Stress response and skin mucus production of greater amberjack (Seriola dumerili) under different rearing conditions

The main objective of this study was to evaluate the stress response of the greater amberjack, Seriola dumerili, by determining plasma and skin mucus cortisol levels, as physiological indicator of fish stress, and skin mucin-2 gene expression as indicator of skin mucin production, under three different potential stressors related to aquaculture practices: temperature, handling and fasting. For trial 1, 45 greater amberjack juveniles (19.5 ± 4.1 g) were divided in three graded rearing temperatures (17, 22 and 26 °C) and maintained for 120 days, in order to define the basal cortisol level for this species after a long-term thermal acclimation within the range described for this species. Trial 2 was conducted under two different rearing densities and two different handling protocols with 222 fish (425.02 ± 36.9 g) divided in 4 treatments by triplicate for 90 days, in order to study the effect of routine handling protocols on the selected stress markers for this species. Trial 3 used 72 fish (302.27 ± 21.4 g) divided in two experimental groups, one fed to apparent satiety and another one subjected to fasting for 10 days (triplicates for each group). After trials 2 and 3, a shallow water challenge was conducted to further study the adaptive response of greater amberjack juveniles to combined stress situations. Plasma and skin mucus were obtained for cortisol determination at 1, 24 and 72 h (trial 2) and 1, 3 and 24 h (trial 3) post stress challenge. Additionally, skin was dissected out for determining muc-2 gene expression. Long-term acclimation to temperature did not affect plasma or mucus cortisol levels and muc-2 gene expression, being basal levels of circulating plasma cortisol around 7.5 ng/mL. A high stocking density (8 kg m⁻³) induced an elevation (p <.05) of plasma cortisol and a reduction (p <.05) of weight gain. No differences were observed for the expression of muc-2 gene, however, a tendency to an up-regulation was observed in fish subjected to the high-density rearing treatment. Short-term fasting induced an increase (p <.05) of plasma and skin mucus cortisol together with a down-regulation of muc-2 gene expression.