 Between the piers along Tribeca, scientists have dropped more than 250 man-made reefs on the riverbed. The structures are three-foot-wide hollow concrete spheres called reef balls and steel cages called gabions that are about the size of a vintage trunk and filled with shells. Twice a year, scientists working with the Hudson River Park Trust pull the structures out of the water to check on the oyster's progress. Carrie Roble is vice president of the Trust's river project.