Dose optimization, Gang magur, Induced breeding, Ovatide
Field Laboratory Complex, Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh
Site selection and duration of the study: This experiment was carried out at Field Laboratory Complex, Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh from March 2017 to July 2018. The aim of the experiment was to evaluate the effects of different dosages of Ovatide hormone for induced breeding of H. menoda. The brood fish were collected from Jaria-Jhanjail point, Kangshariver, under Netrokona district, Bangladesh. Brood fish collection and rearing: A total of 54 live H. menoda were collected during March 2017 and February 2018 from Kangsha river located at Jaria-Jhanjail, Netrokona district, Bangladesh. During the rearing period, the fish were fed with SIS (small indigenous species of fish), and CPC feed, a special feed enriched with protein and vitamin-E was fed at a rate of 4-5% body weight which enhances the gonadal maturation in fishes. Regular manuring, fertilization, and liming were performed whenever necessary. Brood selection: To carry out the induced breeding trial, male and female broods were caught from the brood-rearing ponds using a cast net. The average weight of the male and female were 371±50 g and 778±80 g, respectively. The gravid male and female were selected based on the following criteria. Males with a relatively smaller body, elongated and slender in shape having swollen and reddish urogenital papillae were selected. Contrariwise, the female was identified by having a rounded and protruding abdomen which is soft when touched with fingers and the swollen genital opening sometimes reddish in color. Conditioning: Selected broods were transferred to the cemented circular tank in the hatchery with continuous water showering for about five hours prior to administering hormones. Male and female were kept off-feed during conditioning. Collection and preparation of Ovatide: Ovatide hormones were used as inducing agents. The proper doses of Ovatide hormone were calculated based on the recommended dose and bodyweight of the brood fish using this formula: Amount of ovatide (mL) = Wt. × Pt./ 1000; Where Wt. = the total body weight of the fishes injected (g); Pt. = the rate in mL ovatide injected·kg -1 body weight under a particular treatment. For Ovatide preparation, the powder from synthetic hormone was diluted with distilled water to dissolve it in the contained vial and shaken for a few minutes. The prepared solution was then taken out in a 5 ml disposable syringe for injection. Hormone injection: The hormone solution was administered intramuscularly to the female and male between the dorsal fin and lateral line maintaining about 45° angular position with the body. The injected fish were then released into breeding hapa placed in tanks for synchronized spawning. Observation of reproductive behavior: After injection, both male and female broods were kept together in the spawning tank (temperature 25.5 °C; pH 7.2; dissolved oxygen 6.5 ppm) and their reproductive behavior was closely monitored. Experimental design: This experiment was conducted using commercial Ovatide hormone consisting of three treatments T1, T2, and T3 with three replications each in a Completely Randomized Design (CRD). A total of 54 broodfish were taken and kept at a ratio of 2 male: 1 female in the tanks. Semi artificial (or induced natural) propagation method was adopted which involved synchronized spawning in breeding hapas whereby the injected brood fish (male and female spawners) were placed into a breeding hapa fixed in circular and rectangular tanks (dimensions: 1.22×2.74×0.37 m). Double hapa (upper and lower) were used with the mesh size of the upper hapa (dimensions: 1.35×1.12×0.36 m) being larger than the lower hapa (1.24×1.02×0.36 m). After ovulation of 21-22 h and fertilization, the upper hapa was then removed along with the spent spawners while the fertilized eggs settled in the lower hapa which served as a shelter during the incubation period. Larvae were also hatched in the hapa. Statistical analyses: For statistical analysis of data, one-way analysis of variance (ANOVA) followed by Duncan’s Multiple Range Test (DMRT) at a significance level of P<0.05 was employed. The statistical data analysis was carried out with the aid of the computer software SPSS version 20 and necessary graphs were drawn by MS Excel 2013.