Salvage crews inspected the remaining four of 17 fuel tanks aboard the American Challenger this week, pumping out more than 600 gallons of oil and water and removing seven cubic yards of petroleum-contaminated debris. Oil sheen continues to be seen around the 90-foot ship, though the California Department of Fish and Wildlife has determined that booms are ineffective in containing it amid the crashing surf. The agency has received no reports of oiled wildlife since the Challenger beached on rocks north of Dillon Beach on March 6. The ship, a decommissioned fishing vessel, was being towed to Mexico when its tugboat lost power and the towline snapped.   David Briggs