Assessment of the growth parameters and mortalities of two species of catfish, Synodontis membranaceus (Geoffroy saint- hilaire, 1809) and Synodontis gambiensis (Günther, 1864) in River Benue at Makurdi, Nigeria

Abstract

The growth parameters and mortalities of Synodontis membranaceus and Synodontis gambiensis in river Benue at Makurdi were studied over a 24 month period from January, 2009 to December, 2010. A total number of 106 specimens of S.membranaceus comprising 47 females and 59 males and 109 specimens of S.gambiensis comprising 53 females and 56 males were used for the study. The asymptotic length (L∞) calculated for the two species were 23.50, 23.70 and 23.00cm in the females, males and combined sexes in S.gambiensis and 37.04,28.25 and 35.10 cm in S.membranaceus females, males and combined sexes respectively. The growth rate (K) was found to be (0.310–0.640) in S.membranaceus and (0.500 – 0.571) in S.gambiensis. Growth performance indices (O’) for the two species were 2.212, 2.374 and 2.445 in S.gambiensis females, males and combined sexes and 2.562, 2.689 and 2.869 in the females, males and combined sexes of S.membranaceus respectively. Natural Mortality (M) was found to be 1.1774, 1.3070, and 1.2444 in the females, males and combined sexes in S.gambiensis and 0.9846, 1.3340 and 0.8312 in S.membranaceus females, males and combined sexes respectively. Fishing Mortality (F) was higher than natural mortality (M) except in S.gambiensis females as follows, 1.0096, 2.1540 and 1.3376 in the females, males and combined sexes and 3.0934,1.9850 and 2.2108 in S.membranaceus females, males and combined sexes respectively. Total mortality (Z) was observed to be 2.187, 3.461 and 2.582 in S.gambiensis females, males and combined sexes and 4.078, 3.319 and 3.042 in S.membranaceus females, males and combined sexes respectively. Mean Exploitation ratios for the species were high and exceeded the 0.5 optimal values for sustainable yield. These were S.gambiensis (0.53) and S.membranaceus (0.70). The survival rate of S.gambiensis in the River was found to be much lower than that of S.membranaceus (0.556 in S.gambiensis as against 1.482 in S.membranaceus respectively). The current exploitation rates suggest growth overfishing with selective mortality towards smaller fish sizes, therefore, the protection of the immature fish will be the key factor to preserve the spawning stock.