RILEY, L. G.*; HIRANO, T.; RICHMAN III, N. H.; GRAU, E. G.: Growth Regulation in the Tilapia: Effect of Salinity and 17&alpha-Methyltestosterone (MT) Treatment

Effects of salinity and MT treatment on growth and the growth hormone / insulin-like growth factor-I axis (GH/IGF-I) in the euryhaline tilapia, Oreochromis mossambicus were examined. Animals were raised in either fresh water (FW) or seawater (SW) and treated with or without MT in the feed for 147 days. We observed no difference in plasma GH levels among the groups, despite the fact that animals treated with MT were significantly larger than their respective controls. On the other hand, the FW control fish, which were the smallest, exhibited the lowest GH mRNA levels. Treatment of FW fish with MT and rearing tilapia in SW brought about a significant increase in growth and a concomitant increase in pituitary GH mRNA levels. The combination of SW rearing and MT treatment produced the greatest growth; the effects were at least additive and possibly synergistic. This group also showed the highest levels of pituitary GH mRNA. Furthermore, GH mRNA was positively correlated across treatment groups with fish size, indicating that pituitary GH mRNA levels are more closely associated with growth than plasma GH. We have shown previously that prolactin₁₇₇ (PRL₁₇₇), but not PRL₁₈₈, exhibits growth-promoting actions in FW tilapia. Plasma levels of both PRLs were significantly higher in FW fish than SW fish. Pituitary mRNA levels of both PRLs exhibited similar trends, with significantly higher levels in FW compared to SW fish. MT treatment significantly increased mRNA levels of both PRLs in FW, but had no effect in SW animals. These data clearly indicate that SW rearing and MT-treatment stimulate the GH / IGF-I axis.