Abstract
A study was conducted to find out the aquatic biodiversity in relation to the environmental parameters in three categorised beels: an open beel (Boro beel), a semi-closed beel (Borobila beel) and a culture-based closed beel (Gawha beel) located at the northern Bangladesh from November 2002 to June 2004. A total of 77 freshwater finfish and shellfishes belonged to nine orders, 24 families and 53 genera were identified from these beels. Out of total species, 75 were finfish species (69 indigenous and 6 exotic species) and remaining two were freshwater prawn. Highest 72 fish and prawn species were recorded in Boro beel followed by 58 species in Borobila beel and 38 species in Gawha beel. Macrobrachium spp. and Puntius sophore were found to be dominant in number and in weight, respectively in Boro beel and Borobila beel. Beel resident fish was found to be more dominant than those of migratory and riverine fishes in terms of abundance (% in number and % weight) in all the beels. However, in case of Boro beel riverine fishes were found to be dominant (% in number and % weight). Riverine fish and prawn species were not recorded in Gawha beel. Silver razorbelly minnow (22.32%) and Indian river shad (23.39%) were found to be the two most abundant migratory fish in Gawha beel in terms of numerical (%) and biomass (%) abundance, respectively than that of other beels. Three types of fishers: full-time, part-time and subsistence classified based on their relative time spent for fishing were found in all beels and were found to use four categories of gears. These gears were net, trap, wounding gear and other miscellaneous gears. Highest 40 types of gears were observed in Boro beel while lowest gear (7) recorded in Gawha beel. Maximum number of fish and prawn species were recorded in the catch of full- time fishers using nets found in all the beels. Maximum catch (% number and % weight) of fish and prawn was recorded in Gawha beel by the net. Ber jal (seine net) was found to be dominant gear in terms of number (94.57%) and weight (84.42%) abundance of fishes recorded in Gawha beel where Indian major caps and Chinese carps were stocked and harvested. Maximum catch of subsistence fishers in terms of number (8.28%) and weight (17.38%) composition of fish and prawn was found in Boro beel due to its open access to fishing. Whereas, subsistence fishers has little or no access in Gawha beel as the fishing was more or less controlled by the lessee. Altogether 61 genera of plankton were recorded from these three beels, of which 49 genera represented phytoplankton while zooplankton represented 12 genera. All the beels showed considerably higher phytoplankton diversity than their zooplankton diversity. Quantitative abundance of plankton were found to fluctuate from 1,900 to 6,500, 983 to 3,500 and 383 to 5,267 cells/l in Boro beel, Borobila beel and Gawha beel, respectively. Plankton population was found to correlate significantly with different water quality parameters in three beels although their levels of relationship varied from beel to beel. A total of 31 genera of macrobenthos belonging to 17 families were recorded from three beels. Out of 31 genera, Mollusca dominated with 12 genera followed by Diptera (11 genera) and Oligochaeta with three genera. The diversity of macrobenthic fauna in terms of number of genera was more or less similar in three beels although abundance of macrobenthos represented highest position (1,259 no/m2) in Boro beel. Mollusca (49%), Oligochaeta (44%) and Diptera (48%) were found to be the most abundant macrobenthos recorded in Boro beel, Borobila beel and Gawha beel, respectively. Macrobenthic fauna was highly (P<0.01) correlated with Mollusca (Boro beel), oligochaeta (Borobila beel) and Diptera (Gawha beel). Oligochaeta represented a strong and significant relationship with sediment pH in Borobila beel and Gawha beel. A direct and significant (P<0.05) relationship was found between abundance of Mollusca and organic carbon in Boro beel. Beel sediment texture varied from silt loam to silt clay loam (Boro beel), silt clay to loam (Borobila beel) and silt clay loam to silt clay (Gawha beel). It was observed that values of different water quality parameters were within the acceptable ranges for aquaculture in tropical conditions in all three beels. Environmental parameters showed significant relationship within different parameters but it varied in respect to nature and habitat of the beels. The findings of this study are expected to provide baseline information on growth, production, survival, species abundance and migration of fish, plankton and macrobenthos in three beels for sustainable use of beel fisheries resources.