Protecting the 'Beautiful, Savory Swimmer'

Richard Forward's Blue Crab Research Crucial to Saving Habitats and to Setting Sustainable Catch Limits p.2

   To prevent this from happening, North Carolina officials are enacting new, more stringent catch limits and management policies—safeguards shaped in part by Forward’s groundbreaking studies of the migrations of egg-carrying adult female blue crabs and their recently hatched postlarvae.

  His research has shown that these migrations are predictors of the fishery’s future population size.

  During the warm months, most adult crabs are found in low salinity waters close to where rivers flow into estuaries. As summer wanes, older females go through their terminal molt and slough off their exoskeletons. (Crabs may molt up to 25 times during their two- to threeyear lifespan.)

  The females then mate, store the sperm and—leaving the males and younger, non-mating females behind— head to estuary inlets miles away, where they over-winter.

  In spring, after laying and incubating their eggs, they migrate seaward, each crab laden, on average, with between one million and two million eggs attached to her abdomen.

  “The females undergo what is called ebb tide transport,” Forward says. “They swim primarily at night during ebb tides, when the water current helps carry them seaward, and then sit on the bottom during flood tides, when the water current reverses.” This allows them to move through the estuary toward their hatching grounds in the ocean in a kind of leapfrog mechanism, covering longer distances while conserving their energy.

  Forward and his research collaborators track the females using sonic tags attached harmlessly to the crabs’ backs.

  Called “pingers” because of the noise they make, the tags (which cost about $300 each) emit sound waves audible on underwater hydrophones. After the crabs have been fitted with the tags, researchers release them into the estuary and follow closely behind in a hydrophone-equipped boat, charting the crabs’ speed, direction and trajectory by following the sound of the pings. They also measure the currents, salinity and water temperatures around the crabs as they swim.

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