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Dead Zones and Fisheries

September 5, 2008

Dead zones have been getting a lot of airtime lately. (No, not those spots where your cell phone loses signal right when you’re talking to someone important-we’re talking about those places in the ocean where things actually die.) Why the sudden interest? In 2004 there were only 146 dead zones worldwide-now there are 400 plus, according to a recent paper published in the journal Science. And while cell phone dead zones can be a pain in the ass, the marine variety can hit us where it really hurts-the dinner plate.

Dead zones-those nasty areas of low oxygen water that kill every critter that can’t crawl, slither or swim fast enough to get away-tend to occur in coastal areas where a lot of extra nutrients (a.k.a. poop) are dumped into the water. Coastal waters are already plenty nutritious-receiving enough normal runoff for fisheries to thrive. But by adding poop and chemical fertilizers to the mix, we upset the natural balance. The sudden boost in nutrients leads to an algal bloom and when the algae eventually die, they sink to the bottom and decompose. Because decomposition robs the water of oxygen, immobile and slow-moving critters like oysters, crabs and small fish suffocate. Faster fish (like tuna) can get out before they asphyxiate, but they’re still screwed-they depend on the slow pokes and the slow pokes are now dead. (You can read more about the relationship between poop and dead zones on our post about Burping Cows.)

The Gulf of Mexico dead zone, the best-known dead zone in North America, is a perfect example of this cascade effect. The authors of the paper in Science also reported that the dead zone in the Gulf removes 17,000 metric tons of carbon per year from the demersal fisheries (i.e. those that target fish that live on the bottom, like flounder and crabs) 17,000 metric tons of carbon per year. Hunh?? In other (much clearer) words, thanks to the big-ass dead zone in the Gulf of Mexico, fishermen are missing out on an extra 170,000* metric tons of fish.

Most of the dead zones around the world, like the Gulf of Mexico, are in areas that support important fisheries. One of the worst, however, was in the Black Sea. In the early ‘90s, when the Soviet Union was still going strong, fertilizer runoff from industrial farms in the Black Sea watershed caused a huge dead zone to form. With the fall of the Soviet bloc, the industrial farms shut down and the dead zone disappeared. But now farmers are starting back up in the watershed with their fertilizers and scientists say that the Black Sea dead zone could come back.

The Black Sea offers up some hope that these fertilizer-induced dead zones are fixable-even the Gulf of Mexico dead zone shrinks in years when less water flows down the Mississippi (less water means less polluted runoff). But before you go getting all “anti-farmer,” there’s something you should know: Some dead zones are caused by Mama Nature herself… granted, she’s a little stressed these days.

Areas of low oxygen along the west coasts of North America and southern Africa and in the Bay of Bengal are caused by the forces of nature. These are all upwelling systems where cold, nutrient-rich water from the ocean depths wells up at the coastline to mix with warm, oxygenated surface water. But shifts in weather patterns (which some researchers attribute to climate change) are causing stronger upwelling, which means more nutrients are getting dumped into the system. This leads to the same outcome as we see in the fertilizer-induced dead zones-extra nutrients lead to decomposing algae on the bottom and asphyxiating fishes.

So, it looks like the bottom line here is that poop (and its chemical equivalent) and climate change can have the same effect on our fisheries. How’s that for food for thought?

 

* By the way, 17,000 metric tons of carbon equals 170,000 metric tons of fish because the “wet weight to carbon” ratio for demersal fish is about 10 to 1. Aren’t you glad you asked?

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One Comment leave one →
  1. Abigail permalink
    September 11, 2008 3:57 pm

    Nicely explained, thanks!

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