Gill Na+, K+-ATPase activity, plasma chloride and osmolality in juvenile turbot (Scophthalmus maximus) reared at different temperatures and salinities

The interactive effects of temperature and salinity on gill Na+, K+-ATPase activity, plasma chloride and osmolality of juvenile turbot were investigated by rearing fish at (mean +/- S.D.) 10 +/- 0.2, 14 +/- 0.2, 18 +/- 0.3 and 22 +/- 0.2 degreesC and salinity of 15 +/- 0.4 parts per thousand, 25 +/- 0.4 parts per thousand and 33.5 + 0.1 parts per thousand for 3 months. Gill Na+, K+ -ATPase activity, plasma chloride and osmolality were lowest at 15 parts per thousand, and highest at 33.5 parts per thousand. Highest gill enzyme activity was at 22 degreesC, and overall, there was a positive correlation between temperature and Na (+), K (+) -ATPase activity. Overall, we found the optimal temperature - salinity combinations corresponding to minimum Na+, K+ -ATPase activity (T&S-min (ATPase)) plasma chloride (T&S-min CL) and plasma osmolality (T&S-min OSM) to be 17.6 +/- 2.2 degreesC-15.8 +/- 4.1 parts per thousand, 15.1 +/- 0.5 degreesC - 18.9 +/- 2.0 parts per thousand and 15.6 +/- 3.0 degreesC - 13.6 +/- 3.2 parts per thousand, respectively. Although slightly lower, these values correspond to published optimal temperature-salinity combination for growth of juvenile turbot suggesting some energy-saving effect on osmoregulation in groups reared at low (15 parts per thousand) and moderate (25 parts per thousand) salinities. We hypothesize that the reduced gill Na (+), K+ -ATPase found at intermediate salinities will lead to reduced energy expenditures thereby contributing to higher growth rates seen in previous studies. Our findings may have implications for turbot farming as production may be expanded into new areas, which have previously been regarded as unsuitable for culture of turbot due to low salinity. (C) 2003 Elsevier Science B.V. All rights reserved.