"For a hybrid to exist we need overlapping spawning grounds and timing," said Araujo, noting that chinook usually spawn in September and October, whereas coho traditionally spawn toward the end of October until December. He said when summer droughts extend into fall it can push chinook spawning season back into when coho are also starting to spawn.

Duguid said there are some fish that routinely generate hybrids — such as rainbow trout and cutthroat trout — but not wild salmon.

"Apparently, it rarely occurred in the past and there has never been documentation of hybridization into the second generation," said Duguid.

He said a member of the Cowichan Tribes spotted the first hybrid in the river during an adult fish tagging study being done in partnership between the First Nation and the province.

Duguid said the hybrid fish can sometimes be identified by their abnormal scale arrangement, which is not patterned evenly on either side of their lateral line.

"This is indicative of the fact there may be other abnormalities that we can't see," said Duguid.

He said the hybrids are also identifiable by the degree of spotting on their tails, the shape of their anal fins and the morphology and colouration of their mouths.