Yahia Mahmud

Effects of stocking density on growth, body composition, yield and economic returns of monosex tilapia (Oreochromis niloticus L.) under cage culture system in Kaptai Lake of Bangladesh.

A 120-day research was conducted to evaluate the effects of different stocking densities on growth, body composition, survival, yield and economic returns of monosex male Nile tilapia, Oreochromis niloticus in net cages in Kaptai Lake of Bangladesh. Juvenile monosex tilapia with an average weight of 15.20 ± 0.15 g (mean ± SD) were randomly stocked in 12 floating net cages (3 m × 3 m × 2 m) at densities of 50 fish/m3 (T50), 75 fish/m3 (T75), 100 fish/m3 (T100) and 125 fish/m3 (T125) in triplicate groups. Fish were fed with a commercial pelleted floating feed (29% protein) at 3-5% of body weight, twice daily in all the treatments. The physico-chemical parameters of lake water were within suitable ranges for fish cultured in cages. After 120 days of trial, growth in terms of body final length, final weight, weight gain, percent weight gain, daily weight gain and specific growth rate of fish from T50 were significantly higher than those of fish from T75, T100 and T125. Feed conversion ratio was significantly lower in T50 followed by T75, T100 and T125 consecutively. Survival rate was not significantly different in T50, T75 and T100 while lowest survival was found in T125. Significantly lower amount of body lipid and carbohydrate contents were found in T125 than those of fish from T50, T75 and T100. Gross and net production levels from T100 were significantly higher than those from T50, T75 and T125. However, the benefit cost ratio from T50 was better than those from T75, T100 and T125. The results demonstrated that on the basis of growth and economic return 50 fish/cubic metre was the best stocking density for monosex tilapia culture in cages which might be technically feasible and economically viable.

Impact Assessment of Twenty-Two Days Fishing Ban in the Major Spawning Grounds of Tenualosa ilisha (Hamilton, 1822) on its Spawning Success in Bangladesh

This study was conducted to evaluate the impact of twenty-two days fishing ban from October 22-November 02, 2016 on the spawning success of Hilsa in the major spawning grounds of Hilsa. Fishing ban during the spawning season have substantial role in the successful reproduction of Hilsa found from the present study. Among all the Hilsa captured in and around the spawning grounds of Hilsa, male and female were estimated to make up 34% and 64% respectively of the total catch, suggesting a male to female sex ratio of about 1:1.94. In the year 2016 during the major spawning period in the major spawning grounds 43.93% spent Hilsa was found and calculated egg production was 628291 kg and the estimated Jatka production was 39,268 crore which were found to be much higher than the previous years. Increased production of Hilsa eggs and Jatka indicates a positive impact of twenty-two days fishing ban in the spawning season. Percentages of gravid and oozing Hilsa were also found higher compared to previous fishing ban periods. In the spawning grounds, relatively higher amount of spent Hilsa and Jatka were observed whereas fewer spent Hilsa and Jatka were found in the adjacent areas of spawning grounds. Besides Jatka, spawn and fries of other fish species were also found in higher quantity in and around the spawning areas of Hilsa than previous years which indicates fishing ban might have positive impacts on successful breeding and maintenance of biodiversity of other fish species. Overall, twenty-two days fishing ban was found to be very effective for successful spawning of Hilsa.

On-board Breeding Trial of Hilsa (Tenualosa ilisha, Ham. 1822) and Testing ofLarval Rearing in Bangladesh

The Hilsa shad, Tenualosa ilisha commonly known as Hilsa is one of the most commercially important fish species in South Asian countries. For the conservation of Hilsa it is necessary to establish a standard breeding and culture protocol along with the present Hilsa management activities. The on board breeding trial was conducted during 10 October 2016 to 02 November 2016 which was the peak breeding time of Hilsa for the year 2016. The male and female Hilsa broods were collected from the River Meghna using BFRI experimental net during afternoon and late evening of full moon and new moon time. A total of six breeding trials were conducted in which 13 pairs of selected Hilsa broods were used for the breeding trials. For the breeding trial, both eggs and milt were collected through stripping and then the eggs were mixed with the milt immediately. The fertilized eggs were transferred to a plastic hatching jar for incubation providing mild water circulation, aeration and shade to protect penetration of direct sunlight for controlling temperature i.e., to maintain congenial environment. During the incubation period, the eggs were observed for 24 hrs to study the embryonic and larval development stages of Hilsa. After 4.2-4.5 hrs of fertilization morula stage of embryonic development was identified and after 8-8.5 hrs. of fertilization 18-myotome stage of embryonic development was identified from the sixth breeding trial. After that, no embryonic development of egg was observed up to 12 hrs after fertilization and the fertilized eggs were found to be dead filled with fungus at the end. The water quality parameters were found in good range during the breeding trial though the temperature was found to be fluctuated from the optimum ranges needed for the incubation of fertilized eggs. Although it was not possible to be succeeded completely in artificial breeding of Hilsa, the experience of on board breeding trial in the major breeding ground of River Meghna will give the necessary insight for future works.

Gill Net Selectivity of Hilsa (Tenualosa ilisha) in the Meghna River Estuary ofBangladesh

In this study gill-net selectivity for Tenualosa ilisha in the River Meghna was estimated. Traditional fishing was conducted by using gill-net with 55 mm, 65 mm and 75 mm mesh-size during August 2016 to October 2016. From the Meghna River estuary, a total of 660 specimens of T. ilisha were caught by gill-net during the study period. Mean total lengths were calculated as 24.7 ± 2.7 cm, 31.2 ± 2.78 cm and 34.33 ± 4.69 cm for 55 mm, 65 mm and 75 mm mesh sizes gill-net respectively. Selectivity analysis indicated an optimum catch length of 260.50 mm for the 55 mm mesh size, 328.36 mm for the 65 mm mesh size and 370.99 mm for the 75 mm mesh size gill-nets. The catch percentage of Hilsa by 55 mm, 65 mm and 75 mm mesh size gill-net were 38.78%, 39.10% and 22.12% respectively. The majority of fishes caught by these three-mesh size gill-net were found to be matured. Therefore, 65 mm mesh size were suitable for the sustainable fisheries of T. ilisha in the River Meghna in Bangladesh.