Radio Script
2000

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A Crab's Life
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INTRO: You may think it's impolite to pry into the lives of others. But when it comes to Tanner crabs, scientists have to be Peeping Toms. As Doug Schneider reports in this week's Arctic Science Journeys Radio, knowing the sex lives of Tanner crabs is key to rebuilding the state's commercial fishery.

STORY: Aboard the fishing vessel Big Valley in Alaska's Chiniak Bay, scientist Dr. Brad Stevens stares at a television monitor. The image he sees comes from a camera mounted on a remotely operated vehicle, or ROV, some 500 feet below. Although he's looking for Tanner crabs, on this day, there's not much to see.

STEVENS: "The habitat is mud. This is pretty featureless. I mean it's pretty flat. The animal I'm studying is Tanner crab, Chionoecetes bairdi. That's what these animals like."

Hundreds of thousands of Tanner crabs live on the bottom of Chiniak Bay, just beyond the city lights of Kodiak, a fishing community in the northern Gulf of Alaska.

Although Tanner crabs abound in this bay, populations elsewhere in Alaska aren't faring so well. Fishing for red king crab, Tanner crab, and snow crab has either been closed or dramatically curtailed around the island and in the nearby Bering Sea and Gulf of Alaska. Brad Stevens is a crab biologist with the National Marine Fisheries Service in Kodiak. He says crab populations fluctuate naturally. Learning how to predict these fluctuations begins with understanding what happens to newborn crab, called larvae.

STEVENS: "We need to know more about larval behavior. Once they get up into the water column, what do they do? Do they stay at the surface? Do they swim up and down at night? Do they go up and down with the tides? We don't really know."

Back on the fishing vessel, the ROV continues to look for Tanner crabs. After a dry spell, it hits pay dirt. First one, then another, then three, sometimes four crab are spotted at a time. Adult male crabs the size of serving platters are easily distinguished by their jagged features and huge claws. They scurry sideways, spider-like, as the ROV passes. Female crabs are smaller, with smooth, round shells. They lie half-buried in the mud, their tiny eyes reflecting the camera's light.

Stevens has come to Chiniak Bay twice a week each spring since 1991, dragging the camera-equipped ROV along the bottom to see what the crab are up to. Most of the crabs he sees this time of year are females, gathered together in huge mounds.

STEVENS: "I look at these mounds as sort of larval launching pads. And once they aggregate into mounds, they're very tight. So, think of a football field, and in that area there might have been 100,000 to 200,000 crabs, and really nothing else for miles and miles around."

Scientists don't know why crab pile themselves into these huge mounds or the factors that control the female's release of larvae. Yet timing of the larval release seems to be critical to their survival. Stevens thinks the crabs cue on the spring tides, which are among the largest of the year.

STEVENS: "What the crab seem to be doing is that when we get our minor spring tide in April, they start forming their mounds. They start to release larvae a couple of weeks later. The larval release peaks on the high spring tide and then peters off to the next minor spring tide, and then the mounds dissipate after that."

Stevens believes that females all release their larvae at about the same time because it gives newborn crab a survival advantage. Predators like other crab and fish simply can't eat all of them.

Within about six weeks, the newborn crab will go through two larval stages. After that, they'll settle onto the sea floor. With any luck, this class of 2000 will be a bumper crop, enough for fishermen to catch as soon as they reach commercial size—five or six years from now.

OUTRO: This is Arctic Science Journeys Radio, a production of the Alaska Sea Grant Program and the University of Alaska Fairbanks. I'm Doug Schneider.

For more information about Tanner crabs, visit these Web pages:

Tanner Crabs (ADF&G Wildlife Notebook Series)

Alaska Fisheries Science Center, Kodiak Laboratory

Biological Field Techniques for Chionoecetes Crabs

High Latitude Crabs: Biology, Management, and Economics

 
Thanks to the following individual for help preparing this script:

Dr. Brad Stevens
National Marine Fisheries Service
301 Research Court
Kodiak, Alaska 99615-1638
Ph: 907-481-1726
Email: Bradley.G.Stevens@noaa.gov