The mammals were spotted by tourists and crew on dolphin-watching trips belonging to companies Dream World and Formosamar, believed to be heading west on their annual migration pattern.
They were divided into two groups, among them at least four young females, and sighted last Tuesday, three or four miles off Culatra Island in Olhão, then again on Wednesday, off Armação de Pêra.
A group of killer whales was also spotted at roughly the same time last year, off the coast of Olhão, on 15 July.
Orcas are members of the dolphin family. After mankind, they are the species of mammal that occupies the second largest geographical area on planet earth.
Their food is based on fish, mollusks, birds, turtles, sharks or seals, and they can weigh up to nine tons.
In related news, scientists have spotted what they believe to be the first ever hybrid between a melon-headed whale and a rough-toothed dolphin near Hawaii.
The male ‘wholphin’, which is believed to be close to adult age, was spotted swimming with dolphins near the island of Kauai last year.
Hybrids are extremely rare, with only three ever having been confirmed. This is believed to be the first hybrid between a whale and a dolphin.