Dead zone impacts marine life now, possibly fishermen later

August 06. 2007; Houma Daily Courier

HOUMA — The dead zone in the Gulf of Mexico is good for shrimpers now, because they are scooping up shrimp fleeing the oxygen-depleted water in droves.

But experts say that over time the dead zone could result in smaller catches, both in terms of shrimp size and the overall numbers netted.

The dead zone is a virtually lifeless 5,200 square-mile area of low oxygen reaching from waters off Galveston, Texas, all the way to Plaquemines Parish.

"We’ve created this area the size of New Jersey where marine animals just can’t live," said Matt Rota, water-resources director for the Gulf Restoration Network.

Researchers from Cocodrie’s Louisiana Universities Marine Consortium, or LUMCON, had previously predicted that this year’s dead zone could be the largest ever, perhaps more than 8,500 square miles.

Though smaller than expected, the dead zone is the third-largest ever, and still a long way off from the 2,500 square miles scientists are hoping to eventually shrink it to, said LUMCON Director Nancy Rabalais.

On a recent trip to measure the dead zone, Rabalais said she saw many bottom-dwelling creatures like eels, crabs, shrimp and even schools of string rays paddling near the surface to escape the suffocating waters below.

The animals are obviously stressed, Rabalais said, and being forced to the surface during daylight hours can leave them more vulnerable to predators.

Kevin Craig, a dead-zone researcher with Duke University, said shrimp swim away to escape the dead zone and are pushed inshore, benefiting shrimpers who trawl the bays. Shrimp also tend to crowd around the dead zone’s borders. Craig noted that shrimpers seem to have caught onto the effect, and he frequently sees them trawling along the dead zone’s edges.

And while this can be a boon to business for now, in the long term it could have unwanted side effects.

Shrimping the edges of the dead zone could increase bycatch — the number of unwanted marine animals that get trapped in nets along with shrimp — as other distressed dead-zone inhabitants try to escape suffocation.

And, if shrimp aren’t spending enough time in a quality Gulf habitat, they may begin to lose body fat, Craig said.

"That might mean smaller shrimp, which has obvious economic consequences," Craig said.

Craig also said there are less shrimp in the Gulf, a phenomenon he attributes to the dead zone as well as increased fishing and loss of habitat due to wetland erosion.

"There’s no doubt that there are short-term benefits for fisheries," he said. "The question is, how do these short-term benefits trade off with the long-term effects of habitat loss?"

If oxygen stays low long enough, there could eventually be mortal consequences for some species on the lower end of the food chain, Rabalais said. Even animals that burrow in mud bottoms will eventually die off, she said, explaining that after time they’ll emerge in search of oxygen. Sea anemones will lay open and less responsive, brittle stars will contort their bodies to try to bend toward the surface, and clams will reach out of their shells, trying to reach oxygen.

Some of these species, like brittle stars, which reproduce by laying eggs, would take a long time to recover, Rabalais said.

And while fish, shrimp, crabs and other swimming animals can escape the dead zone, the larvae of the bottom-dwelling animals they feed on will be greatly reduced.

"If the oxygen has been low enough, even the following spring populations won’t recover enough," Rabalais said. "It takes generations to move back into that area."

The dead zone has had a large presence off the Louisiana coast for nearly two decades, though scientists have noted appearances on and off since the 1950s.

Increasing pollution in the Mississippi River is the source of the problem, researchers say. Fertilizer runoff from farms and treated sewage pumped into the river are just a few of the sources.

Increased corn production to make ethanol and other biofuels also could be a factor, Rota said. U.S. farmers planted 19 percent more corn in 2007 than they did in previous years, according to USDA statistics.

"The amping up of ethanol production without incentives for cleaner farming, well, it doesn’t bode well," Rota said.

The dead zone is formed when warm river waters flow onto the top of the Gulf of Mexico carrying excess nitrogen and phosphorous that, when mixed with sunlight and warm summer waters, fuel enormous algae blooms on the water’s surface.

These blooms eventually die and sink to the bottom to decay in large numbers. That process sucks all of the oxygen out of water at lower depths, and bottom-dwelling critters, like shrimp, crabs and mollusks, must either escape or suffocate.

"These animals — shrimp and crabs — they have a variety of habitats, so we’re looking at the entire life cycle, and where the major life events occur," Craig said. "But here we have a huge chunk of real estate basically being taken out of production. Over the long term it seems that would have to have some sort of effect."