Effect of administering sustained-release thyroxine microparticles on reproductive performance and egg quality in tilapia (Oreochromis mossambicus) broodstock

Biodegradable sustained-release microparticles containing thyroxine (T-4) w ere developed to study the influence of hormone manipulation on broodstock reproductive performance and the quality of their eggs. Female tilapia (Ore ochromis mossambicus) were injected with either microparticles containing T -4 (2.5 mu g g-l body weight) or an equivalent amount of blank microparticl es. The administration of sustained-release T-4 microparticles generated a large increase in serum T-4 levels soon after injection (day 2); this was f ollowed by a gradual decline in serum T-4, but the level remained significa ntly higher than in the controls during the 20-day study period. Significan tly higher T-4 levels in eggs and newly hatched larvae were also observed i n the groups from females injected with sustained-release T-4 microparticle s compared with those injected with blank microparticles. However, no signi ficant differences were observed in the spawning intervals, egg number per spawn, egg diameter, or percentage fertilizability and hatchability. Morpho metric examination of the newly-hatched larvae revealed that in the group o f females injected with T-4 microparticles, the mean total length and the s tandard length were significantly increased, whereas no clear effect was ob served in other parameters such as yolk area and eye diameter. These data i ndicate that manipulation of the T-4 hormone level in broodstock through su stained release did not affect the reproductive performance significantly, but resulted in an improvement in the quality of their eggs as shown by the significantly larger larvae at hatch.