EFFECTS OF SALINITY ON THE IONIC BALANCE AND GROWTH OF JUVENILE TURBOT

The effects of salinity changes (27, 19 and 10 parts per thousand) on seawater-adapted juvenile turbot were studied on their plasma osmolarity and ion concentrations, on oxygen consumption, on gill Na+,K+-ATPase activity after 3 months and on growth parameters. All plasma concentrations (except chloride) were unchanged, suggesting that fish were well adapted to their environment. Oxygen consumption was significantly decreased in the 19 and 10 parts per thousand groups, where fish weighed significantly more 105 days after transfer than fish maintained in sea water. These results, and the Fact that apparent food conversion rates were lower in a diluted environment, suggest that on a long term schedule growth conditions could be improved by adaptation to brackish waters (salinities between 10 and 19 parts per thousand). The effects of transfer from sea water to 27, 19, 10 and 5 parts per thousand were also followed during the first 3 weeks. With salinity 10 parts per thousand a steady state was reached on day 21 with all plasma values within the same range. The significant differences observed in osmolarity, plasma ion concentrations and Na+,K+-ATPase activity 3 weeks after transfer of juveniles to 5 parts per thousand salinity, compared with transfers inhigher salinities, suggest that there is a threshold of acclimation of turbot to a hypotonic environment. (C) 1995 The Fisheries Society of the British Isles