Agricultural Runoff: Bad For Us, Bad For Otters

Runoff from farmland near the Santa Maria River drains from ditches to the river.

Runoff from farmland near the Santa Maria River drains from ditches to the river. (Photo credit: The Otter Project)

OTTER FACT: Scientists attribute up to 40 percent of southern sea otter mortality to infectious diseases alone, many of which are known to have anthropogenic (human) causes and land-sea linkages.

The Central Coast of California is one of the most productive agricultural areas in the United States. But all that productivity comes at a price to sea otters, and to us. Pesticides used to keep crop-destroying insects at bay and nutrients that increase crop yields foul the rivers that eventually empty into the sea, often right where otters are most at home. The Salinas and Santa Maria Rivers, located where the biggest population of southern sea otters live, are classified as highly toxic, according to Steve Shimek of the Monterey-based The Otter Project. And while the issues otters face as a result of such intensive farming could spell disaster for the furry species, they also stand guard as harbingers of what we humans could face. “Otters are simply out ahead of people,” Shimek says. “Otters eat many of the same things that people like to eat, and otters are steeped in the same water that we swim in when we go to the beach.”

The industrialization of agriculture in the last 50 years —nowhere more evident than on the Central Coast—has been a boon for consumers. As farms have gotten bigger and more efficient, food prices have dropped, and exotic greens, fruits and vegetables are now ubiquitous in supermarkets around the country. But how is such productivity possible? The answer often comes in series of letters that form cryptic chemical names.

The miracle pesticide used to be DDT, which decimated insect pests. Otters are still fighting to break their connection with the noxious pesticide decades after it was banned. Because of their chemical structures, pesticides like DDT attach to sediment. When we flood agricultural lands with water, tons (literally) of this laced sediment washes into rivers. The rivers eventually flow to—you guessed it—the ocean, where shellfish and other filter-feeders siphon suspended sediment particles from the water in their search for food, concentrating the levels of pesticides. Ultimately, this makes for a toxic meal for otters.

“The levels [of DDT] that we’re finding in sea otters are levels that are high enough to kill other animals that are in the same family,” says Shimek. New pesticides have been developed and are currently used by conventional farmers, he adds—chemicals that are supposed to breakdown more quickly. Still, there are a lot of unknowns regarding even these modern pesticides. “There’s lots of science that is looking at the legacy pesticides, but nobody’s even looking at the modern pesticides, so we have no idea,” warns Shimek.

Other problems crop up when farmers flood their fields with irrigated water. Many Central Coast agricultural lands are downstream from intensive farming further inland. This means that the ground water that is the source of irrigation often was already used for agriculture, meaning it likely has a considerable amount of nitrates already in it. Shimek explains: “They are watering with pre-fertilized water, and it takes a modern, sophisticated grower to realize that they can actually save money if they account for the amount of nutrients already in the water.” In fact, he adds, in some cases, the water can have 13 times the allowable amount of nutrients for drinking water, and for rural families dependent on wells, that’s exactly what this water is. Then, when farmers add more fertilizer, they’re bumping up the nutrient load even more. “You kind of have a mess,” Shimek says.

Aerial of Monterey Bay coastline, Salinas River and surrounding agricultural land.

Aerial of Monterey Bay coastline, Salinas River and surrounding agricultural land.

Ideally, plants receive enough water to reach their roots and no more. In practice, Shimek says, it’s easier to simply saturate the soil with nitrate-loaded water. A lot of this water runs off through rivers into the ocean, where the nitrates do just what they do for farmers—encourage plant growth. Only this time, instead of bolstering edible salad and strawberries and asparagus, the nitrates touch off harmful algal blooms. These blooms can cause a host of problems and release the neurotoxin domoic acid into the water where, again, it’s concentrated in mussels and clams, which eventually are eaten by otters. Blue-green algae also cause microcystin intoxication, a potent liver infection that has killed at least 21 otters in the last few years and 11 in 2007 alone.

Shimek and The Otter Project recently led a coalition that fought for legislation to cut down the amounts of toxic material and nutrients that end up in the ocean. “Alhtough it’s far from perfect…what we were able to pass on the central coast was probably the strictest regulation of agriculture in the United States,” he says. Irrigation water, he points out, doesn’t have to meet drinking water standards for pollution levels under the federal Clean Water Act, a loophole he blames for leading to rampant agricultural pollution on America’s coasts. But in California, irrigation must meet stricter standards, and Shimek and other otter advocates leveraged this distinction into getting the limits put in place for the Central Coast

On a smaller scale, California’s coast might have been able to handle the pesticides and nitrates. But a side effect of large-scale agriculture has been the loss of wetlands, not just in California, but around the world. These wet lands act as scrubbers of freshwater before it reaches the ocean, trapping excess sediment particles and the harmful agents connected to them. But too much sediment and too much water clogs the system, choking wetlands’ ability to filter this water before it reaches the sea, not to mention the loss of habitat: wetlands serve as nurseries for countless species of fish, birds, mammals (including otters) and other animals.

Otters are only the most visible species affected by unchecked industrial agriculture. They’re a keystone species, the centerpiece of the ecosystem on which the other members of the system depend—the kelp, for example, that need the sea otter to keep urchin populations down. And now, they’re signaling that we humans need to do something about the farms that grow the food we eat, whether it’s insisting on organic food that requires no pesticides or buying local groceries that don’t need copious amounts of fertilizer to be coaxed from the soil. Unfortunately, it’s not limited to just our farming practices. The stuff we dump into the sea from motor oil to plastics will come back to haunt us. As Shimek puts it, “Otters are telling us that stuff—whether it be pathogens or chemicals—is washing into the water and killing them. And frankly, I think it’s that simple.”


Related Links

The Otter Project Took on Big-Ag, and Guess What… We Won!

Surface Tension: After three years of indecision, a new ag runoff rule takes effect.  Both enviers and farmers say it’s not good enough.

EPA Finalizes California’s List of Polluted Waters

EPA Fact Sheet: Protecting Water Quality from Agricultural Runoff


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About is dedicated to raising awareness about California's threatened sea otters and the role research plays in the species recovery and conservation. It's a collaboration of the Monterey Bay Aquarium, U.C. Santa Cruz, U.C. Davis, U.S. Geological Survey, U.S. Fish & Wildlife Service, California Department of Fish and Wildlife, among others. is also home of the world's first HD live stream of southern sea otters in the wild.