Estimating Intermolt Duration in Giant Crabs (Pseudocarcinus gigas)

Estimates of intermolt duration of giant crabs, based on tag-recapture methodology, are used in evaluating management options. However, several short-comings of tag-recovery data have been noted including the low number of tags inserted in legal-sized animals and that the unusually long intermolt duration requires long periods of time-at-large. This need led to the evaluation of alternative methods to estimate intermolt duration. Reproduction in female giant crabs occurs in annual cycles, although females occasionally 'skip' a reproductive season and do not become ovigerous; it has been noted previously that this appears to be associated with molting. Thus the proportion of females that do not participate in reproduction may indicate the proportion molting. We tried this approach with a sample of 342 females and measured the number that were 'skipping' a reproductive season by computerised tomography scanning (CT-scanning) of their ovaries prior to the extrusion of eggs. From the inferred proportion molting intermolt duration was estimated at 9 years for mature size classes, however 95% confidence limits were broad (6.8-13.1 years). This estimate does, however, corroborate those previously reported from studies in which tag and recapture methods were employed. Radiometric ageing (228Th/ 228Ra) of carapaces was also undertaken with the focus of this work on testing an assumption of the method, rather than describing the intermoult duration of a population. We tested the assumption that there is negligible exchange of radionuclides during intermolt in the exoskeleton, which is critical for reliable estimation of intermoult. SEM images of the internal structure of the exoskeleton indicated that exchange of material within the exoskeleton was unlikely and the majority of radiometric assays were consistent with this observation. Radiometric age was estimated by gamma spectroscopy, which allowed rapid analysis compared to previously reported methodology. This rapid processing may facilitate broader application of radiometric ageing to crustacean research.