Estimated mortalities and potential yield ofudfreshwater shark (Wallago attu) from theud'Dhir beel' (riverine floodplain lake) ecosystemudof the Brahmaputra basin, Assam, India

摘要

The annual instantaneous natural mortality as well as the total mortality ratesudand the potential yield of Wallago attu (Bloch and Schneider) were estimatedudfrom the 'Dhir beel' ecosystem of the Brahmaputra basin, Assam, India. Theudinstantaneous natural mortality (M) as estimated by different methods wasudfound to be 0.83 (Pauly) and 1.15 (Cushing). However, the realistic value ofudM for W. attu in the 'Dhir beel' was found to be 1.55 (assumed value). Theudaverage instantaneous total mortality (Z) values calculated by various methodsudwere 3.44 (Jackson), 3.50 (Jones and Van Zalinge) and 3.70 (Beverton andudHolt). The annual Z for the period 1980-'90 ranged from 1.45 (1988-'89) toud5.14 (1986-'87) for M = 0.83; 1.69 (1988-'89) to 4.95 (1986-'87) for M = 1.15udand 2.0 (1988-'89) to 4.67 (1986-'87) for M = 1.55. The potential yield perudrecruit (Y1) for W. attu in the 'Dhir beel' was estimated to be 440.0 gudcorresponding to a length (1) of 70.80 cm and age (t) of 1.16 year as per theudanalytical model of Beverton and Holt. However, it was 427.78 g for 1 = 73.55udcm and t =1.22 year as per the Kutty and Qasim model. The biomass perudrecruit attained a peak at 1 = 62.63 cm and t = 0.98 year as per Jones' lengthudcohort analysis. The potential yield per recruit was estimated for threeuddifferent values of natural mortality, M(=0.83, 1.15 and 1.55) as a functionudof exploitation ratio (E) for the observed age at first capture, t = 0.36 yearudcorresponding to lc = 30.0 cm and also for the age at first recruitment, t =ud0.21 year corresponding to lr = 17.5 cm. In view of the highly predatory habit,udthe length a-; first capture, lc = 30.0 cm can be considered to be the optimumudsize at first exploitation of W. attu in 'Dhir beel'. The maximum sustainableudyield (MSY) for W. attu has been estimated to be 10,458 kg for the optimumudeffort at MSY (fmgy) of 182 men/day as per Schaefer and the MSY of 8,646udkg at fms of 182 men/day as per Fox. The fms of 182 men/day could be moreudeconomical for the W. attu fishery in the 'Dhir beel'.