Comparing Independent Approaches to Estimate Age at Size of the Jonah Crab (Cancer borealis): Corroborating Gastric Mill Band Counts as a Direct Aging Method

Knowing the age of an organism is essential in understanding the dynamics and management of wild populations. Age determination has been an especially long-standing challenge in the study of crustaceans, since they posed a unique challenge, shedding all calcified structures with each molt. The Jonah crab (Cancer borealis), is one of many commercially harvested crustaceans for which no absolute aging method has yet been established. It is an ecologically important species and a newly managed fishery in New England and Atlantic Canada. The recent increase of commercial fisheries for this species has highlighted the large data gap of growth and aging information, resulting in high uncertainty of understanding their population dynamics. Direct aging methods for marine species typically rely on counting growth bands preserved in calcified structures. Recent evidence suggests that bands present in the gastric mill of the foregut of lobsters, crabs, and crayfish may accumulate with age. This study compares three independent aging methods to estimate the age-size relationship for Jonah crab through: (1) length frequency analysis of crabs sampled periodically in wild nursery populations including young-of-year crabs, (2) building a probabilistic growth model informed with data from a laboratory growth study, (3) and finally applying the method of direct gastric mill band counts from crabs collected in two contrasting temperature regimes along Maine’s coast. This is the first comprehensive study on the growth of juvenile Jonah crab and providing additional data on mature crabs, enabling absolute age estimates to be determined for the Jonah crab for the first time. Passive settlement collectors and monthly size frequency analysis of juvenile crabs clearly documented the transition from postlarval stages into their benthic phase occurring between July and September. Definitions of these juvenile size classes provided from this study, allows annual size frequency data to be applied as an index of Jonah crab recruitment. The growth study and subsequent molt model have highlighted the variability in growth and will enable management to determine age at legal size for males and information about age at maturity. This study also provides additional supporting evidence that the bands observed in the gastric mill ossicles of crustaceans can be used as direct age indicators. First, I used known age Jonah crabs up to two years old to validate this method. Second, the gastric mill band count method has been corroborated using two indirect aging methods providing a direct one-to-one relationship with age. The regional comparison of band count conducted here demonstrates that the number of bands are not solely dependent upon the size of the animals but are likely age indicators. The results from this study provide fisheries management and scientists a tool to improve their understanding of growth, longevity, and age at specific life history events enabling a better understanding of the biology of the Jonah crab