Establishment of hatchery-reared broodstock of spotted scat (scatophagus argus linnaeus, 1766) and its reproductive performance

Abstract

Establishment of cultured spotted scat (Scatophagus argus Linnaeus, 1766) in hatchery and its reproductive performance for mass seed production in the future were determined. Four studies were investigated: 1) effects of water salinity on the artificial insemination of spotted scat and the survival rate of the larvae, 2) effects of water salinity on reproductive performance of female spotted scat, 3) timing for oocyte recruitment and reproductive performance of female hatchery-reared spotted scat after artificial insemination and 4) the maturation and possible protandrous sex change of hatchery-reared spotted scat. The first study, effects of eight water salinity levels (0, 5, 10, 15, 20, 25, 30 and 35 ppt) on five artificial insemination of spotted scat caught in Songkhla Lake and larval rearing were investigated. The results showed that female broodfish held at 0 ppt produced the lowest ovulation rate (20%) while significantly (p<0.05) greater ovulation rates (80 - 100%) were observed at other water salinity levels. The fertilization and hatching rates were also significantly higher (p<0.05) at 25-35 ppt than at other water salinity levels (62.8 - 66.9% and 53.6 - 54.9 % respectively). A high survival rate of 15-day-old larvae was observed at 10 and 15 ppt (40.50-41.00%). The second study, effect of water salinity on the reproductive performance, serum 17ẞ- estradiol (E2) levels and osmolality of female hatchery-reared spotted scat were investigated. Female broodfish in resting stage and mature male were selected and maintained in 1-m3 tanks with holding water salinities of 5, 15, 25 and 33 ppt at density of 10 fish/tank (4 males and 6 females) for 4 months. The results showed that none of the reproductive parameters including the E2 profile and blood osmolality differed significantly between treatments except that the ovulation rate of the fish held in water salinities of 15 and 25 ppt were significantly higher than those of other salinities (p<0.05). The third study, timing for oocyte recruitment and reproductive performance of spotted scat from 5 consecutive inseminations were determined. The results showed that the timing for oocyte recruitment was 41.6 ± 6.8 days. Most reproductive performance parameters of the first and the second artificial insemination were not significantly different (p>0.05), except for oocyte diameter before hormone injection, ovulation rate and viable egg of recruited females (381.3 ± 16.0 μm, 37.5 ± 26.2 % and 54.8 ± 21.2%, respectively) that were smaller/lower than those of the former matured female broodfish (422.8 ± 12.1 μm, 72.9 ± 17.0 % and 81.3±7.2%, respectively). Histological investigation of the ovarian found hydrated oocytes, pre-vitellogenic oocytes, vitellogenic oocytes and post-vitellogenic oocytes were predominantly located in the mature ovaries of 1, 15, 30 and 45 days after stripping, respectively. The final study, maturation and the possible protandrous sex change (male in early age, and female when elder) of hatchery-reared spotted scat was undertaken. Mature male fish were selected for 2 rearing regimes in different 28 m3 tanks: in tank 1, only 50 males were reared for 18 months, and in tank 2, both males and females were reared together at a ratio of 1:2.5 (70 fish in total) for 12 months. Maturation and sex change were investigated every 3 months. The reproductive performance of two rearing regimes was not significantly different (p>0.05) except milt volume of the male fish in tank 1 seemed to have decreased gradually. There was no finding of protandrous sex change in both culture regimes. These studies indicate that spotted scat is an excellent euryhaline species. However, artificial insemination of spotted scat should be conducted at high salinities while lower water salinity levels should be applied for larval rearing following development of the larvae. Broodstock rearing can be applied wide range of water salinity which was unaffected oocyte development and egg quality. Mature female scat recruited new oocyte clutch after stripping which was indifference in most of reproductive performance compare with the former clutch. The evidence of sex change was not found in this study. Obtained data are useful for spotted scat broodstock establishment in hatchery for mass seed production planning which support commercial aquaculture in the future.