The case of the missing Chesapeake Bay blue crabs | WYPR

The case of the missing Chesapeake Bay blue crabs

Jul 21, 2016

  Over the years, scientists have learned more about the Atlantic blue crab than just about any other species in the Chesapeake Bay. But there’s at least one mystery that still has them stumped. What happened to the millions of young crabs that vanished in 2012, what should have been a bumper year?

Tom Miller, a professor at the University of Maryland Center for Environmental Science Chesapeake Biological Lab in Solomons, says they have "two general ideas as to what might have happened to them." Maybe they weren’t there in the first place, even though a survey the previous year said they were. Or maybe they were there, but "something caused them to die at an unexpectedly high rate."

It all started with the 2011 winter dredge survey. Bay scientists have been dredging hibernating crabs from Chesapeake mud every winter since 1990 to check in on the population, and they've had a pretty good record of accuracy for predicting crab harvests. In 2011 they estimated an astounding 800 million crabs, the most in nearly 20 years. Some 600 million of those were juvenile crabs that should have been ready for harvest by the summer of 2012. But for some reason, those crabs disappeared.

Was the survey wrong?

Miller built a chronology using data from all around the bay. No, he found, the survey was accurate. And that left him with no smoking gun and no bodies. Still, he came up with some ideas.

"In the southern regions it was a perfectly normal year," he said. "But there was something odd from the middle of the bay north going on that was causing an unusually high mortality or unusually low survival of crabs between the winter of 2012 and late summer."

Rom Lipcius, Miller's opposite number at the Virginia Institute of Marine Science in Gloucester Point, says his number one suspect is red drum, a finfish that loves crab as much as people do. In 2012 as the juvenile blue crab population hit historical highs, so did red drum.

And they can be "voracious predators," Lipcius says. "In some cases large red drum can have upwards of 100 or more juvenile crabs in a single fish."

To test out the theory, three young VIMS scientists vacuumed up samples of juvenile crabs from the York River, tethered them to a pipe, much like that poor goat in Jurassic Park, and used a Go-Pro to record what ate them, which, it turned out, was mainly other crabs.

Mandy Bromilow, one of Lipcius’ students, collected other fish from the bay and the river and said she found "juvenile blue crabs in the stomachs of striped bass and Atlantic croaker."

But Miller says figuring out crabs is difficult, despite all the study.

"Just when you think you understand, nature can throw a curve and remind you you're not quite as smart as you think you are."

For Lipcius, the mystery continues. He says scientists can identify the most likely causes, but they’ll never really know for sure. And he compares it to a mystery from the 1990s that television has revisited lately.

"It's not unlike will we ever really know whether or not O.J. did it"

The scientists’ reports on the missing blue crabs of 2012 are due out at the end of this year.