In a new study, researchers from North Carolina State University compared numbers of juvenile blue crabs across three nursery habitats in Pamlico Sound, both pre- and post-fishery decline, and found that while adult populations declined and have remained low, juvenile populations remained the same during both periods. The work points to a potential population bottleneck for crabs post-nursery but pre-maturity.
The Albemarle-Pamlico Estuarine System (APES) supports the majority of North Carolina's blue crab population and provides key nursery habitat. Larval blue crabs are released by mature female crabs from narrow inlets along the Outer Banks during the late spring, progressing through several planktonic molts in the Atlantic Ocean before returning to the estuary in the fall through a combination of wind- and storm-driven transport. The newly arrived juvenile crabs settle in near-shore habitats, like seagrass beds and shallow marsh peat habitats, along the eastern and western shores.
"These juveniles hang out in the nurseries until they're basically big enough to pick a fight and win, then they move into the rest of the estuary," says Erin Voigt, a Ph.D. candidate at NC State and first author of the study.
The North Carolina blue crab fishery showed a decline in blue crab populations in the early 2000s, leading to protective measures and a 50% reduction in crab fishing. However, the adult crab population hasn't rebounded.
"The normal explanation for situations like these is recruitment overfishing, which just means that adults have been overfished to the point that they cannot produce enough young to get back to pre-collapse numbers," Voigt says. "We wanted to determine if that was the case here."
Voigt looked at the density of juvenile crabs - defined as having carapace widths between 2.2 and 20 millimeters - in three nursery habitats in the APES: patchy and ephemeral seagrass beds on the western shore, western shallow detrital habitats composed of marsh peat mats, and the eastern seagrass beds. She compared the densities across two time periods, 1996 - 1999, pre-fisheries decline, and 2017 - 2019, post-fisheries decline.
The comparison showed that the number of juveniles in the APES nursery habitats were the same both pre- and post-decline, although the western seagrass habitats had almost four times more crabs than the eastern one, which was traditionally considered the primary nursery habitat.
"The main takeaway here is that the decline in adult populations isn't directly due to recruitment problems," Voigt says. "Even if some of these juveniles are coming down from the Chesapeake or up from the Gulf populations, the numbers indicate that there should be significantly more adult crabs in the area.
"There is most likely a bottleneck we aren't aware of that's happening somewhere between their arrival in the habitats and reaching adulthood. If we can find that bottleneck it may help the numbers improve."
The study appears in Fisheries Oceanography and was supported by North Carolina Sea Grant; the Southeastern Climate Adaptation Science Center; Albemarle-Pamlico National Estuarine Partnership; Curtis and Edith Munson Foundation; North Carolina Division of Marine Fisheries and the Center for Marine Sciences and Technology, NC State University. David Eggleston, professor of marine, earth and atmospheric sciences at NC State, is the corresponding author. Former NC State Ph.D. student Lisa Etherington also contributed to the work.
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