Gulf 'dead zone' reduces brown shrimp catch

The oxygen-starved "dead zone" along Louisiana's coastline is causing a measurable reduction in the number of brown shrimp caught by Louisiana and Texas fishers, according to an ongoing study by federal scientists.

Preliminary results from the study were presented recently in New Orleans by Rick Hart, a biologist at the National Oceanic and Atmospheric Administration Fisheries Galveston research laboratory. He addressed the Water Environment Federation conference in New Orleans.

The study used shrimp landings data reported by state wildlife agencies and by NOAA Fisheries for each of nine geographic zones in Texas and Louisiana. Researchers compared that with the size of viable brown shrimp nursery areas in Louisiana wetlands and to the area affected by hypoxia, the scientific name for low-oxygen conditions.

Nutrient-rich water from the 31 states in the Mississippi River watershed enters the Gulf of Mexico from the mouths of the Mississippi and Atchafalaya rivers during the spring and summer, forming a freshwater layer over the saltier Gulf waters.

The nutrients create huge blooms of algae that eventually die and sink to the Gulf bottom, where they decompose, using up oxygen trapped in the saltier layer.

Oxygen levels of less than 2 parts per million -- the level at which many species of fish and shellfish die or avoid the area -- have been measured in bottom waters over as much as 8,000 square miles of the Gulf off Louisiana's coastline in recent years. Scientists believe the condition has expanded dramatically since the increase in the use of agricultural fertilizers in the Midwest in the early 1950s.

Brown shrimp can be killed by oxygen levels that low, so they avoid those areas, according to past NOAA Fisheries research.

Though scientists have long suspected that hypoxia has affected landings of shrimp and other fish species, they have been hesitant to blame low oxygen until other possible reasons were eliminated.

The study previewed by Hart deals with one of the major issues that confounded researchers: the annual size of the area used as a nursery by brown shrimp in Louisiana wetlands.

Adult brown shrimp live in deep Gulf waters off Louisiana and Texas, where they spawn. The eggs and recently hatched larvae drift into the Louisiana wetlands, where the shrimp grow to juvenile size before beginning their offshore migration. The entire cycle takes about six months.

The in-shore area preferred by brown shrimp consists of a mix of wetland vegetation and water with a salt content of 10 parts per million. The new study indicates that the larger that area is each year, the larger the number of shrimp caught.

Hart found that larger hypoxia areas corresponded to smaller catches of shrimp, whether the inshore maturation area was large or small.

The study does not include white shrimp, the other commercially significant shrimp species caught by Louisiana and Texas fishers, Hart said, because that species tends to stay closer to shore and out of waters affected by hypoxia.

Exploring other possibilities

Jim Nance, a co-author of the study and chief of the fishery management branch at the Galveston laboratory, said there still is not enough evidence to say that the catch reduction is the result of hypoxia killing the shrimp.

"The models aren't that sophisticated to jump there yet," he said. "It may be that shrimp are moving to areas that are just not conducive to harvesting."

He said the next step in the study is to try to factor in the effort used by individual fishers to catch shrimp. Shrimpers are believed to be forced to travel longer distances to catch each pound of shrimp in years with larger hypoxia areas, but their efforts also may be affected by the price of fuel.

So far, Nance said, it seems as if hypoxia does have a greater effect.

"We've gone back through data for 1986 through 2003, and there seems to be a good relationship between hypoxia and landings," he said. "And that's taking into account that often you had many other different market factors going on. Fuel prices bounced around, but the relationship holds up."

A paper describing the research conducted by Hart, Nance and co-author Roger Zimmerman, director of the Galveston laboratory, is expected to be published in a peer-reviewed scientific magazine later this year, Hart said.

By Mark Schleifstein
Staff writer Times-Picayune