Two incidents were observed within the past month, one off Washington state and the other in British Columbia's Strait of Georgia, The Vancouver Sun reported Saturday.
While biologists see whales kill porpoises locally about twice a year, the actual number of porpoises killed is probably higher, said John Ford, a whale expert with Canada's federal fisheries department.
One theory is that the killer whales see the porpoises as an opportunity for play -- a "cat and mouse" game with deadly consequences, the Sun reported.
Another theory is the whales are trying to prop up the porpoises as they do with their own young, though the porpoises ultimately succumb to shock, injury, exhaustion or death, Ford said.
"It could be a maternal-driven behavior that is misdirected toward another species," Ford said.