Toan said he first encountered the whales on July 26 when he was taking tourists to Hon Trau Islet, about 8 kilometers from the shore.
He described the whales as "blue whales" but experts from Vietnam Institute of Oceanography said careful observation is needed to accurately determine the species.
Vo Van Quang from the institute said recent photos suggested there were whales from two species in De Gi.
According to folklore, whales are auspicious creatures because fishermen have a good harvest when they appear.