The dynamics of an exploited population of a siluroid catfish (Schilbe intermidius Reupell 1832) in the Cross River, Nigeria

We studied the breeding biology, growth, mortality, recruitment pattern and yield per recruit of a freshwater siluroid catfish (Schilbe intermedius) in the Cross River, Nigeria. Monthly variation in mean gonadosomatic and mean condition indices shows that the species breeds twice in a year (March and September). We used 12 consecutive months length-frequency data together with FiSAT software to study the population dynamics of this catfish which is exploited by artisanal fishermen. Fitting the seasonalized von Bertalanffy growth function to our length-frequency data gives the following growth parameters: L-infinity = 27.5 cm total length, K = 0.29 yr(-1), C = 0.5, WP = 0.46. Using the seasonalized length converted catch curve, we estimated the instantaneous total mortality coefficient Z as 1.85 yr(-1). The instantaneous natural mortality coefficient M was 0.81 yr(-1) while the instantaneous fishing mortality coefficient F was 1.04 yr(-1), giving the current exploitation rate E = 0.56. From the analysis of probability of capture of each length class we estimated the length at first capture L-c to be 10.9 cm. The relative yield per recruit analysis predicted the maximum exploitation rate E-max = 0.53. The current exploitation rate E is more than this predicted maximum E-max. Thus, the species is on the verge of being overexploited. (C) 1999 Elsevier Science B.V. All rights reserved.