RELATIONSHIPS AMONG RATION, SALINITY, 17-ALPHA-METHYLTESTOSTERONE ANDGROWTH IN THE EURYHALINE TILAPIA, OREOCHROMIS-MOSSAMBICUS

The effects of long-term 17 alpha-methyltestosterone (MT) treatment (1 0 mg MT kg(-1) feed) and ration size on the growth of the euryhaline t ilapia, Oreochromis mossambicus, in fresh water (FW) and seawater (SW) were examined. Tilapia were fed a limited daily ration, starting init ially at 18% of body weight for yolk-sac fry and decreasing gradually to 4% at 126 days. Eight treatments were employed, Control and MT trea tment groups were replicated in FW and SW using either fish fed the li mited daily ration described above or fish fed twice that ration, On l imited ration, control fish in SW grew to twice the size of their FW c ounterparts. Treatment with MT increased growth in both FW and in SW t ilapia. Doubling the feeding ration produced a significant increase in growth in SW, but not in EW fish. The greatest growth was observed in SW, MT-treated fish on double ration which grew to > 4.2 X the size o f EW control fish on a limited ration. Routine metabolic rate was meas ured in tilapia reared from the yolk-sac fry stage in FW or SW. The ra te of oxygen consumption in the SW tilapia was 72.4 +/- 11.3 mg kg(-1) h(-1) (mean +/- SE, n = 6) and 148.2 +/- 9.2 mg kg(-1) h(-1) in FW fi sh (n = 6). These data suggest that the reduction in routine metabolis m in O. mossambicus in SW may account for the increase in growth of th e SW animals over their FW counterparts.