Abstract
The research was carried out to assess the ecological and biological factors of hilsa shad riverine habitats, food and feeding habits, and reproductive biology, estimation of population parameters and stock assessment with management recommendation. The study was done in four sites (Site 1: Meghna river at Chandpur; Site 2: Meghana river at Daulatkhan, Bhola; Site 3: Tetulia river near Lalmohon; and Site 4: Andhermanik river, Kalapara, Patuakhali District) from January to December 2014. Water temperature, DO, pH, alkalinity, chlorophyll-a, water nutrients (NO3-N, NO2-N, PO4-P) were found within suitable or acceptable ranges for fishes in all the sites, except NH3-N. Conductivity, TDS, alkalinity, pH, NH3-N and chlorophyll-a parameters were found significantly different between Andhermanik river and the other three sites. Natural food availability in terms of quantity and quality (genus composition) was higher in Meghna-Tetulia river basin in comparison to Andhermanik river and it varied significantly. A total of 76 genera of plankton, of which, 58 genera of phytoplankton and 18 genera of zooplankton, were found in the Mehna river at Chandpur Site, whereas 61 genera (44 of phytoplankton and 17 of zooplankton), 60 genera (43 of phytoplankton and 17 of zooplankton), and 31 genera (24 of phytoplankton and 7 of zooplankton) were observed at Sites 2, 3 & 4, respectively. The mean plankton abundance were 36,996 cells 1-1, 35,929 cells 1-1, 32,556 cells 1-1, 4,020 cells 1-1, where, phytoplankton abundance were 34,795 cells 1-1, 34,142 cells 1-1, 30,612 cells 1-1, 3,225 cells 1-1, and zooplankton were 2,201 cells 1-1, 1,787 cells 1-1, 1,943 cells 1-1, 795 cells 1-1 in Site 1, Site 2, Site 3, Site 4 respectively. Fifty six genera of phytoplankton and seventeen genera of zooplankton were observed in hilsa guts. Hilsa fed mainly on phytoplankton (98.08%), with a small quantity of zooplankton (1.92%). Chlorophyceae appeared highest (58.07%) among the phytoplankton, followed by Bacillariophyceae (38.60%), Cyanophyceae (1.25%), Euglenophyceae (0.1%), Xanthophyceae (0.03%) and Dinophyceae (0.03%). Among the zooplankton, Cladocera (0.77%) dominated in the gut contents, followed by Rotifera (0.56%), Copepoda (0.52%) and Protozoa (0.06%). The food choice of hilsa was analysed using the 'Ivlev's electivity index' that showed hilsa preferred phytoplankton to zooplankton. Bacillariophyceae (diatom) was the most preferable food organism for hilsa especially at the early stages of life cycle. It also showed that the neutral selections for Chlorophyceae and feeding preference moved towards Chlorophyceae when hilsa reached adult stages. The fish responded positively to Bacillariophyceae, Chlorophyceae, while negatively to Cyanophyceae, Euglenophyceae and other groups. Hilsa showed positive selection for Amphora, Coscinodisscus, Cyclotella, Nitzchia, Tabellaria, Melosira, Tetraedon, Ulothrix, Microspora and Bosmina, while a negative or neutral selection for the rest of the plankton. The relationship of pooled data of total length and body weight for male showed isometric growth and that were BW-0.0106TL2990 (R2 =0.9749), whereas, the relationship for female and unsexed groups showed positive allometry and that were BW=0.0076TL3.108 (R2=0.9788) for female; BW=0.0088TL3.049 (R2=0.9743) for unsexed fish population. The sex ratio of male and female was 1:0.53 which differed significantly (x2= 100.69, p<0.05) from the expected 1:1 ratio. Number of male was prominent in the smaller size, whereas, female dominated in larger size. Minimum length at sexual maturity of male and female were 17 cm and 22 cm TL in this study, while mean length at maturity (0.5 probability) were 21.55 cm and 28.01 cm TL. The relationship between fecundity and total length, fecundity and body weight were expressed as F (in million) = 0.000006TL 3.33 (r2=0.902) and 0.00164BW-0.01 (r2=0.839) respectively and both the relationships were statistically significant (p<0.05). T. ilisha spawned throughout the year and spawning activity reached peak between September and October. Parameters of the von Bertalanffy growth model for male and female were TL 44.2 cm and 54.3 cm, K 1.10 and 0.92 per year respectively employing electronic length frequency analysis (ELEFAN I). Parameters of the von Bertalanffy growth model in terms of body weight for male and female were BW- 880.82 g and 1882.98 g, K= 1.10 and 0.906 per year, and to= - 0.007 and -0.017 years. Natural, fishing, and total mortalities for male and female were 1.69, 1.64, 3.326 per year and 1.41, 2.13, 3.547 per year respectively. The maximum yield per recruit obtained for male and female was 72.95 g and 169.95 g at fishing mortalities of 1.65 and 1.50 per year respectively. Total relative weight of male was 90.37 kg at the age of 1.01 years, while female attained 196.16 kg at the age of 1.22 years if 1000 individuals in each case were recruited to the fishery. Results from the study may be used in the modernization of the existing hilsa fisheries management action plan in Bangladesh.