Ionic and osmotic regulation capabilities of juvenile Gulf of Mexico sturgeon, Acipenser oxyrinchus de sotoi

The salinity tolerance, and hydromineral regulation capabilities of three size groups (small 110-170 g; medium 230-290 g, large 460-700 g; n = 48 for each group) of 13-month-old juvenile Gulf of Mexico sturgeon were investigated. Fish (n = 6 for each salinity) were transferred directly from freshwater (FW) to a series of experimental salinity treatments (0, 5, 10, 15, 20, 25, 30, and 35 parts per thousand (ppt)). Fish were also acclimated in brackish water (20 ppt) for 2 weeks and transferred to a salinity of 34 ppt. In this condition juvenile Gulf of Mexico sturgeon adapted to saltwater (SW) and maintained their :hydromineral balance. FW adapted sturgeon (n = 6) had an average blood hemotocrit of 28.2 +/- 0.8%, plasma osmolality of 260.7 +/- 1.6 mOsm kg(-1) H2O, and plasma ion concentrations of 135.7 +/- 1.2 mM l(-1) Na+, 106.9 +/- 1.9 mEq l(-1) Cl-, and 2.9 +/- 0.1 mM l(-1) K+. In SW adapted sturgeon (n = 8) blood parameters averaged 26.9 +/- 0.7% for hematocrit, 294.2 +/- 2.3 mOsm kg(-1) H2O for osmolality, 152.0 +/- 1.7 mM l(-1) Na+, 149.2 +/- 1.4 mEq l(-1) for Cl-, and 3.1 +/- 0.1 mM l(-1) K+. The method of transfer (abrupt or slow acclimation) directly affected fish survival and the time they took to achieve ionic and osmotic regulation. This SW adaptation appears to be related to body size, the larger the fish the easier the adaptation process. A threshold size of about 170 g was apparent for the fish to adapt to saltwater after 2 weeks of acclimation. Chloride cells were present in both FW and SW adapted sturgeon with SW and brackish water fish having chloride cells significantly (P < 0.05) more numerous (561 +/- 53 and 598 +/- 45 cells mm(-2)) and larger in size (41.0 +/- 3.85 and 34.2 +/- 4.49 mu m(2)) than FW adapted sturgeon (10 +/- 1.0 cells mm(-2) and 22 +/- 2.53 mu m(2)). Few chloride cells were observed in the opercular membrane, however, none were found in the pseudobranch and spiracle. (C) 1998 Elsevier Science Inc. All rights reserved.