Nutrient runoff and the "dead zones" they create are a growing problem around the world. But what's their impact on fish?
A new study published in the journal Proceedings of the National Academy of Sciences suggests that this pollution reduces fish populations by disrupting the near-shore areas where the animals spend their fishy youths.
Marine ecologist Brent Hughes at the University of California-Santa Cruz and colleagues spent years tracking the health of a fish called English sole, in a salt marsh called Elkhorn Slough. It's found in Monterey Bay, which lies on the Pacific Coast, south of Santa Cruz. He discovered that for the most part, populations of these (and other fish) suffered when nutrient levels were high. This was accompanied by declines in the number of fish that grew large enough to move offshore and into the open ocean.
When nutrients found in fertilizers and sewage, specifically nitrogen and phosphorous, accumulate in the water, this leads to the development of algae blooms, which turn the water green and die shortly thereafter. As this material decomposes, it consumes oxygen, creating a so-called "dead zone" in the water, Hughes explains. This can kill fish or force them to move elsewhere, although the extent of this effect isn't always easy to quantify, he says.
Perhaps counterintuitively, large rainfalls can help reduce the problems caused by nutrient runoff, by flushing all the material further out to sea, Hughes notes. For that reason, El Niño may be good for young fish in California's estuaries. When El Niño conditions are in place, water temperatures in the tropical Pacific are above average, which has wide-ranging effects on climate around the globe, including more precipitation in parts of the Golden State.
But continuing agricultural land development and urbanization creates more nutrient pollution, leading to dead zones that are becoming more common every year, which is bad news for fish, Hughes says. There are steps humans could take to limit nutrient runoff—such as using less fertilizer in agriculture, developing better sewage treatment plants and planting trees along drainage ditches and water bodies—and, thereby help fish populations, the study suggests.
