Testing the ontogenetic migration hypothesis in the emerging Rhode Island Jonah crab fishery using stable isotope analysis

In southern New England (USA), rapid ocean warming in recent decades has caused substantial redistributions of fishes, invertebrates, and the fisheries they support. The rapid emergence of the warm water-tolerant Jonah crab Cancer borealis , once discarded as bycatch from the now declining lobster fishery, illustrates a prime example of climate-adaptive shifts in southern New England fisheries. However, limited data exist on the basic life history of Jonah crabs, despite their growing economic and societal value. Here, we tested what we call the ontogenetic migration hypothesis, i.e. the observed size differential between crabs caught in inshore (smaller terminal size) and offshore (larger terminal size) fishing areas is due to ontogenetic migration from inshore to offshore. We tested this hypothesis using an isotopic clock approach comparing isotope values of carapace (record of residence >1 yr ago) to muscle (local residence signature) of Jonah crabs across a broad carapace width spectrum (71-153 mm) from inshore (n = 59 crab) and offshore (n = 64 crab) fishing areas. We did not see isotopic evidence of population-wide ontogenetic migration between inshore and offshore fishing areas; however, there was isotopic evidence that an ecologically relevant proportion (~25%) of the crabs collected from the offshore fishing area migrated there after past residence in the inshore fishing zone. This work provides key data on critical life history characteristics around movement ecology and migration capacity needed to understand the connectivity of Jonah crabs in southern New England.