Morphometric relationships of blue swimming crab (Portunus pelagicus, Linnaeus, 1758) caught in Negombo coastal waters in Sri Lanka.

Length-weight relationships provide information about the blue swimming crab (Portunus pelagicus) population for successful stock management. Morphometric measurements of Two hundred fifty (250) crab samples were obtained from April to November 2019, of which 128 were males, and 122 were females, showing a sex ratio of 1:1. Chi-square test indicated that the sex ratio was not significantly different from the expected value of 1:1 at 5% level of significance (χ2cal = 0.144, χ2tab = 3.84). Carapace width (CW) was measured at the nearest 0.001 cm using a vernier calliper, and total body weight (W) was measured at the nearest gram (g) using a digital scale. The carapace width of males ranged from 7.28 - 15.7 cm, whereas it varied from 3.06 - 15.8 cm in females. The body weight of male crabs varied from 22.45 - 334.6 g, and females had a range from 23.0 - 275.35 g. Length-weight relationships for male and female crabs were W=0.0495CW3.0469 (R2=0.8526) and W=0.063CW2.9428 (R2=0.8179) respectively. Student’s t-test showed that the growth of male crabs was positive allometric (t = 5.64, p<0.05) and negative allometric (t = -8.21, p<0.05) for females. The mean condition factors for males and females were 5.68±0.12 and 5.65±0.12, respectively, indicating no significant difference between males and females (p=0.882) by two-sample t-tests. Providing baseline data, the present study recommends further studies for successful utilization, development and management of this crab species as it has been recognized as a commercially valuable species with a high market demand.