A humpback whale is swimming in the St. Lawrence River in Montreal

A humpback whale has reached Montreal by way of the St. Lawrence River, according to a marine mammal expert.

The whale was spotted underneath the Pont de Québec earlier this week, swimming upstream. By late Saturday morning, it was near the Jacques-Cartier Bridge in Montreal.

"It's a very unusual situation," said Robert Michaud, the co-ordinator for the Quebec Marine Mammal Emergency Network.

Michaud said that, as far as local marine biologists are aware, this is the first time a humpback whale made its way into Montreal waters.

"This is a first for a humpback whale. We already had a beluga in the Old Port in 2012, a minke whale reluctantly a few years ago and more frequently seals also ventured to Montreal," Michaud said.

A group of journalists and passersby gathered near waterfront by the bridge Saturday to see the whale. It surfaced every couple minutes, either spraying or flapping its tail, to gasps from the crowd.

The animal appeared to be swimming around in the area, but slowly heading west.

"We don't know why this animal made this journey. There are several hypotheses," Michaud said, adding that the whale could have been following fish because it was hungry or confused.

"We have to recognize that mammals do that. Humans, whales and land mammals, sometimes they are vagrants that go in unusual places."

"These journeys are usually a series of mistakes. But what is sure is that this animal doesn't belong to this habitat."

The whale appears to have travelled from Tadoussac, where it lives in salt water. Michaud said it can survive in fresh water, but the food and water around Montreal won't be as healthy.

There is also more marine traffic, which could cause it stress or other harm.

Michaud said people could face a fine if they get closer than 100 metres away, but he suggested keeping at least a 200-metre distance.

Humpback whales are one of the large marine mammal species, measuring up to 15 metres long. They are generally agile and gentle, Michaud said, but if one is stressed it could become dangerous for a small watercraft.

A team from Michaud's network is on the water monitoring it and agents from Fisheries and Oceans Canada have been following it for the last two days.

He said the whale should be fine if it spends just a few days in this habitat, but that experts are hoping it will start heading back by Sunday.