by Kristen Minogue
This summer and fall, biologists at the Smithsonian Environmental Research Center are looking to tag 10,000 blue crabs in Chesapeake Bay. They’re pursuing the project in spite of the two-year slump the crabs have suffered in the latest reports of the Chesapeake Bay Stock Assessment Committee. They’re hoping some of those crabs will help answer two unresolved questions on the path to recovery: the role of recreational crabbing, and the struggling population of adult females.
Every year watermen on Chesapeake Bay haul in between 40 and 110 million pounds of blue crabs on trotlines or in crab pots. The vast majority come from commercial watermen who rely on the crustaceans for their livelihoods. But recreational crabbers also take their share, and today no one knows exactly how large or small that share is.
“We really have very little idea how big the recreational fishery is now,” says Matt Ogburn, a postdoc at SERC’s Fish and Invertebrate Ecology Lab. That’s a problem for managers working to keep the fishery at sustainable levels. Managers rely on those numbers to estimate what fraction of the blue crab population is harvested each year. If the estimates are off, they cannot determine if blue crabs are in danger of being overharvested.
The latest figure estimates that the recreational harvest is roughly 8 percent the size of the commercial harvest. But that estimate dates back to 2002, and scientists have no clue if it’s still accurate. If he had to guess, Ogburn would suspect the current estimate is too low, because the Bay’s human population—especially on the waterfront—has grown. “But we really have no idea. It’s old.”
The tagging project could change that. In late June, Ogburn and the other ecologists in the lab began scouring rivers and Bay waters throughout Maryland looking for crabs. Because they go out with watermen, they keep watermen hours—leaving before dawn at 4 or 5 in the morning and returning in the early afternoon. (Since they’re releasing the crabs they find, they reimburse the watermen for their time.)
The ecologists attach a pink tag to any crab that could be legally caught in a fishery. The tag contains several pieces of information: a unique tag number, the phone number of the Smithsonian Environmental Research Center’s blue crab hotline, a website where tags can be reported, and notification of a reward, either $5 or $50, for reporting the tag. Ideally, watermen who find the crabs will call the number or report to the Website, letting SERC scientists know when and where they caught it, whether they are commercial or recreational crabbers, and what kind of crabbing gear they used.
In fall, the lab will expand the project to cover the rest of the Bay and the Potomac River. They’ll also switch focus, targeting exclusively females. Adult females dipped into a depleted state for the first time in more than a decade, according to the latest Winter Dredge Survey—and as spawners of the next generation, those crabs are critical for sustaining the population. By tagging them, the lab can get a better sense of their migration patterns and help managers determine which areas to target.
They’ll also track crabs using their shells. When female crabs become mature adults, they undergo one final molt. During that molt they absorb trace elements from the water in whatever part of the Bay they inhabit. Female crabs then migrate from various regions of the Bay to salty waters near the Bay’s mouth. Scientists can figure out where each female migrated from using the trace elements in her shell. While biologists have done this kind of “trace element tracking” for fish, this is the first time it has been attempted on a large scale for crustaceans.
Whether the lab reaches their 10,000-crab target depends largely on the overall blue crab numbers. “I don‘t know if we’ll be able to tag that many, because that population’s pretty low,” Ogburn says. “But that’s the goal.”
They’ll tag as many as they can catch. If they don’t make it to 10,000, though, that in itself will send another message to Chesapeake stewards: Six years after showing the first signs of recovery, the blue crab is still in need of saving.
Photo of watermen used with permission under Creative Commons License BY-SA 2.0.