|
Crab Pod
INTRO: Animals, fish and birds have devised lots of ways to avoid being eaten by hungry predators. As Doug Schneider reports in this week's Arctic Science Journeys Radio, even the red king crabs living on the sea floor off Alaska have a trick or two for confusing predators. STORY: Several times each week, crab researchers Pete Cummiskey and Eric Munk make the short drive from the fishing town of Kodiak, Alaska, out past the U.S. Coast Guard base to Womens Bay, a picturesque inlet that at this time of year is surrounded by snowcapped hills and mountains. They launch their small inflatable skiff and motor a short ways off shore. Cutting the engine, they lower a device called a sonar signal receiver into the water. The receiver picks up the sound from tiny transmitters glued to the shells of four juvenile red king crab. After a few minutes, Munk and Cummiskey figure they're close to the tagged crabs. They put scuba tanks over their dry suits and slip over the side into the ocean. Forty feet beneath the surface, Munk and Cummiskey have found more than the four crab they fitted with transmitters. They've homed in on a giant, tangled ball of crab—literally thousands of them—on the sea floor. Cummiskey has seen this behavior before. It's called a crab pod. CUMMISKEY: "Pods are large aggregations of juvenile king crab. The crab pile up in huge piles of thousands and thousands of crab." Cummiskey and Munk have been following these crab since last February, when they were not much bigger than a quarter. CUMMISKEY: "Originally we located this pod of crab and they were too small to tag. We were able to keep up with them until they molted into a larger size and then we put our smallest tags on and hoped it would work. It seems to have." Back on the sea floor, Munk videotapes the pod as Cummiskey uses a meter-long stick to measure the volume of the pod. From that, they estimate the number of crab to be around 3,000. Most have shells only three to four inches across. In a few years, however, their shells will be eight inches or larger, making them the biggest crabs in Alaska.
But at this stage, they're vulnerable to a host of predators, including sea otters and fish, such as large halibut and sculpin. Which is why these young crab stick together, says Cummiskey. CUMMISKEY: "The thought is that it is a predator avoidance thing, that if they are all in one spot the predator would have a hard time finding that group of crab. And if they do find it, then they can only eat so many. It's kind of like herding or schooling and those sorts of behaviors in birds and other animals." Cummiskey says king crab have other ways of outwitting predators. If the pod were attacked by a sea otter, for example, the crab might choose to flee—sort of every crab for himself. When that happens, the scrambling crab kick up a cloud of mud and silt. Predators have a hard time seeing them, much less catching them. As these crab mature, they'll rely on the pod less and less. Cummiskey believes that's because adult crab aren't as vulnerable to predators, but also because larger crab need more food, so they'll spent more time walking the sea floor in search of things to eat. Cummiskey says foraging offers some crab a chance to really explore. CUMMISKEY: "Generally they forage in a fairly tight-knit group. But we've seen these crab find each other from different parts of the bay. We've put two tags in one part of the bay and on the same day put two tags in another part of the bay in small groups of crab. Three or four days later some of those tags have switched places. So they find each other on the bottom." It used to be that red king crab lived in the waters around Kodiak Island in unbelievable numbers. Today, however, red king crab are scarce, even here in Womens Bay, a spot long believed to be a nursery for crab. No one knows for sure why, which is where Pete Cummiskey and Eric Munk come in. Both are scientists with the National Marine Fisheries Service, the federal agency charged with finding ways to help king crab stocks recover. CUMMISKEY: "In order to have big king crab you have to have little king crab. In order to have successful recruitment of little king crab you need to have the right conditions. We're trying to define those conditions, some of the areas that crab utilize and some of the behavioral aspects. How often do they molt? How much do they grow when they molt? How long does it take them to become sexually mature? That's what we're trying to do." Cummiskey says just coming to Womens Bay each week to see how this crab pod is doing, is important. For example, they've noticed that the number of juvenile red king crab in the bay has gone way up in the last two years. It's too soon to say whether that means red king crab are on the rebound, but it's a trend he hopes will continue. 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. Audio version and related Web sites Thanks to the following individuals for help preparing this script: Peter Cummiskey Eric Munk Arctic Science Journeys is a radio service highlighting science, culture, and the environment of the circumpolar north. Produced by the Alaska Sea Grant College Program and the University of Alaska Fairbanks.
Alaska Sea Grant In the News The URL for
this page is http://seagrant.uaf.edu/news/ |
Listen to story on RealAudio Related Web sites Kodiak Fisheries Research Center High Latitude Crabs: Biology, Management, and Economics (proceedings from crab symposium) |
