A huge "dead zone" of water that has spread across the Gulf of Mexico may be contributing to an unusual spate of shark bites along the Texas coast.
In the last 30 years, the dead zone has been an annual event, fed by the rising use of nitrate based fertilizers.
The extensive area of uninhabitable water may be contributing indirectly to a rise in shark bites in Texas waters.
Three people have been bitten by sharks along the upper Texas coast this year - which is a higher number than normal.
The dead zone has spread across 5,800 square miles (15,020 sq km) of the Gulf of Mexico and is so devoid of oxygen that sea life cannot live in it
Texas border
Nancy Rabalais from the Louisiana Universities Marine Consortium said the dead zone extended from the mouth of the Mississippi River, in south-eastern Louisiana, to near the Texas border 250 miles (400 km) west.
She also claimed the dead zone is closer to the shore than usual this year, because of winds and currents.
"Fish and swimming crabs escape from the dead zone," said Dr Rabalais. "Anything else dies."
In the last 30 years, the dead zone has become an annual summer phenomenon, because farmers in the Mississippi watershed are using more nitrate-based fertilizers, Dr Rabalais said.
The nitrates, carried into the Gulf's warm summer waters by the river, feed algae blooms that use up oxygen and make the water inhospitable to other forms of life.
The dead zone's size has varied each year depending on weather conditions, but on average it is about 5,000 square miles (12,950 sq km), and remains in place until late September or early October.
Shark bites
Virtually nothing is being done to stop the flow of nitrates into the river, meaning the dead zone will reappear every year, Rabalais said.
The dead zone forces fish to seek better water, which may be a reason for the recent shark bites on Texas beaches.
Three people have been bitten by sharks along the upper Texas coast this year - a high number for a state that has recorded only 18 shark attacks since 1980.
Terry Stelly, an ecosystem biologist with the Texas Parks and Wildlife Department, said increasing numbers of sharks have been found in recent years in the waters along the Texas-Louisiana border, near the edge of the dead zone.
"The chances are good that sharks were looking for higher dissolved oxygen in the water," he said.
Rabalais agreed: "The higher number of sharks in shallow waters may very likely be due to the low oxygen being close to the shore at the time of the attacks.
"The available habitat for the sharks is definitely less when the low oxygen is so widespread."
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