Tertiary stress responses in Senegalese sole (Solea senegalensis Kaup, 1858) to osmotic challenge: Implications for osmoregulation, energy metabolism and growth

The effects of three different environmental salinities (15, 25 and 39‰) on growth, osmoregulation and energy metabolism of juvenile Senegalese sole (Solea senegalensis Kaup, 1858) after a period of 10 weeks were investigated. Immature sole (n = 150, 39 ± 1 g mean initial body weight) were randomly divided in 6 groups of 25 fish and reared under three different environmental salinities in an open system. Growth, weight gain, specific growth rate and estimated feed intake decreased in salinities lower than 39‰, with the most profound effects observed at 15‰. Branchial Na+,K+-ATPase activity correlated positively with environmental salinity, while renal Na+,K+-ATPase activity was not altered. Plasma electrolyte concentrations did not change in the salinity range tested, indicating that osmoregulatory capacities were unaffected. Plasma cortisol levels were higher in salinities different than 39‰ (15 > 25 > 39‰). Plasma glucose, non-esterified fatty acid, triglyceride, and protein levels were decreased in the lower salinities (25 and 15‰), whereas that of lactate was increased two-fold in the 15‰-exposed group only. Measured in tissues (liver, gill, kidney, white muscle), the most profound changes in metabolite levels were generally found in the group exposed to 15‰, compared to the 25‰- and 39‰-groups. A similar pattern was found for the activities of enzymes involved in energy metabolism. Taken together, our data suggest that the poor growth rate of 15‰-exposed fish is caused by decreased feed intake and the subsequent reallocation of energy sources that was non-sufficient to keep similar growth in fish exposed to 15‰ salinity compared with fish exposed to 25 and 39‰ salinity.