STUDIES ON THE REPRODUCTIVE PHYSIOLOGY OF MILKFISH (CHANOS CHANOS FORSSKAL)

Abstract

The morphological changes and associated steroid hormone profiles during gonad development of captive milkfish (Chanos chanos Forsskal) were investigated. Histologically, six ovarian and testicular stages could be distinguished. Serum estradiol (E2) and testosterone (T) levels were low and relatively constant during the long immature stage. Serum E2 and T levels gradually increased as vitellogenesis progressed, peaked in mature females, and dropped in spent fish. In males, serum T level increased during spermatogenesis, peaked in spermiating males, and dropped in spent males (to levels found in immature fish). T and 11-ketotestosterone (11-kT) appeared to be related to the degree of spermiation. Significant positive correlations were obtained between GSI and serum E2 or T levels, and between oocyte diameter and serum E2 and T. Although serum levels of E2, T, and progesterone (P) showed seasonal fluctuations in 3-. 4- and 5-year old immature milkfish, no distinct differences in patterns were seen. In broodstock that matured on its fifth year, a slight peak of T in July-September and of E2 in October-December during two consecutive years preceding puberty may be significant. Significantly higher serum T level was found in immature 4-year old fish compared to 2and 3-year old milkfish. Significant diurnal changes in serum levels of E2, T and p which varied at different seasons were observed in 4-year old immature and 6-year old refractory fish but not in 3-year old fish. In the 4- and 6-year old fish, amplitudes of diurnal T and P levels were higher in January than during the start of the breeding season in April. Semipurified salmon gonadotropin (SGA), carpgonadotropin (cGtH) and N6, 2’-o-dibutyryladenosine 3’4’-cyclinc monophosphate (dbcAMP) stimulated in vitro secretion of T but not E2 in ovarian fragments from immature 3-, 4- and 5-year old and refractory 6-year old milkfish. The T output was about 10-fold greater in response to dbcAMP than to the gonadotropins. The results suggest that in oocytes of immature milkfish. The theca has limited GtH receptors while the granulosa has none. The steroidogenic capacity of granulosa also appears to be undeveloped while that of theca is developed. Chronic administration of T and analogues of gonadotropin-releasing hormone (GnRHa) to immature or regressed fish did not significantly improve percentage of maturation. However, T + luteinizing hormone-releasing hormone analogue (LHRHa) treatment seemed to have supported vitellogenesis in maturing females. Testosterone treatment appeared to have induced pituitary GtH synthesis in immature fish. Further, LHRHa ± pimozide (PIM) stimulated GtH secretion in T-implanted fish. No significant differences in steroid levels were found between hormone-treated and control fish but significant differences were observed at different sampling times in both hormone-treated and control fish. Stress as a result of handling and timing of hormone administration were likely factors influencing the results of these experiments. The efficacy of human chorionic gonadotropin (hCG), and different GnRHa preparations, and of different methods of administration, in inducing final oocyte maturation, ovulation and spawning in milkfish were compared. A single GnRHa injection (10 ug/kg), GnRHa pellet implant (100 ug/fish) and hCG injection (1000 IU/kg) were equally effective in inducing spawning of females having oocyte diameters greater than 0.70 mm. Significantly fewer females with oocyte diameters less than 0.70 mm were induced to spawn.