Integrating different aging methods to model the dynamics of hard-to-age crab growth: Age at size estimates for the Jonah crab (Cancer borealis)

Age determination of an organism is essential in understanding the dynamics and management of wild populations. This has been an especially long-standing challenge in the study of crustaceans due to the loss of calcified structures during ecdysis. In this study we developed the first probabilistic growth model for the Jonah crab (Cancer borealis Stimpson, 1859), one of the many commercially harvested crustaceans for which no absolute aging data has yet been established. The growth model utilized indirect aging estimates from conventional laboratory growth studies and size-frequency analysis of field collections to estimate the growth from the time of settlement (age 0) to legal harvestable size. Our results suggest highly variable growth rates resulting in growth from settlement to harvestable size of 121 mm carapace width ranging from 4 to 9 years. To our knowledge these are the first absolute age estimates of Jonah crabs. We also used this study as an opportunity to evaluate the utility of band counts in the gastric mill ossicles as a promising, yet controversial, method of direct age determination for crustaceans. The gastric mill band count method was applied to directly estimate the age of 150 Jonah crabs collected from this same region. The ages estimated from the gastric mill band count method correlated strongly with the independent model-based estimates (R²=0.96). Thus, despite the admittedly poor understanding of the mechanism by which bands form, our observations are consistent with the hypothesis that they may be a useful independent age indicator in this species.