Standardization of oocyte size during artificial fertilization and optimization of stocking density during indoor larval and outdoor nursery rearing of captive spotted scat (Scatophagus argus) for a viable juvenile production system

Abstract

The present study describes the viable and efficient hatchery seed production technique to maximize spotted scat (Scatophagus argus) juvenile production in captivity during the breeding season. Broodstock fish maintained in the earthen pond of hatchery were selected according to varying mean oocyte diameter (MOD) ranging from 351 to 400 (T1), 401–450 (T2), 451–500 (T3), and > 500 μm (T4) according to natural vitellogenesis progression. Breeding pairs were formed as two males: one female. Females (n = 24) were induced with a double dose of hCG (2000 IU kg−1 body weight day−1) and a single dose of LHRHa (400 μg kg−1 body weight) each at 24 h interval, whereas males (n = 48) received a single dose of LHRHa (200 μg kg−1 body weight). Females in T4 were spawned through dry stripping earliest at 30 h post hormone induction, whereas females of the T1 group spawned last at 33 h. The largest (P < 0.005) diameter (718.33 ± 3.28 μm) of fertilized eggs was observed in T4, whereas the smallest (652.33 ± 18.68 μm) fertilized eggs were observed in T1 post-fertilization. Significantly highest absolute and relative fecundity was found in T5 (1060.67 ± 45.36 and 213,239.7 ± 17,324.9, respectively), whereas T1 and T2 exhibited no significant difference. The dry stripped eggs were fertilized with milt and incubated for 22 h for hatching. Females in T4 achieved the greatest fertilization (98.67 ± 0.88) and hatching (97 ± 2.08) percentages significantly. The initial oocyte size range of 351–400 μm in T1 led to the lowest fertilization (17 ± 3.46) and hatching (10.67 ± 3.92) percentage. Subsequently, scat juvenile production techniques were standardized by conducting two experiments on stocking densities of larvae and fry. Hatchlings were first reared in the mesocosm larval rearing system in an indoor hatchery with four different stocking density of 2 (SD2), 4 (SD4), 6 (SD6), 8 (SD8) larvae L−1 for 30 days. Stocking density of 4 and 6 larvae L−1 achieved the highest survival of 53.05 ± 1.01% and 51.03 ± 1.23%, respectively. On completion of metamorphosis (30 dph), fry were shifted to hapa based rearing in a pond with three different stocking density of 2000 (SD2000), 3000 (SD3000), 5000 (SD5000) fry m−3 for 30 days. Highest survival percentage of 78.94 ± 2.78 found in SD3000. Present study standardized the operation protocol for a commercial hatchery aiming for juvenile scat production by selecting females with initial oocyte size >500 μm diameter for artificial fertilization. Seed production can be optimized by adopting a stocking density of 4–6 larvae L−1 in the indoor larval rearing and 3000 fry m−3 in outdoor hapa based nursery systems.