Use of a Durometer to Differentiate Between Soft- and Hard-Shelled Mud Crabs (Scylla serrata)

Abstract

Commercial mud crab fishers in the Northern Territory, Australia, can harvest both sexes of Scylla serrata year-round. The combination of seasonal moulting and legislation that prohibits the retention of Commercially Unsuitable Crabs (CUCs or “soft crabs”) can result in many individuals being returned to the water at particular times. Concerns regarding consistency in the mandatory manual shell hardness test prompted an evaluation of a durometer as a quantitative assessment tool. Variability in durometer readings from different positions on the exoskeleton made it necessary to impose standardized test sites for each sex: the center of the second abdominal plates for males, and the intersections of perpendicular lines spanning the carapace of females (i.e., a transverse line between the seventh marginal spines and two longitudinal lines originating from each of the third marginal spines). While the vast majority of CUCs returned lower durometer readings than Commercially Acceptable Crabs (CACs), there were some instances where CUCs returned slightly higher readings than CACs (and vice versa). This imperfect matching of hand and durometer test values led to the development of a graphical aid for determining optimal durometer limit values that minimizes the retention and release of CUCs and CACs, respectively.

Keywords:

• durometer
• mud crab
• Scylla serrata
• shell hardness
• moulting
• fisheries management

Acknowledgments

This work was funded through a Science and Innovation Award for Young People in Agriculture, Fisheries and Forestry conferred to Michael Phelan by the Bureau of Resource Sciences. The assistance of Vu Van Nguyen and Tam Van Nguyen, who collected soft crabs under scientific permits, was instrumental to the success of the project, as was the help of Sherwood Thorbjornsen who transported these crabs to Darwin. Chai Eam, Peter Fraser, and Greg Quail kindly allowed their crabs to be measured and tested. Chris Errity and Paul Williams are thanked for assisting with the measurement and testing of mud crabs. Robert Brown, Blair Grace, Andria Handley, and Thor Saunders are thanked for proofing the draft manuscript. The authors also acknowledge Mark Hearnden's assistance with statistical analyses.