SODIUM AND CALCIUM BALANCE IN MOZAMBIQUE TILAPIA, OREOCHROMIS-MOSSAMBICUS, RAISED AT DIFFERENT SALINITIES

Mozambique tilapia, Oreochromis mossambicus, born and raised in five s alinities, viz. (relatively soft) fresh water, 25, 50, 75% and full-st rength sea-water, were analyzed for ionoregulatory performance (in par ticular sodium and calcium handling) and growth. This tilapia regulate s its blood serum mineral composition rather effectively; however, in sea-water serum concentrations oi sodium, chloride and calcium (in mal es only) were increased, as was the serum osmolarity. In sea-water, th e total body sodium pool was significantly enlarged. With increasing s alinity, sodium turnover increased. Serum calcium levels and the total body calcium pool were more strictly controlled than those of sodium. The lowest density of chloride cells in opercular epithelium and the lowest branchial Na+-K+-ATPase activity were observed in 50% sea-water ; these values were higher in fish kept in waters of lower or higher s alinities. Fish grew more rapidly in brackish water. Fish kept in brac kish water appeared to depend on food-related calcium for growth as br anchial calcium uptake provides no more than 20% of growth related Ca accumulation. (C) 1998 Elsevier Science Inc.