Increasing numbers of unprovoked shark attacks on swimmers could be linked to the formation of immense oxygen-starved dead zones in the world’s oceans, according to American scientists.
Three people have been bitten by sharks off the Texas coast this year, compared to only 18 attacks in the past 24 years, leading researchers to believe they may be linked to a dead zone which this year has spread across 15 020 square kilometres of the Gulf of Mexico .
The zone, caused by the runoff of sewage and nitrate-based fertilisers from intensive farming, is deprived of oxygen and kills all fish within it. It also forces marine life near it to seek better water and come closer inshore. The one in the Gulf of Mexico this year is much closer to shore.
Terry Stelly, an ecosystem biologist with the Texas parks and wildlife department, said increasing numbers of sharks had been recorded in recent years in the waters along the Texas-Louisiana border, near the edge of the dead zone. ”The chances are good they [sharks] are looking for higher dissolved oxygen in the water”, he said.
Nancy Rabalais, of Louisiana Universities Marine Consortium said: ”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.”
According to the UN environment programme (Unep), there could now be more than 150 dead zones around the world: more than twice as many as 15 years ago. The largest are off the coasts of South America, China, Japan, south-east Australia and New Zealand.
The number of unprovoked shark attacks has also risen in the same period, but this is usually attributed to the growth of tourism in shark-infested regions. The Gulf of Mexico’s dead zone this year extends 400 kilometres west from the mouth of the Mississippi river in south-eastern Louisiana to near the Texas border.
Its size varies each year depending on weather conditions but averages about 12 950 square kilometres and remains in place until late September or early October.
Almost nothing is being done to stop the flow of nitrates into the river.
Klaus Toepfer, Unep’s executive director, said: ”Humankind is engaged in a gigantic experiment as a result of the inefficient … over-use of fertilisers, the discharge of untreated sewage and the ever-rising emissions from vehicles and factories.
”The nitrogen and phosphorous from these sources are being discharged into the environment, triggering sometimes irreversible effects.” – Guardian Unlimited Â