Juvenile length-at-age data reveal that spanner crabs (Ranina ranina) grow slowly

Abstract

The spanner crab (Ranina ranina) is a widespread and abundant brachyuran in offshore sand substrata of the Indo-Pacific region. Little is known of this species’ biology, population dynamics and ecology, despite it being the target of commercial fishing operations in many areas. Previous studies of R. ranina growth using length-frequency analysis of samples collected with commercial fishing gear have derived widely divergent estimates of growth parameters. The estimated time taken to reach 100 mm rostral carapace length (minimum legally exploited size in Queensland, Australia) in those studies has ranged from 1.75 to 8.83 years for females and from 1.08 to 3.58 years for males. Our data show that the commercial fishing apparatus used in those studies is size selective and catches only adult crabs. The resulting size bias in samples collected using that apparatus precludes the application of length-frequency-based techniques to estimate growth parameters from those samples. We devised a new dredge to collect samples of juvenile R. ranina and to calculate juvenile growth rates from modal progression in those samples. We combined those data with estimated mean maximum lengths (L_∞) of 121.7 mm for females and 155.9 mm for males from commercial catch data to model other von Bertalanffy growth parameters using bootstrap methods. Those modelled parameters (K=0.29, T₀=−0.24 for females; K=0.23, T₀=−0.25 for males) indicate that R. ranina grows more slowly than most previous estimates suggest, with females requiring an average of 6.35 years and males 4.31 years to reach 100 mm rostral carapace length. This slow growth is consistent with the slow metabolism of R. ranina, and indicates that this species would be likely to recover slowly from overexploitation.