Growth patterns, Sex ratios and Fecundity estimates in Blue Crab (Callinectes amnicola) from Yewa River, Southwest Nigeria.

Abstract

The present study investigates the growth patterns, sex ratios and fecundity estimates in Blue crab, Callinectes amnicola from Yewa River, Southwest Nigeria, between June 2011 and May 2012. Specimens were collected from Yewa River with basket traps (with non returned valve), gill and cast nets at depths between 1 and 15 metres. Morphometric data, body weight measurement, sex and number of ripe eggs (fecundity) were estimated from the crabs following standard methods. A total of 250 individuals measuring between 5.2 and 14.8 (10.75±1.98) cm long and weighing 8.2-165 (74.16±33.5) g in body weight were caught from Yewa River. The length-weight relationships were LogW=Log-0.96+2.72LogL (r=0.95) for males and LogW=Log-0.48+2.27LogL (r=0.95) for females. Growth was a negative allometry in C. amnicola with males exhibiting a better growth pattern. Condition factor variations were 2.14-9.48 (5.92±1.03) in males and 2.99-8.51 (5.49±0.95) in females. A total of 102 males and 148 females were encountered, given a sex ratio of 1male:1.45females. The sex ratio was significantly different (X²_{cal =8.46}> X²_{tab (df= 1,?=0.05) =3.86}) from the expected and theoretical 1male:1 female ratio. Fecundity estimates ranged from 260,000 to 2,150,692 (1,269,345.25±592.39) eggs for mature females. The linear relationships between fecundity/carapace length; and fecundity/body weight were LogF=Log0.383+3.21LogL (r=0.463) and LogF=Log0.333+1.38LogW (r=0.461) respectively. Conclusively, blue crab (C. amnicola) in Yewa River exhibited variations in their growth patterns, sex ratios and fecundity estimates when compared with the counterpact reports from the neighbouring water bodies.

Key Words: Length–weight, condition factor, morphometry, allometry, estuary.