July: Traveling Crabs

Chesapeake Almanac Podcast Episode and Transcript

Episode 17
Copyright © John Page Williams Jr. All rights reserved.

This is John Page Williams with another reading from Chesapeake Almanac. The month is July and the title of this chapter is "Traveling Crabs."

"The Maryland crab is different," said the man in the tackle shop at Solomons. "Its meat is sweet, like a lobster's. Virginia crabs just aren't the same animal."

He said that nearly 50 years ago, when we were all more naive and more provincial about our Bay. The Atlantic blue crab grows best in the fresher waters of upper estuaries, but it spawns in late spring and summer in salty waters near the estuaries' mouth. The man in Solomons was right to the extent that "his" crabs grew fat in excellent Maryland crab-growing habitat. Even so, virtually all of them had been born in Virginia between the mouth of the Rappahannock and Hampton Roads, and they had done a surprising amount of traveling before they made it up to the Patuxent River.

Any waterman working the spring and fall migrations and any biologist looking at catch survey data could see the general pattern, and William W. Warner's Beautiful Swimmers has done a lot to get the message across to the general public. Our beloved blue crabs are true creatures of the estuary, and their life cycle sends them from one end of the system to the other, rivers and main Bay alike. They are not Maryland or Virginia crabs. They are Chesapeake crabs.

Because the blue crab is so valuable to the economy of the Bay country, its population fluctuations are watched closely by scientists, state fishery managers, biologists, watermen, and seafood packers. As much as possible, the latter two base their business strategies on the expected catch for any given season. For years, the trawl net survey conducted by Willard Van Engel at the Virginia Institute of Marine Science ( or VIMS) served as the foundation for predictions. The crab harvest has a history of varying widely from year to year, and there have always been years when the survey did not predict the catch well at all. In June 1972 tropical storm Agnes led to just such a situation.

Before then, it was believed that a high summer river flow, especially from the James, would flush crab larvae out of the Bay and into the Atlantic, killing most of that year class of crabs. Agnes poured fresh water down the Bay's rivers, including a particularly heavy flow from the James (the river crested some 28 feet above flood stage at Richmond). According to the prevailing theory that year's larvae would have been swept out to the ocean and lost. In 1973, however, the Bay was full of 1972 crabs. Something had happened at the mouth of the Bay that the scientists did not know about. Observations that do not fit accepted theory are opportunities. This kind of occurrence is what advances scientific knowledge.

The problem was too large for one scientist or even one laboratory to attack alone. If larvae were being swept out to sea, tracing them would require physical oceanographers who could study the currents at the Bay mouth and out on the continental shelf. But the larvae might not stay in the same position in the currents. If they swam up or down, they could find themselves traveling first one way and then another as they changed current levels. Thus the study needed biologists to study the behavior of crab larvae in detail. Lots of data had to be collected aboard research vessels in the Bay and in the ocean, and it had to be fitted in with all the observations that had already been made over the years. A team was developed that included biologists and physical oceanographers from the University of Maryland, the University of Delaware, Old Dominion University, and VIMS. Funding was supplied in large part by the Sea Grant programs in the three states.

For nearly 10 years, the team pieced the puzzle together. It appears now that larval crabs tend to swim away from the force of gravity and toward light. Thus, when they hatch on the bottom in the spawning grounds at the Bay's mouth, they migrate toward the surface. This movement puts them in the fresher water layers that move generally seaward, rather than the saltier deep layers that move generally into the Bay.

Virtually the whole year class travels out into the Atlantic over the continental shelf. This is the normal condition, not just the result of a big storm like Agnes. To some extent, the oceanographers have been able to model water temperature, wind speed and direction, and current conditions on the shelf to predict the movement of the larvae. "When it works, it's almost eerie," said one scientist. "We feed our computer some wind and current data, and it gives us GPS coordinates. We run forty miles offshore to those numbers, put down the plankton net, and there they are." It is the ultimate needle-in-the-haystack game: the larvae are microscopic in size.

Movements on the continental shelf depend on weather patterns. It's probable that some of the patterns send larvae into the Chesapeake, while others push them into Delaware and Chincoteague bays, and into the bays behind the Virginia barrier islands. Research at Old Dominion is concentrating especially on how the crabs move back into the Chesapeake. It seems almost certain that they develop another behavior pattern that allows them to ride some combination of deep currents to their estuarine nursery grounds in the fall. Another team, from VIMS, is working on how the young crabs use those nurseries.

The patterns are large. Concentrations of larvae at the mouth of the Bay can reach 30,000 per cubic meter. The area of water involved is huge, and the current patterns vary each year with the weather. There is plenty to intrigue scientists, and us lay Bay lovers as well.

One thing is sure, though. The blue crab's behavior is much more complex than the fellow in the Solomons tackle shop ever imagined. Fifty years ago, the area of the continental shelf where the larvae that would be his Patuxent River crabs spent their first summer were international waters. Chesapeake crabs don't care anything about politics.

For more on these complicated Bay critters, see our Blue Crabs web page.

For more happenings around the Bay in July see our other Chesapeake Almanac podcasts and read our blog post "Life in an Eelgrass Bed."

Subscribe to this podcast at https://chesapeake-almanac.captivate.fm/listen

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