Mud Crab (Scylla serrata) population changes in Koggala Lagoon, Sri Lanka since construction of the groyne system

Authors

  • G. P. Amarasekara Faculty of Fisheries and Marine Sciences and Technology, University of Ruhuna, Matara, Sri Lanka
  • T. Priyadarshana Faculty of Fisheries and Marine Sciences and Technology, University of Ruhuna, Matara, Sri Lanka
  • J. Manatunge Department of Civil Engineering, Faculty of Engineering, University of Moratuwa, Moratuwa, Sri Lanka
  • N. Tanaka Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan
  • G. L. Gunaratne School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, Perth, Western Australia

Keywords:

crab net, crab pot, exploitation, vulnerability status

Abstract

This study was conducted to determine the present status of the Mud Crab (Scylla serrata Forskal) population in Koggala Lagoon, including details on size at maturity, sex ratio and carapace width - body weight relationship. This Crab is the most common edible Crab species found in the lagoon, comprising 93.10% of total catch from crab pots and 89.86% of total catch from crab nets. Sex ratios (male:female) were 1:0.86 and 1:0.82 in crab pots and nets, respectively. Female crabs of various sizes were randomly sampled to determine size at maturity. The results indicate that size at maturity of females was 12.17 cm carapace width. Size at maturity of males could not be determined using external characteristics. Population parameters were estimated from length-frequency data collected in 2009 and 2011. Asymptotic carapace length (L∞) and growth constant (K) were estimated to be 20.12 cm, 0.91 yr−1 in 2009 and 19.27 cm, 3.2 yr−1 in 2011, respectively. Exploitation rates were 0.32 in 2009 and 0.44 in 2011. There was a significant difference in Crab size between harvests by the crab nets versus traditional crab pots. Results indicate that S. serrata population is declining due to over-fishing as of increased fishing gear efficiency from traditional crab pots to modern crab nets. Commercial catches of crab net and crab pots comprised 85.7% and 72.0% of immature females, respectively, based on the size at first maturity revealed by this study. In order to maintain natural health of the Mud Crab population, immediate management action is needed to regulate fishing in Koggala Lagoon.

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Published

2016-01-02

Issue

Vol. 19 No. 1 (2016)

Section

Research article

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