It's a bleak time to be a seafood lover.
It seems like every day, reports of worldwide overfishing clog our news feed, warning us that soon, we'll no longer be able to enjoy tasty halibut steaks, crispy battered cod, and lunchtime spicy tuna rolls. But fishing isn't the only human activity that's leading to massive losses of the marine species we love to eat: our chemical-intensive farming practices, as well as our propensity to pollute the air with greenhouse gases, are creating massive ocean "dead zones."
These huge swaths of water contain such a low concentration of oxygen that fish can't survive in them: they either migrate out of them, or die. In 2001, a Narragansett Bay dead zone killed off 4.5 billion mussels, about 80 percent of the total population of bivalves. The Gulf of Mexico is home to the world's second-largest dead zone—it's currently about the size of Connecticut—and each year, the area's well-established shrimp industry faces crustacean losses that cost it between $300 and $500 million each year. And the problem isn't unique to US waters: the world's largest dead zone is located in the Baltic Sea, where both cod and the small sprats that they eat are perishing.
Worldwide, ocean dead zones have doubled in frequency every ten years since the 1960s, and there are now more than 400 of these oxygen-depleted areas around the globe. The primary cause of dead zones is agricultural runoff: during times of heavy rain, nitrogen and phosphorus found in both chemical and natural fertilizers—aka manure—washes into these bodies of water. There, the excess levels of these nutrients cause algae to grow more robustly than usual; when the algae dies, it sinks and decomposes in the water. Its decomposition process uses up oxygen, reducing what's available for fish and shellfish to breathe. They die, and we're left wondering what to make for dinner.
Keryn Gedan is a researcher and professor at the University of Maryland's biology department. Recently, she and fellow biologist Andrew Altieri examined the growth of dead zones for a paper published in last November's issue of Global Change Biology. Living in Maryland, Gedan's work led her to the Chesapeake Bay, which is plagued with dead zones; half of the nitrogen pollution that enters the bay is a result of agricultural runoff. Gedan said that seeing huge populations of dead fish in the Bay is off-putting, to say the least.
"I've seen fish kills in the estuaries where I've worked," she said, "and they make a big impression. They have an important effect on human well-being; they tend to scare people away from the water."
The way we farm might be the primary cause of the acceleration of ocean dead zones, but the rest of what do isn't helping, either: the billions of tons of carbon dioxide that we spew into the atmosphere each year, accelerating global warming, is deadening our oceans, too. Climate change's effect on ocean dead zones was the primary focus of Gedan's paper, and what she and Altieri found wasn't heartening: about 94 percent of worldwide dead zones will warm by 3.6° Fahrenheit or more by the end of the century. That's bad news for several reasons, Gedan said.
"First, warmer water holds less oxygen; that's just basic chemistry," she said. "And at the same time, organisms living in warmer temperatures demand more oxygen. So that's kind of a major double whammy."
Plus, Gedan said, climate change makes for a hotter, longer spring and summer season, giving warm temperature-loving algae an even lengthier growing season: More of them will bloom, and die, and use up all the oxygen that's needed by fish and shellfish.
When it comes to agricultural overdependence on chemical fertilizers, Gedan was hesitant to criticize current farming practices. "We have to feed the people," she said. The key to limiting the growth of dead zones—which is still possible, according to Gedan—is to work with farmers on best management practices, which might include cutting back on fertilizers but which also means leaving "vegetative barriers" in place. Instead of clearing all forests, grasslands, fields, and wetlands before sowing a crop, farmers should make sure to leave some natural habitat in place, Gedan said. As phosphorus- and nitrogen-heavy runoff passes through those ecosystems, the nutrients get caught up in the plants and soil "instead of just charging right into the water," Gedan said.
"It's all about figuring out the best ways to make changes that will have the least negative impacts on our environment," she said.
The future of our happy-hour plate of oysters depends on it.