Two recent studies blame farm runoff for separately causing problems in rivers and the ocean that have injured fishing around the world.
Last week, the prestigious journal Science reported that there are now more than 400 dead zones in the ocean, double the number the United Nations reported just two years ago.
Scientists Robert Diaz of the Virginia Institute of Marine Science and Rutger Rosenberg of the University of Gothenburg in Sweden, found 405 dead zones in coastal waters worldwide today, affecting about 95,000 square miles of ocean, an area about half the size of California.
These rapidly multiplying dead zones occur when fertilizer runoff dumps excess nutrients, such as nitrogen and phosphorous, into coastal waters, providing food for algae. When these microscopic plants die and sink to the ocean bottom, bacteria feed on them and subsequently consume all the oxygen dissolved in the water, summarized online science writer Andrea Thompson reported.
This leaves fish and other bottom-dwelling sea creatures without enough oxygen to survive, causing mass die-offs and displacements. Typically, the researchers noted, these events aren”t noticed until they threaten valuable fish stocks.
The largest dead zone in the United States sits in the Gulf of Mexico at the mouth of the Mississippi River and is about the size of New Jersey. Scientists have predicted that the Gulf dead zone could grow larger than ever this summer. Hotter weather increases the size of dead zones.
Scientists reported in Science that most of the newest dead ocean areas are found in the Southern Ocean off South America, Africa and Asia. The Baltic Sea is the worst spot in the world.
“If we screw up the energy flow within our systems we could end up with no crabs, no shrimp, no fish. That is where these dead zones are heading unless we stop their growth,” Diaz said in a telephone interview with the Associated Press.
Removal of wetlands in many urban areas is another cause. Wetland areas serve to remove toxins from water before it enters the ocean.
San Francisco Bay used to create a large dead zone off Northern California. But that issue has gotten dramatically better due to environmental regulations (especially of sewage treatment plants) in the 1970s through 1990s.
According to Diaz”s survey, the few dead spots along the California coast develop only periodically where water circulation is limited. They include the inland portion of Elkhorn Slough near Moss Landing in Monterey County and Alamitos Bay at the mouth of the San Gabriel River near Long Beach.
However, because the problem is so severe in the Gulf of Mexico and in the Atlantic, studies have not focused on the northern Pacific.
New low-oxygen areas have been reported in Samish Bay of Puget Sound, Yaquina Bay in Oregon, prawn culture ponds in Taiwan, the San Martin River in northern Spain and some fjords in Norway, Diaz said.
A portion of Big Glory Bay in New Zealand became hypoxic, or oxygen starved, after salmon farming cages were set up, but began recovering when the cages were removed, he told the Associated Press.
The Bush Administration is vigorously pushing offshore fish farms.
There were just 69 dead zones found in the first study, done during the 1960s.
Another recent study may be less frightening globally but may have more impact on salmon in Northern California
A weak mix of pesticides in river water dampens a salmon”s sense of smell, say researchers. In experiments, steelhead rainbow trout exposed to low levels of 10 common agricultural pesticides could not perceive changes in levels of a predator”s scent, NewScientist online reported.
Salmon have much lower success rates in getting to the ocean than in past years. Predators, such as the invasive pike minnow, are blamed, along with low water levels, warmer waters that cause disease and other factors.
A depressed sense of smell might also keep fish from finding mates and food, the researchers found. Trout are closely related to salmon, and, though the theory needs more study, pesticides may be a cause of plummeting salmon stocks in Canada and the U.S., lead scientist Keith Tierney said.
