Marine scientists are struggling to explain a significant change in the migration pattern of whales visiting Victorian bays this year.

The Dolphin Research Institute observed a number of humpback whales that stayed in Western Port and Port Phillip bays throughout the migration season instead of travelling north as usual.

A spokesman for the institute, Jeff Weir, says marine biologists are unsure what has caused the change in the whales' movement pattern. He noted changes in their behavior as well. He says the scientists also observed the animals competing for the first time, off Mount Martha and Mornington.

"These are big animals, the size of tourist buses, playing demolition derby under the water and then sometimes reaching the surface banging into each other, jostling for position in the pod," he said.

Weir says it is the kind of behaviour normally seen when the animals are mating in northern Australia.

"I mean, it possibly is simply numbers of whales are picking up again and the populations are increasing," he said.

"Maybe they're just spreading themselves more evenly around our coast. The critical thing will be to monitor it in the years to come."

The Dolphin Research team is set to gather more data about the apparent changes in the whales migration pattern and behavior.