Abstract
To provide practical feeding management guidelines preceding a stressful episode during farming practices, European sea bass juveniles (initial weight: 72.3 g) were fed for 60-days different fish meal (FM) and fish oil (FO) dietary levels [high (30% FM, 15% FO, FM30/FO15), intermediate (20% FM, 7% FO, FM20/FO7), and low (10% FM, 3% FO, FM10/FO3)] in triplicate conditions. Fish were then fasted for 36 h and exposed to a 2-h acute crowding (80 kg m–3 biomass). Plasma biochemistry, skin mucus parameters and gene expression of stress and immune-related genes were performed before, at 2 and 24 h after crowding. At the end of the trial, the FM10/FO3 group showed lower final body weight, weight gain, and specific growth rate compared to the other treatments. Most of the plasma parameters were mainly affected by crowding condition rather than diet; however, after stress, lactate was higher in the FM30/FO15 group compared to the other treatments. Similarly, protease, antiprotease, peroxidase and lysozyme in skin mucus were mostly affected by crowding conditions, while fish fed FM10/FO3 displayed higher skin mucosal IgM and bactericidal activity against Vibrio anguillarum and V. harveyi. Most of the stress-related genes considered (hsp70 and gr-1 in the brain; hsp70, gr-1 and gr-2 in the head kidney), showed an overall expression pattern that increased over time after stress, in addition, hsp70 in the head kidney was also up-regulated in fish fed FM30/FO15 after stress. Higher plasmatic lactate together with the up-regulation of some stress-related transcripts suggest a higher reactivity to acute crowding of the stress-response mechanism in fish fed high FM and FO dietary levels. Otherwise, the higher skin mucosal IgM and bactericidal activity observed in fish fed FM10/FO3 dietary levels seems to indicate that acute crowding was able to activate a higher pro-inflammatory response in this treatment. Overall, the results of the present study seem to indicate that 10% FM and 3% FO dietary levels might affect stress and immune responses.
Highlights
Nowadays, fish farmers are urged to apply feeding management practices that implement feed formulations which address both optimal growth and fish welfare issues, especially for coping with exposures to various types of stress (Boyd et al, 2020; Reverter et al, 2021)
No data are reported on esterase activity in fish skin mucus in response to acute crowding; our results indicated a positive response to stress of this mucus component in all dietary groups
10% FM, 3% fish oil (FO) dietary inclusion levels showed a moderate reduction in growth performance, an effect that seemed to be related to the higher Feed conversion ratio (FCR) reported in fish fed this diet
Summary
Under current aquaculture intensive rearing conditions, fish are continuously exposed to many stressors both biotic and abiotic; among them, husbandryrelated stressors such as handling, crowding, transportation, or confinement are the most important (Tort and Teles, 2012) When experienced, these stressors are well-known to cause a wide range of physiological alterations, worsening overall fish performance (Barton et al, 2005; Bagni et al, 2007; Castro et al, 2018; Balasch and Tort, 2019). In order to find practical feeding management guidelines for fish farmers, the aim of this study was to evaluate the effect of different FM and FO dietary levels on growth performance, feed efficiency and, stress and immune-related biomarkers in European sea bass exposed to acute crowding. Fish responses to acute confinement were evaluated by analysis of different humoral immune biomarkers from skin mucus, plasma biochemistry, and gene expression of stress and immune-related genes in order to provide insight into the overall effects of experimental diets on fish welfare
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