ANAHEIM, Calif -- About 80 percent of the commercially important seafood species along the Southeastern United States spend parts of their lives in estuaries, brackish coastal nursery areas that show signs of being degraded by human activities, according to a Duke University marine ecologist.
"When salt marsh habitat and production is lost, when estuarine creeks and rivers are anoxic (oxygen depleted), when seagrass beds are disappearing, all those things are telling us that the habitat on which these organisms depend is being degraded," said Larry Crowder, a professor at Duke's Nicholas School of the Environment Marine Laboratory in Beaufort, N.C.
"One reason we're at the table talking about habitat is because many of these fish stocks are heavily over-harvested," he added. "We have to examine what are the habitat effects and how serious they are."
Crowder spoke in an interview before delivering the same message in a report prepared for presentation Sunday at the American Association for the Advancement of Science's annual meeting. He said affected fish and shellfish include such well-known species as flounder, spot, croaker, menhaden, blue crab and shrimp.
The Duke researcher, formerly a faculty member at N.C. State University in Raleigh, spent about seven years in a National Oceanic and Atmospheric Administration-funded study of the life histories of estuary dependent fish that range between Cape Canaveral, Fla., and Cape Hatteras, N.C.
He is currently studying how marine life respond to changing conditions in North Carolina's largest estuarine system, Pamlico Sound, as well as on the Neuse River that supplies the sound with part of its upstream water.
Research suggests that after spawning out on the continental shelf in the winter, many Southeastern species then migrate shoreward into estuaries the following spring. They then spend the first vital year or two of their lives there, in the bays, tidal creeks and river mouths that pocket the Southeastern coastline.
In contrast to the winter months, when offshore Gulf Stream waters are comparatively warmer and richer in nutrients, estuaries offer juvenile marine life better sanctuaries and more plentiful food supplies in the spring and summer, Crowder said.
"They make the transition into an estuary at about the time when it becomes warmer and more productive than the continental shelf," he said. "They evolved that lifestyle for a number of reasons. Basically, their life history tracks productivity cycles."
Some scientists also have proposed an "out-welling hypothesis," which suggests that organic material emanating from estuarine salt marshes also finds its way to the offshore waters, where it provides nutrients to animals that live there, Crowder added.
"The out-welling hypothesis provides a lot of impetus for the protection of salt marsh habitats because those not only support the nursery phase of food webs, but their continental shelf phase as well," he said.
A second hypothesis posits that such nutrients do not diffuse directly from estuaries to the open continental shelf ocean, but rather get there indirectly within the bodies of the fish that move back out there after their juvenile phase. Crowder said that second hypothesis might be especially relevant in North Carolina estuaries, because outward water movement there is restricted by the barrier island chain known as the Outer Banks.
Both scenarios would make estuaries another kind of nutrient source for ocean life in addition to the well-known upwelling zones off California, Africa and Chile, he said.
Crowder's bottom line is that "if we want to keep these systems working to our benefit in terms of fisheries production, we need to protect the nursery area habitats so that fish can feed and grow successfully in there.
"Having much of the Chesapeake anoxic is not protecting the habitats of the fish," he said. "Losing seagrass or salt marsh is not doing the job, nor is losing mangroves."
Scientists have gathered increasing evidence that estuaries like the Chesapeake Bay and North Carolina's Pamlico and Albemarle sounds are being degraded by nitrogen from agricultural operations, lawn fertilizers and sewage, the destruction of wetlands for construction projects and other causes.
Excess nitrogen can cause microscopic plants to "bloom" explosively in estuarine systems and then rob the water of oxygen when they die and decay.
CONTACT:
James Rattray
919-613-8017
Jrattray@duke.edu
Note to editors: Larry Crowder's talk at the America Association for the Advancement of Science annual meeting is scheduled for a session beginning at 9 a.m. Sunday in the Anaheim Marriott Hotel's Orange County Ballroom, Salon 3.