Quantifying Shell Hardness in the Southern Rock Lobster (Jasus edwardsii)

Abstract

Southern rock lobsters Jasus edwardsii that are about to moult or have recently moulted have reduced market value due to higher mortality in live transport, higher cannibalism and lower meat recovery. Limiting the landing of softer shelled lobsters is desirable to maintain product quality; however, attempts to set closed seasons or to introduce industry self-regulation have been hampered by subjectivity in measuring shell hardness. We evaluated the use of durometers as a tool for collecting objective measurements of shell hardness. Of the ten locations on the exoskeleton that were evaluated, two locations along the lateral suture line of the exoskeleton gave most consistent readings and underwent largest changes through the moult. The effects of several factors on durometer readings were evaluated: sex, temperature (ambient plus elevated 3 degrees C), location (from around the coast), and size. No effect on durometer readings could be detected from any of these actors with the exception of lobster size. A cumulative logistic function fitted to grouped data provides a method for predicting time before or after moulting from durometer readings. Although durometers appear to be a valuable tool for research on closed seasons, there was not sufficient precision for grading of individual lobsters by industry.