 MIGRATION INVESTIGATION -- UNDERSTANDING THE MIGRATORY BEHAVIOR OF FEMALE BLUE CRABS COULD PROVE BENEFICIAL TO PROTECTING THE SPECIES Orlando Sentinel -- Sunday, April 8, 2001 By: Steven Ford, Sentinel Staff Writer The blue crab might be as ubiquitous as it is delicious, especially here in Florida, where the predatory crustacean is the mainstay of an almost $11 million fishing industry in the state. And although scientists already know a lot about the life cycle of the Callinectes sapidus, a researcher at the Florida Institute of Technology in Melbourne hopes to learn more about the decapod's migratory behavior and gain a better understanding of how human factors can affect the species. The blue crab thrives in abundance in Atlantic and Gulf of Mexico waters and can be found from Nova Scotia to northern Argentina. Scientists know how the animals live, travel and reproduce. They also know that precise water-salinity levels are needed for successful spawning and that female blue crabs use tidal currents to migrate to specific salt-water habitats to accomplish this. What scientists don't know, however, is what exactly triggers this migratory behavior. Next month, Florida Tech professor Richard Tankersley will track crabs to better understand this behavior. He and his students will monitor blue crabs off the coast of Brevard County and North Carolina. If scientists can better understand the factors involved in this migration, Tankersley says, they can know more fully the ways in which altering blue crab habitats will affect the species. "One of the byproducts of this, from a fishery perspective, is to determine where these spawning areas might be and possibly set aside these areas for protection," Tankersley said. Blue crabs typically live in the brackish waters of estuaries, where the crustaceans go through a series of moltings as they shed their exoskeleton in order to grow. However, the eggs carried by fertilized female crabs (called sponge crabs because of the sponge mass of eggs attached to the female's shell) require a higher saltwater density in order to hatch. Therefore, female crabs migrate to saltier waters to spawn. To accomplish this, sponge crabs with late-stage eggs use a unique mode of tidal transportation. During outgoing ebb tides, the crabs tread water near the surface and allow the tide to move them down the estuary and toward offshore waters. During incoming flood tides, however, the crabs drop toward the bottom to avoid being pushed back toward shore. "They sort of leap-frog their way down the estuary," Tankersley said, calling the phenomenon "selected tidal stream transport." Using a $109,000 grant from the National Science Foundation, Tankersley and his students will track this migration by attaching ultrasonic transmitters to female crabs. By monitoring the frequency characteristics of the ultrasonic "pings" produced by the transmitters, the researchers will detect the path and depth of the crabs as they migrate. "One of the things that we're trying to look at is how currents influence where [the crabs] end up when they go to spawn," Tankersley said. "Because, even though they may be coming from a variety of different locations, we think that there may be a migratory corridor -- and area where they all are concentrated because the currents concentrate to that position." And human factors -- such as inlet dredging, for example -- can affect such currents, he said. Similar migratory behavior is demonstrated by other aquatic species, Tankersley said, adding that, "Even though it seems to be fairly widespread, we don't understand well enough the underlying mechanism that controls it." About 20 individual crabs will be tracked during the two-year study. Tankersley and his students will track crabs in the Indian River Lagoon near the Sebastian Inlet to gather data from a lagoonal environment. Working with researchers at Duke University and the University of North Carolina, Tankersley and his students also will track blue crabs off the North Carolina coast to gather data from a different inlet environment. Blue crabs long have been a seafood staple on restaurant menus up and down Florida's coasts. But in recent years, commercial crabbers here have reported a decrease in their catch. Anne McMillen-Jackson, an associate research scientist at the Florida Marine Research Institute in St. Petersburg, said that, although it is difficult to quantify the abundance of blue crabs in Florida waters, data on commercial landings have shown a general decrease in the number harvested. A severe decline in the blue-crab population of Chesapeake Bay has led to harvest restrictions and some fishing bans in that area. And it is a situation, say both McMillen-Jackson and Tankersley, that can have a bearing on Florida, too. Citing the recent crab decline in the Chesapeake Bay, Tankersley said, "I think that there's the potential that, as those fisheries begin to decline, there would be additional pressure on the Florida fishery. "So I think that understanding some of these phenomena and understanding more about the life history of these animals is critical to making sure that we protect the fishery here in Florida and we don't run into the same problems that they have run into elsewhere." Copyright (c) 2001, Sentinel Communications Co. |