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Low oxygen areas related to likely to BP spill shinking

Growing Hypoxic Zones Reduce Habitat for Billfish and Tuna habitats
"Growing Hypoxic Zones Reduce Habitat for Billfish and Tuna"

ScienceDaily (Dec. 23, 2010) Billfish and tuna, important commercial and recreational fish species, may be more vulnerable to fishing pressure because of shrinking habitat, according to a new study published by scientists from NOAA, The Billfish Foundation, and University of Miami Rosenstiel School of Marine and Atmospheric Science.

An expanding zone of low oxygen, known as a hypoxic zone, in the Atlantic Ocean is encroaching upon these species' preferred oxygen-abundant habitat, forcing them into shallower waters where they are more likely to be caught.

During the study, published recently in the journal Fisheries Oceanography, scientists tagged 79 sailfish and blue marlin with satellite tracking devices in the western North Atlantic, off south Florida and the Caribbean; and eastern tropical Atlantic, off the coast of West Africa. The pop off archival satellite tags monitored horizontal and vertical movement patterns. Researchers confirmed that billfish prefer oxygen rich waters closer to the surface and will actively avoid waters low in oxygen.

While these hypoxic zones occur naturally in many areas of the world's tropical and equatorial oceans, scientists are concerned because these zones are expanding and occurring closer to the sea surface, and are expected to continue to grow as sea temperatures rise.

"The hypoxic zone off West Africa, which covers virtually all the equatorial waters in the Atlantic Ocean, is roughly the size of the continental United States, and it's growing," said Dr. Eric D. Prince, NOAA's Fisheries Service research fishery biologist. "With the current cycle of climate change and accelerated global warming, we expect the size of this zone to increase, further reducing the available habitat for these fish."

Less available habitat can lead to more fish being caught since the fish are concentrated near the surface. Higher catch rates from these areas may give the false appearance of more abundant fish stocks. The shrinking availability of habitat and resulting increases to catch rates are important factors for scientists to consider when doing population assessments.

Researchers forecast that climate change and its associated rise in ocean temperatures will further increase the expansion of hypoxic zones in the world's oceans. As water temperature increases, the amount of oxygen dissolved in water decreases, further squeezing billfish into dwindling available habitat and exposing them to even higher levels of exploitation.

Dead Zone in Gulf Linked to Oil
Jessica Marshall
By Jessica Marshall
Fri Jul 9, 2010 08:21 AM ET

An unusual low oxygen zone in Gulf of Mexico waters off the Alabama shore has persisted for more than a month, and evidence points to the ongoing Deepwater Horizon oil spill as the cause.

Oil spills can deplete oxygen in water by providing a source of food to microbes that grow on oil and consume oxygen in the process.

Researchers can't say how low oxygen levels will affect the region's ecosystem in the long term, but for now, most animals that can swim away have left the area. Plankton in the zone have died.

The researchers measured low oxygen levels along the entire 40-mile stretch they sampled around Dauphin Island, Ala., from about 40 miles offshore to within a mile or two of the shoreline. The bottom layer of water was oxygen-depleted at depths of about 30 feet close to shore to 100 feet further out, along the continental shelf -- a rim of shallow water tracing the coast from Mississippi to Florida.

"It's not little local pockets," said Monty Graham of the Dauphin Island Sea Lab, who is tracking the zone. "It's over a regional scale. It wouldn't surprise me if there were a band of low oxygen over that entire area between the Mississippi River and Apalachicola, Florida."

"The low oxygen was pushing up very close to the shore," he added.

His team trawled the waters to survey wildlife.

"In the low oxygen, all we got were sea stars. The fishes were up against the shore. In the same time frame people started to see large sharks cruising in unprecedented numbers up against the shore. The low oxygen has pushed the prey up against the shore and the sharks are cruising to take advantage of that."

Like sea stars, plankton can't escape the oxygen-depleted waters. "In the bottom layer, all the plankton were a white color," Graham said. "The others were nice and pink-looking. The ones that were white at the bottom would indicate that they were probably dead for a while."

This dead zone is unusual because of its location, but a massive low oxygen zone now about the size of New Jersey has surrounded the mouth of the Mississippi River for decades. This area results from fertilizer runoff and other nutrients from upriver washing into the Gulf and stimulating growth of algae, which decay, consuming the oxygen in the water and making it inhospitable to life.

"The dead zone that forms because of excess nutrients in the area is a huge one," said Nancy Rabalais of the Louisiana Universities Marine Consortium in Chauvin who studies the permanent dead zone. "Hypoxia east of the river does occur," she said, where Graham has measured it. "But it's pretty patchy and it comes and goes."

In the Gulf of Mexico the low oxygen levels are widespread and they have lasted. "It's been over four weeks and it's persisted and that's usual," Graham said.

When the researchers first found the low oxygen levels, they could not say whether they were attributable to the oil spill. Nutrients from the rivers feeding into Mobile Bay near Dauphin Island could have been carrying excess nutrients. This spring was especially wet, so unusually high river flows were a possibility.

Now the researchers have more evidence suggesting the rivers are an unlikely culprit. The low oxygen levels have persisted past the time of the seasonal freshwater surge into the Gulf; the zone is larger than would be expected from river-borne nutrients; and the low-oxygen waters seem to be arriving via a tongue of cold water that has pushed up from offshore carrying elevated levels of methane, Graham said.

The team is still waiting for test results measuring oil levels in the oxygen-depleted waters, which would help pinpoint oil as the cause.

Long-term effects are uncertain. "I don't think anybody can predict anything with confidence about what the ecosystem will do," Graham said.

The good news is that the shallow waters turn over relatively quickly, so once the spill is stopped, oxygen levels should rise. In deeper waters, where researchers have measured decreased oxygen levels, it might be a different story.

"The effects off the shelf might be longer," Graham said. "If you drive the oxygen down at 1200 meters (3600 feet), there is nothing to replace that oxygen rapidly. You might see a low oxygen signature for years, maybe even decades.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by National Oceanic and Atmospheric Administration.

Journal Reference:

1. Eric D. Prince, Jiangang Luo, C. Phillip Goodyear, John P. Hoolihan, Derke Snodgrass, Eric S. Orbesen, Joseph E. Serafy, Mauricio Ortiz and Michael J. Schirripa. Ocean scale hypoxia-based habitat compression of Atlantic istiophorid billfishes. Fisheries Oceanography, Volume 19, Issue 6 DOI: 10.1111/j.1365-2419.2010.00556