Fish can avoid dead zones, but they wreak havoc on the lower end of the food chain.
A Virginia marine biologist is studying a silent killer in oceans and bays. It's a phenomena that, just two centuries ago, was widely reported as a miracle in places like Mobile, Ala.
"The publishers of the [Mobile Register] said that all good citizens of Mobile should run down to the shore; that God has given them a gift of crabs and fish," says Robert Diaz, a marine biologist at the College of William and Mary. "What the paper didn't know is, it was reporting for the first time, something called a jubilee."
But this miracle, this jubilee, was caused by dead zones, and as you can probably guess, a dead zone isn't a good thing. A dead zone is an area in a river or bay that doesn't have enough oxygen in the water to sustain the presence of fish, crabs, and shrimp.
"And rather than being a natural phenomenon, they are related to human activity almost exclusively," says Diaz. "The biggest problem comes from sewage and nutrients that are running off of agricultural lands."
When an area starts to lose oxygen, the fish and crabs are able to just swim away, but the clams and creatures that the bigger guys eat, can't leave. Instead, they suffocate. Eventually, oxygen might return, and with it come the fish and crabs. When they do, there's no food to be had.
It's a process that bore itself out in the Black Sea in the 1960s. There were fisheries everywhere, at least until a dead zone developed.
"By the '80s, most of the bottom fisheries were no longer in existence," says Diaz. "The fishermen who worked in this area had to go somewhere else."
While this collapse of fisheries hasn't happened yet in the U.S., there's a real fear that it might. "If we keep things the way we're going, by the year 2050, we're looking at close to half a billion dollars in economic losses," says Diaz.
In 2011, a dead zone in the Chesapeake Bay was among the worst on record, stretching from the Baltimore Harbor to below the Potomac. High temperatures and heavy rains, along with persistent problems with pollution, were said to be largely to blame.
The good news? As long as we can strike before a major disaster, dead zones are reversible. If we could reduce agricultural runoff by 20-25 percent, Diaz says, we would see significant reductions. He's optimistic that cities will recognize the problem and take measures to stop it.
One great example is Tampa Bay. Dead zones were causing them to lose sea grasses, which are important not only for fisheries, but also for recreation and tourism: "So, through a series of organizations and volunteer reductions, you saw an improvement in water quality — if you take away the stressers, these systems do recover."