Determination of Moulting Events in Rock Lobsters from Pleopod Clipping

Caleb Gardner,David J Mills

doi:10.1371/journal.pone.0074146

Abstract

Rock lobster growth is routinely measured for research to optimise management measures such as size limits and quotas. The process of estimating growth is complicated in crustaceans as growth only occurs when the animal moults. As data are typically collected by tag-recapture methods, the timing of moulting events can bias results. For example, if annual moulting events take place within a very short time-at-large after tagging, or if time-at-large is long and no moulting occurs. Classifying data into cases where moulting has / has not occurred during time-at-large can be required and can generally be determined by change in size between release and recapture. However, in old or slow growth individuals the moult increment can be too small to provide surety that moulting has occurred. A method that has been used since the 1970’s to determine moulting in rock lobsters involves clipping the distal portion of a pleopod so that any regeneration observed at recapture can be used as evidence of a moult. We examined the use of this method in both tank and long-duration field trials within a marine protected area, which provided access to large animals with smaller growth increments. Our results emphasised that determination of moulting by change in size was unreliable with larger lobsters and that pleopod clipping can assist in identifying moulting events. However, regeneration was an unreliable measure of moulting if clipping occurred less than three months before the moult.

Highlights

Rock lobsters are Australia’s most valuable fisheries species group with the gross value of annual production in 2011 estimated at $AUD390 million [1]
In many cases growth is treated as being continuous so that classical growth parameters can be fitted. This approach has been adopted successfully in some southern rock lobster (Jasus edwardsii) fisheries where time-at-large and the number of recaptures are sufficient to create a reasonable approximation of continuity, despite discontinuous growth caused by moulting [2]
Both sex and site appeared to influence the rate of redevelopment of pleopods, measurement of the effect of sex is confounded by the time of year the moulting occurs (Females in April/May, Males in September/October) and associated differences in water temperature

Summary

Introduction

Rock lobsters are Australia’s most valuable fisheries species group with the gross value of annual production in 2011 estimated at $AUD390 million [1]. In many cases growth is treated as being continuous so that classical growth parameters can be fitted This approach has been adopted successfully in some southern rock lobster (Jasus edwardsii) fisheries where time-at-large and the number of recaptures are sufficient to create a reasonable approximation of continuity, despite discontinuous growth caused by moulting [2]. The discontinuous nature of lobster growth is a critical consideration and must be examined explicitly in tagging data A common situation is where both moulting and research sampling occur in a single annual pulse. This can lead to data sets where some tagged individuals recaptured after 12 months had not moulted, while others had undergone two moults [3]. A first step in examining these issues in tagging data is establishing whether moulting has occurred between tagging and recapture events

Methods

Results

Conclusion