With fruit standing in for tar balls, the U.S. Coast Guard will put its oil-spill fighting capabilities to the test Thursday during an exercise that will temporarily close Bear Cut, the channel between Virginia Key and Key Biscayne.

The exercise to block the cut with booms comes in response to an increasing risk of oil reaching South Florida shores — particularly from Cuban waters, where the government is partnering with foreign companies to drill exploratory wells.

The Cuba drilling, combined with lessons from the infamous BP blowout in the Gulf of Mexico, showed that spill response plans drafted years ago were in need of revamping, said Capt. John Slaughter, chief of planning and force readiness for the Coast Guard seventh district.

Old plans were drawn up to tackle groundings or sinking near shore, which would send broad slicks toward South Florida beaches and inlets. BP showed the more likely threat may come from far off in the form of thick mats of goo and heavy tar balls, Slaughter said. No one is certain when the booms were last tested, he said, but it’s likely been a decade or more.

“One of the things that came up was, “Geez, does this stuff really work?’ We all looked at each other and said, “Well, on paper it does’’ but we decided to go ahead and test it,’’ Slaughter said.

The Coast Guard, working with state agencies and a private clean-up crew, will string boom to see how well it performs in powerful currents that sweep through the 1,950-foot wide pass between Virginia Key and Key Biscayne. They plan to use 7,500 feet of boom, shaped in a vee-like funnel on the west side of the cut. Exactly 240 lemons, grapefruits and oranges will be dumped from the bridge to mimic tar balls.

Slaughter said the Coast Guard has gotten approval from both state and federal agencies to deploy the fruit, which is easy to spot and won’t do any environmental damage if some escape.

Florida has some 84 inlets to protect but Coast Guard chose Bear Cut for its test because its width, strong water flow and mangrove shoreline make it “a difficult one to protect,’’ Slaughter said. Responders will try to determine the best strategy for setting the boom.

One of the lessons from the BP spill, said Slaughter, was that it was much easier to clean oil on beaches then from sensitive mangroves or marshes fed by inlets.

If there were a real spill, simply protecting Bear Cut alone obviously wouldn’t save the bay. There are miles of open ocean, tidal sea grass flats and channels flowing into Biscayne Bay south of Key Biscayne. But protecting vulnerable inlets like Bear Cut, Slaughter said, would allow the Coast Guard and clean-up crews to focus skimming boats and other resources on more challenging areas.

The exercise will close the Bear Cut for boaters from 6 a.m. until 5 p.m. Thursday. Marine patrols will be on the scene to enforce the closure.