Vessels continue working to clean up more than 88,000 gallons of oil that leaked from a Shell flow line into the Gulf of Mexico last Thursday, some 90 miles off Timbalier Island, Louisiana.

More than 51,000 gallons of the oil-water mixture have been sucked out of the Gulf since the leak was reported Thursday morning, the Coast Guard said.

The spill has angered environmental groups that argue offshore drilling needs to be banned because of the hazards it poses to wildlife. On Sunday, activists plan to march in Washington D.C. to demand an end to drilling and will use Thursday's Shell spill as further proof.

"It's unacceptable that oil spills have been permitted to become the status quo in the Gulf," said Michael Brune, the Sierra Club's executive director, in a statement. "We have allowed the region to be perpetually treated as a sacrifice zone."

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Shell did not directly respond to the complaints of environmental groups. In a statement, Kimberly Windon, a Shell spokeswoman, said, "No release is acceptable, and safety remains our priority as we respond to this incident."

Spills happen every year in the Gulf. This new spill is classified as medium in size under U.S. Coast Guard guidelines.

Since 2012, there have been 147 spills and about 516,900 gallons of oil spilled in the Gulf, according to figures from the Bureau of Safety and Environmental Enforcement, the agency that oversees drilling.

But scrutiny of the offshore industry's spills has increased since BP's catastrophic oil spill in 2010 when an out-of-control well leaked for 87 days, releasing millions of gallons of oil. Eleven workers were killed and many more injured when the blowout of the well caused the Deepwater Horizon drilling rig to explode and sink.

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On Friday, Shell said it sent a remotely operated vehicle down to the seafloor to inspect where the leak came from and found a flow line as the source.

The flow line was connected to four wells and Shell's Brutus platform, which floats in seas that are about a half mile deep in the Green Canyon area of the Gulf.

Shell said the leak was contained and that the wells on the site were shut in. It also said drilling at the site was stopped.

BSEE, the regulatory agency, said there have been no reports of injuries. In a statement, the agency said its investigators were at the Brutus platform to find out what caused the leak. The agency said it would review Shell's repair plans.

BSEE Director Brian Salerno said the agency would assess if improvements to "subsea infrastructure technology" should be made as a result of the incident.

MORE ON WEATHER.COM: The 2010 Deepwater Horizon Oil Spill

A dead crab is seen in a piece of marsh ravaged by the effects of the BP oil spill on April 17, 2011, in Ocean Springs, Mississippi, a year after the devastating incident. The spill occurred after the Deepwater Horizon rig exploded on April 20, 2010, killing 11 workers and dumping millions of barrels of oil into the Gulf of Mexico.