VANCOUVER - Whale watching companies in the Salish Sea report unusually large groups of humpback whales are becoming a frequent sight off B.C.'s south coast.

The Pacific Whale Watch Association says the number of whales is unprecedented around the southern end of Vancouver Island.

Association executive director Michael Harris says humpback whales were a rare sight off the south coast just 20 years ago, but have become increasingly common over the last three or four years.

He says whales usually travel in groups of two or three, but the latest sightings are unique because they are in groups of up to 20, mirroring conditions he says occur only off Alaska or Hawaii.

Rhonda Reidy, a naturalist and whale watching boat captain, is about to begin a PhD study of what are termed the "comeback humpbacks" of the Salish Sea, arguing shifts in oceanographic and ecological conditions may be affecting the food chain.

She says humpbacks can feed on krill and small schooling fish, such as sardine, anchovy and herring, but their exact diet isn't known, and she believes more data could explain the robust return.