group with minimal water flow (MF, < 0.1 BL·s^-1). All swimming conditions resulted in lower activities of superoxide dismutase (SOD), glutathione reductase (GR), and glutathione-S-transferase (GST) in the liver compared to the MF group, while steady swimming (SL and SH) led to higher reduced glutathione/oxidized glutathione ratio (GSH/GSSG) compared to the MF condition. Swimming also differently modulated the antioxidant enzyme activities in red and white muscles. The OH condition increased lipid peroxidation (LPO), catalase (CAT) and glutathione peroxidase (GPx) activities in the red muscle, decreasing the GSH/GSSG ratio, whereas the SL condition led to increased GSH. Oscillating swimming conditions (OL and OH) led to lower CAT activity in the white muscle, although GPx activity was increased. The GSH/GSSG ratio in white muscle was increased in all swimming conditions. Liver and skeletal muscle antioxidant status was modulated by exercise, highlighting the importance of adequate swimming conditions to minimize oxidative stress in gilthead seabream.

The swimming activity, although an essential trait in the life cycle of fish, is still poorly understood in farmed fish. The current study aimed to investigate the impact of short-term induced swimming on the immune and antioxidant defence systems in European eel (Anguilla anguilla). Sixteen male yellow European eels (total length: 39.9 ± 0.7 cm; body weight: 108.8 ± 6.1 g) were individually placed in swimming flumes and divided into two groups: i) no swimming (n = 8); and ii) induced-swimming (n = 8) at 0.3 body lengths (BL)·s^-1 for 7 h. Swimming resulted in a 2-fold lower cortisol concentration in plasma, whereas plasma glucose, lactate, and several immune-related parameters did not present variations between groups. Interestingly, swimming led to higher lysozyme, peroxidase, and protease activities in skin mucus, whereas bactericidal activity did not show differences among groups. Additionally, the gene expression of interleukin 1 beta showed an up-regulation in the skin of fish with induced swimming, while no differences were observed in the head-kidney or gills. Furthermore, modulation of the antioxidant status was observed in the liver and posterior skeletal muscle after induced swimming. Fish subjected to swimming showed lower lipid peroxidation and higher reduced glutathione levels, increasing the reduced/oxidized glutathione ratio. However, no variations in the antioxidant status were observed between groups in the anterior skeletal muscle. This study showed modulation of immune and oxidative stress markers in European eels upon short-term induced swimming compared to non-swimming fish.

Promoting swimming in aquaculture has been proposed to enhance fish welfare. This study investigated the effects of 6 h of swimming on stress, immune, metabolic, and antioxidant status of juvenile rainbow trout ( Oncorhynchus mykiss ). Fish ( n = 8) were individually subjected to one of four experimental conditions: minimal water flow (control, < 0.1 body-lengths (BL)·s⁻ ¹ ); low-speed (L, 0.8 BL·s⁻ ¹ ); high-speed (H, 2.3 BL·s⁻ ¹ ); and oscillating speeds (O, 0.8/2.3 BL·s⁻ ¹ ). Plasma cortisol levels were lower in swimming groups (L, H, O) than that in the control group, indicating reduced stress. Lysozyme and peroxidase activities increased in skin mucus of fish from L and H groups, reflecting enhanced mucosal immunity. Expression of cytokines ( tnfα , il1β ), inflammation modulators ( nfκb1 ), and immune-related ( lyz , ctsd ) genes in head-kidney, gills, and heart did not vary among groups. In red muscle, increased citrate synthase activity across swimming conditions reflected enhanced aerobic metabolism, while cytochrome c oxidase activity was higher in fish from H and O groups, indicating elevated mitochondrial oxidative capacity. Lactate dehydrogenase activity was higher in red muscle in the O group, reflecting greater reliance on anaerobic metabolism under oscillating swimming. Increased GSH/GSSG ratio in red and white muscle of L and H groups indicated improved antioxidant status. Nonetheless, higher lipid peroxidation levels in the liver suggest increased oxidative stress in the O group, indicating that oscillating swimming may impose metabolic and oxidative challenges. Results show that short-term induced steady swimming under L and H conditions could be used beforehand when applying procedures that decrease welfare in farmed rainbow trout, such as handling or transport, aiming to reduce stress, improving immune and antioxidant responses.