The Black Sea Fleet is far bigger than Ukraine"s modest navy. The Royal United Services Institution (Rusi), a security think tank, says Russia"s six Black Sea submarines "substantially outmatch the regionally deployed subsurface assets" of other navies in the region. The submarines have fired at Ukraine – an additional source of acoustic disturbance, along with helicopter motors, underwater explosions, and mines. Russia has fired hundreds of missiles from the Black Sea, and any that fell in the sea would have displaced cetaceans, Popov says. In spring, he and his colleagues recorded the highest density and abundance of cetaceans off the Bulgarian coast for six years.