Abstract
A research experiment was conducted in fifteen oxbow lakes from July 2001 to December 2003 to find out some management aspects of culture-based fisheries in oxbow lakes in Bangladesh. Some important water quality parameters of these lakes were studied; and the values were found within the acceptable ranges which indicated that most of the lakes are productive for fish culture. Fish culture in oxbow lakes is practiced by conversion open water fisheries into a culture-based fisheries through screening the inlets and outlets. Three species of Indian major carps (rohu Labeo rohita, catla Catla catla and mrigal Cirrhinus cirrhosus,) and three exotic carps (silver carp Hypophthalmichthys molitrix, grass carp Ctenopharyngodon idella and common carp Cyprinus carpio carpio) are regularly stocked and harvested. Two years mean stocking density of 15 lakes ranged from 1,672 to 9,621 (mean 4187) fingerlings ha', yield varied between 91 and 1,465 (mean 713) kg ha", fishing intensity varied from 2.53 to 25.24 (mean 13.00) boat-days ha1 yr and recovery from 13 to 87% (mean 41%) were found. Simple and multiple regression between carp fish yield, stocking density, fishing intensity, recovery percentage, SWA, mean depth and different productivity variables (Secchi depth, total alkalinity, TDS, nitrate nitrogen, temperature, pH, phosphate-phosphorus, total phosphorus, chlorophyll-a) was established. The results indicate that stocking density, Secchi depth, chlorophyll-a, fishing intensity and recovery percentage may be used as more dependable yield predictor in culture-based fisheries in tropical water while Secchi depth may be reliable in water with very low turbidity. Yield-stocking density relationship established stocking density should be within 9736 fingerlings ha for maximum yield for culture-based fisheries management, where biodiversity is not concern. Economic analysis showed that the total average cost per ha of all lakes was Tk. 17,441 in which the fixed and variable cost comprised 18.03% and 81.97%. In cost concern, fingerling was the most important item accounting for 40.72% of the total cost. The findings thus indicate that the profitability of culture-based fisheries in oxbow lakes can be increased substantially if cost of fingerling purchase is reduced. The total average gross return per ha of all lakes was Tk. 26,465 in which stocked fish, non-stocked indigenous fish species (NIFS) and others comprised 89.92%, 9.72% and 0.36% respectively. Without considering opportunity cost the gross margin per ha was Tk. 12,535 and the net lake income per ha was Tk. 1,088. Since, the gross margin and net lake income are both positive, lake fishing appeared to be profitable in both short and long run situation. Benefit cost ratio came out to be 1.62. Different stocking and harvesting regimes are practiced in these oxbow lakes. These management regimes include high, moderate & low stocking, multiple, continuous & single stocking, and continuous & periodic harvesting. Economic performance in different management system showed that multiple stocking and continuous harvesting is the best for management of oxbow lakes in Bangladesh. A study on the impact of culture-based fisheries on biodiversity of NIFS and environment was carried out in nine selected oxbow lakes among earlier fifteen lakes from August 2002 to July 2003. Fishers' catch were regularly sampled to monitor the yield and species composition (number and weight) of NIFS. Two types of fishing gears are used almost exclusively to catch stocked carp: purse seine nets (kochals) and brush parks (komars), while a variety of fishing gears (nets, traps and wounding gears) were found to be operated for fishing of NIFS. Yield of NIFS in the nine selected lakes varied between 65 kg ha' (Sastar) and 407 kg ha1 (Bukbhara). Except that for Kaliganga lake in Kushtia, the single dominant species was Indian River shad Gudusia chapra contributing to about 10.75 to 72.37% of total NIFS catch. The other dominant species harvested by different gears were Aorichthys seenghala, a variety of prawn species (Macrobrachium sp.), Glossogobius giuris, Parambassis spp., Puntius spp., Channa spp., Mystus spp., Mastacembelus and Macrognathus spp., Amblypharyngodon mola and Nandus nandus. Time series analysis of data and the results of the study indicate that where high density, (≥5000 fingerlings ha') multiple stocking and continuous harvesting in Nasti, Porapara and Marufdia lakes have been used over the last 7 years, the number of NIFS recorded during the study period were between 18 and 23, while in lakes where the average stocking densities were ≤2000 fingerlings ha', species diversity varied between 38 and 43. It was also observed that intensively managed (with high density stocking) culture-based fisheries in oxbow lakes led to the change of aquatic habitats by cleaning of aquatic vegetation and frequent fishing. Around 4,000 fingerlings ha" may be the proper stocking density for CBFM. In the light of above findings, stocking programmes should consider as essential for the rational balance between increasing fish yield and maintaining environmental integrity of the water body brought under CBFM.