Relationships among ration, salinity, 17α-methyltestosterone and growth in the euryhaline tilapia, Oreochromis mossambicus

The effects of long-term 17α-methyltestosterone (MT) treatment (10 mg MT kg −1 feed) and ration size on the growth of the euryhaline tilapia, Oreochromis mossambicus , in fresh water (FW) and seawater (SW) were examined. Tilapia were fed a limited daily ration, starting initially at 18% of body weight for yolk-sac fry and decreasing gradually to 4% at 126 days. Eight treatments were employed. Control and MT treatment groups were replicated in FW and SW using either fish fed the limited daily ration described above or fish fed twice that ration. On limited ration, control fish in SW grew to twice the size of their FW counterparts. Treatment with MT increased growth in both FW and in SW tilapia. Doubling the feeding ration produced a significant increase in growth in SW, but not in FW fish. The greatest growth was observed in SW, MT-treated fish on double ration which grew to >4.2 × the size of FW control fish on a limited ration. Routine metabolic rate was measured in tilapia reared from the yolk-sac fry stage in FW or SW. The rate 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 fish ( n = 6). These data suggest that the reduction in routine metabolism in O. mossambicus in SW may account for the increase in growth of the SW animals over their FW counterparts.