Breeding season, length-weight relationship and condition factor of introduced fish, Tilapia zillii Gervais 1948 (Pisces: Cichlidae) in Lake Zwai, Ethiopia

Abstract

Breeding season, length-weight relationship and condition factor of Tilapia zillii were studied in Lake Zwai from samples collected over 12 months between January and December, 2001 using gillnets of 60 and 100 mm stretched mesh. State of gonad maturation and Gonadosomatic index (GSI) values showed that T. zillii in Lake Zwai breeds all year round with peak activities between April and September. The relationship between total length (TL) (5.5–32 cm) and total weight (TW) (3.8–676.9 g) of the fish was found to be curvilinear and represented as TW = 0.02TL^2.9813, R² = 0.99, P < 0.001, n = 848. Mean monthly Fulton condition factors (FCF) and Relative condition factors (RCF) of the population of T. zillii were 2.06 and 1.01, respectively. Two factor ANOVA indicated a significant seasonal fluctuation (p < 0.001) in the condition of both sexes, but not between female and male T. zillii (p = 0.127 and 0.135 for FCF and RCF, respectively). Poor body conditions coincided with time of peak breeding activity. The interaction effects of sex and month on Fulton's and Relative condition factors were not significant (p=0.127 and 0.138 respectively).

Key words/phrases: Breeding season, Ethiopia, Lake Zwai, length-weight relationship, Tilapia zillii

SINET: Ethiopian Journal of Science Vol.26(2) 2003: 115-122