Scientists aboard a U.S. research ship have started an around-the-clock search for elusive signs of oil lurking beneath the Gulf of Mexico’s surface in what they jokingly call “Operation Dipstick.” As debate rages among scientists over how much oil remains in the water after BP Plc’s massive oil spill, their research vessel circles above the blown-out Macondo well, some 40 miles southeast of the mouth of the Mississippi River. Oil is not visible on the surface around the well, but as waters reopen to fishing, many question what the crude will do to this season’s fish, shrimp and oyster catch, as well as its long-term effect on marine life.

Commercial fishermen can now trawl Louisiana’s waters for white shrimp as the season opened on Monday, but questions linger about the effects BP Plc’s Gulf of Mexico oil spill will have on the harvest. Some state waters have been open for brown shrimping since the well ruptured on April 20, but the overall catch has been down from previous years partly because a number of boats are signed up with BP’s oil spill clean-up program. The plump, sweet white shrimp are typically larger than brown shrimp and more desired by chefs. The U.S. government has said that seafood pulled from the areas of the Gulf of Mexico that is open to fishing is safe to eat despite all the oil that gushed into the ocean. More than a fifth of federal waters in the Gulf remain closed due to fear of oil contaminating the seafood.

BP made promising strides in its latest bid to capture some of the oil spewing from its ruptured deep-sea well in the Gulf of Mexico, while President Barack Obama called off an overseas trip and prepared for another visit on Friday to the spill-stricken U.S. Gulf Coast. After failing days ago to plug the well, BP Plc managed on Thursday to shear away the gushing well pipe a mile below the ocean surface, then lowered a containment cap over the jagged hole left atop the crippled wellhead assembly in its latest bid to curtail the oil flow.