Maturity induction and extended spawning kinetics of milkfish (Chanos chanos) administered with combined GnRHa and 17α-methyl testosterone pellet at varied frequencies

Abstract

Inclusion of sustained-release androgen-gonadotrophin releasing hormone analogue implants into fertility treatment regimes of male and female milkfish has been proposed to benefit artificial maturation outcomes. This study aimed to investigate scope of co-administration of the androgen, 17α –methyl testosterone (17 α -MT) in combination with GnRHa as a single hormone pellet at varied frequencies in male and female milkfish to affect maturation, egg quality, spawning and subsequent larval fitness. Cholesterol pellet combined with GnRHa and 17 α–MT in equal amount was found superior to individual treatment in breeding responses (Experiment-1, 2015). In second experiment during 2016–2019 we evaluated effectiveness of chronic (10 implantations/year- December, January–September) and assisted (04 implantation/year- Dec, Jan, April, July) implantation of combined hormone pellet to compare breeding responses during trimester of implantation/breeding period divided as early (January–March), maximum (April–June) and extended (July–September) phase. Assisted hormone implantation helped to achieve desired percentage of unimodal distribution of mature oocytes (650–750 μm) reaching final oocyte maturation (FOM) stage in milkfish along with higher spawning frequency. On the contrary, chronic implantation caused an increased percentage of vitellogenic oocytes with bimodal distribution (250–500 μm and 500–600 μm) in ovary with reduced spawning events. Application of combined hormone implants, both assisted and chronic facilitated sustained release of both gonadotropin releasing hor- mone analogue (GnRHa) and androgen (17 α – MT) together and thereby induced entire reproductive axis resulting in extended spawning frequency for seven months (February–September). We also observed that milkfish breeding was aligned with abiotic factors such as higher water temperature and salinity along with synchrony to full moon or new moon. We conclude that assisted type of implantation pattern significantly improved maturation, milting male %, average fecundity, egg diameter, fertilization rate, hatching rate and larval length compared to that of chronic implantation with added benefit of reduced handling stress. This study represents the first report of milkfish captive maturation and hatchery seed production in tropical climate using combined hormone pellet administration