Size, Length-Weight Relationships, Reproduction And Trophic Biology Of Chrysichthys nigrodigitatus And Chrysichthys auratus (Siluriformes: Bagridae) In A Natural West African Lake

Size, length-weight relationships, reproduction and trophic biology of Chrysichthys auratus and C. nigrodigitatus were investigated from May, 1990 to July, 1991 as part of the bio-limnological study programme of Agulu lake. C. auratus was more abundant and significantly longer (SL) than C. nigrodigitatus. As an assemblage, the population of Chrysichthys species exhibited isometric length-weight relationship (LWR) (b = 3.109±0.281) thus obeying the ‘cube' law. Fecundity ranged between 625 and 2184 oocytes (mean 1163±461 oocytes) in C. auratus and between 733 and 1988 oocytes (mean 1261±384 oocytes) in C. nigrodigitatus. Mean exponent (b= 1.765±0.382) of the length-fecundity relationship (LFR) is less than 3 indicating that the Chrysichthys species exhibit substantial fish size - egg size changes. The mean weight exponent (b= 0.56±0.23) of the weight-fecundity relationship (WFR) is significantly different from 1 showing that fecundity does not relate linearly with weight. Standard length (SL) is the best predictor of fecundity in C. auratus while in C. nigrodigitatus it is body weight (Bw). Breeding in C. auratus occurred between April and September. The diameter of ripe oocytes was 2.04 ± 0.32 mm in C. auratus and 2.1±0.36 mm in C. nigrodigitatus. The species fed on both plant and animal materials with the animal component dominating [56.8% and 62.1% percentage frequency of occurrence (%FO); 53.6% and 42.6% volumetric index (VI) for C. auratus and C. nigrodigitatus respectively]. The high Spearman's rank coefficient (r = 0.75) and the Raabe's factor (R = 59.0%, %FO; 56.8%, VI) show strong similarity and overlap in the food types of the two species. Similarly, Raabe's factor (R = 62.7%) indicates trophic niche overlap between the small- [8 - 14 cm standard length (SL)] and large-sized (> 15 cm SL) individuals of C. auratus. Key Words: Size, morphometric relationships, reproduction, food, Chrysichthys species Bio-Research Vol.2(1) 2004: 47-58

 NM Inyang, HMG Ezenwaji