Coastal Dead Zones May Lead To Ecosystem-based Fisheries Management

Larry Crowder, who is Stephen Toth Professor of Marine Biology at Duke's Nicholas School of the Environment and Earth Sciences, will discuss these potential long range impacts of nutrient runoff from distant farm fields at a 1:45 p.m. Feb. 20 symposium during the American Association for the Advancement of Science's 2005 annual meeting in Washington, D.C.

The symposium will take place in Workshop Room E of Exhibit Hall B North at the Marriott Wardman Park Hotel.

Crowder's research group has been studying a so-called "dead zone" that forms annually off Louisiana in the Gulf of Mexico. Water within this approximately 20,000 square-kilometer-wide bottom feature is robbed of much of its dissolved oxygen each spring in a biological response to farm fertilizer runoff that may originate far upstream along the Mississippi River, he said.

After entering the Gulf, these fertilizer nutrients fuel population explosions among microscopic marine plants. As the marine algae grow and then die, they consume most available oxygen in bottom layers.

Crowder described how fish and shrimp can evade death simply by relocating to the zone's edge. "Basically, it's the same as the way forest wildlife might aggregate on the edge of a forest fire," Crowder said in an interview. And shrimpers and fishermen exploit that knowledge by positioning their nets at the zones' periphery too, he added.

Crowder's group is studying whether this annual convergence may result in overfishing, in significant upsurges in inadvertent nettings of other untargeted marine species, called "bycatch," or in other less obvious delayed effects that may reduce commercial production.

"People have been aware for almost 20 years now that these low-oxygen zones form," he said. "But, until our work, there hasn't been a serious look at impacts on target species, shrimp, or on the fish and sea turtles that are taken as bycatch ."

Crowder said that his group's initial findings are pointing to various ways that living along dead zone edges may lower growth rates of some Gulf fish and shellfish.

"None of these lines of evidence in themselves would make a compelling case," he said. "But when you pull them all together it makes for a pretty interesting case."

His previous published studies of analogous low-oxygen zones that develop in North Carolina's Neuse River documented 30 percent reduction in growth among croakers living on the zones' margins, Crowder said. That translates into 40 percent reductions in the numbers of harvestable-sized fish.

Such zones in the Neuse have also been implicated in fish kills when wind conditions and currents conspire to trap the fish on the bottom in a way that makes it hard for them to escape the low-oxygen waters, he added.

In the Gulf, "to date no one has been able to make a link between the dead zone and the production of valuable fisheries," Crowder said. "People say, 'so what?' What impact does it have on fisheries, if any?"

If scientists do document such an impact, regulators might someday invoke a new emerging "ecosystem-based" approach to fisheries management, Crowder said. "If we are going to ask farmers in the Mississippi drainage basin to reduce nutrient loading, we need to show that it will solve the problem.

"In this symposium, I will show that things we do to the ecosystem in terms of nutrient additions can ramify to effects on fisheries," he said. "So we will have to manage fisheries in the context of changes generated somewhere up in a watershed, thousands of miles away."