By Christopher Intagliata | Scientific American
February 8, 2017

The low-oxygen waters of the dead zone in the Gulf of Mexico result in smaller shrimp, and a spike in large shrimp prices. Christopher Intagliata reports.

Every spring, the Mississippi River dumps tens of thousands of tons of nutrient runoff into the Gulf of Mexico. Add temperature, current and wind to that pollution, and you have the Western Hemisphere’s largest stretch of oxygen-poor waters—a so-called “dead zone.”That dead zone hits the Gulf’s famed—and financially important—brown shrimp fisheries. And it does two things: first, the low oxygen slows down the shrimps’ growth.

“The other thing that occurs is what I like to call the burning building effect.” Martin Smith, an environmental economist at Duke University. “The shrimp try to avoid the low oxygen so they swim out of these areas of depleted oxygen. As a result they end up kind of aggregating on the edges. They kind of line up outside the deoxygenated waters. And that’s why I call it the burning building effect. If you’re in a burning building you’re running to get out of the fire, you don’t keep running when you get outside, you stop and you take a breath.”

Fishermen flock to where those shrimp “take a breath.” And shrimp get caught earlier in the season. So combine these two effects—slower growth and earlier catches—and the result is a haul of more small shrimp, and fewer large and jumbo shrimp. Meaning the price on big shrimp temporarily goes up. Supply and demand, right?

Smith and his team studied that link—between the dead zone and a spike in large shrimp prices—using 20 years of shrimp pricing data. Their analysis is in the Proceedings of the National Academy of Sciences. [Martin D. Smith, Seafood prices reveal impacts of a major ecological disturbance]

The brown shrimp fishery in the Gulf was once the most valuable in the U.S. Now, Smith says, we can measure the true cost of that nutrient runoff. “We can start to ask questions like, how much does the shrimp industry lose as a result of this problem, and how does that compare to what it would cost to control nutrient flows coming from food prediction upstream in the Mississippi watershed?” In other words—whether there might be some net economic benefit to keeping the water environmentally protected.