PHYSIOLOGICAL AND RESPIRATORY RESPONSES OF THE MOZAMBIQUE TILAPIA (OREOCHROMIS-MOSSAMBICUS) TO SALINITY ACCLIMATION

We have examined several physiological variables related to salinity a cclimation in the euryhaline tilapia, Oreochromis mossambicus. Tilapia reared in fresh water (FW) were transferred to FW, isosmotic salinity (ISO, 12 parts per thousand) and 75% seawater (SW, 25 parts per thous and). Oxygen consumption, plasma levels of cortisol, growth hormone (G H), prolactins (tPRL(177) and tPRL(188)), glucose, ions (Na+, K+, Cl-) , and gill Na+,K+-ATPase activities were measured for up to 4 days in each salinity treatment. Plasma Na+ and Cl- concentrations were elevat ed 1 day after transfer to SW, but returned to FW values on day 4. Pla sma cortisol and glucose levels were higher in FW and ISO than in SW 1 day after transfer. Plasma GH levels of tilapia in SW increased above FW and ISO values after 4 days, whereas plasma PRL levels decreased i n ISO and SW compared to FW at 4 days. These results are consistent wi th the possible osmoregulation roles of GH and PRL in SW and FW, respe ctively. Girl Na+,K+-ATPase activity of tilapia in SW increased more t han 2-fold over the FW value after 4 days, but activity of this enzyme did not change in ISO. Oxygen consumption rates of tilapia in SW were significantly elevated 4 days after transfer compared to FW and ISO. The results of this study indicate that the physiological changes asso ciated with SW acclimation in tilapia represents a significant short t erm energetic cost, and may account for as much as 20% of total body m etabolism after 4 days in SW. (C) 1997 Elsevier Science Inc.