Growth parameters and mortality rates of giant river-catfish <i>Sperata seenghala </i> from the Indus river, Pakistan

Abstract

Length frequency of <i>Sperata seenghala</i>¹ from the Indus River were collected during the months of February, March, April, May and June in 2011. The sample consisted of <i>n</i>= 133 pairs of length and weight data. The length ranged from 46 to 113 cm TL (65.661±15.995 cm), while the weight varied from 500 to 7500 g TW (1806.391±1481.456 g). Length-weight relationship was calculated as <i>a</i>= 0.04, <i>b</i>= 3.047 (<i>R</i>²= 0.99). The von Bertalanffy growth function parameters for <i>S. seenghala</i> were <i>L_∞</i>= 117.60 (TL cm) and <i>K</i>= 0.370 year^-1, while the age at zero length was <i>t₀</i>= -0.513 year. The total instantaneous mortality rate (<i>Z</i>) was estimated at 95% CI of 0.77-1.22 (<i>Z</i>= 1.00 year^-1) in the Indus River. The natural mortality (<i>M</i>) was computed as <i>M</i>= 0.556 year^-1 at an average annual temperature of surface water of 21^oC, whereas the fishing mortality was estimated as <i>F</i>= 0.444 year^-1. Therefore, exploitation ratio (<i>E</i>= F/Z) was calculated as <i>E</i>= 0.444. The growth performances indices for <i>L_∞</i> and <i>W_∞</i> were computed <i>Φ'</i>= 3.709 and <i>Φ</i>= 2.175 respectively. When <i>t_c</i> was at 1, the yield per recruit analysis found that <i>F_max</i> was at 0.85 year^-1 and <i>F_0.1</i>at 0.75 year^-1. The current age at first capture was about 1 year, the <i>F_current</i>rate was 0.444 year^-1. Thus the current fishing mortality rate was smaller than <i>F_opt</i>= 0.556 year^-1, <i>F_max</i>= 0.85 year^-1 and <i>F_0.1</i>= 0.75 year^-1. Therefore, we can assume that the current stock of this important fishery resource is not over-fished.