Sockeye are returning to the Fraser River in huge numbers. (CBC)

The number of sockeye salmon returning to the Fraser River are expected to hit levels not seen in nearly 100 years, the Pacific Salmon Commission says.

It's estimated that the Fraser sockeye return will be more than 25 million fish, the largest return since 1913, the commission said in a news release Tuesday.

The near record numbers come after very low sockeye returns in recent years, especially in 2009 when fewer than 1.5 million of the fish returned after 11 million had been forecast.

A mysterious species

The lack of fish prompted speculation about the possible permanent disappearance of the highly prized sockeye and served as a reminder of how little is known about the species.

There is no way to monitor the salmon in the middle of their life cycle when they're out in the ocean, which is when the survival rates are established, said Carl Walters, a professor at UBC's Fisheries Centre.

"We don't understand the mechanisms at all," said Walters. "We think there's some complicated kind of delayed ecological interaction effects that a big run can cause poor survival down the road and maybe low runs can cause good survival down the road."

The 2009 return was so disturbing that the federal government created a public inquiry to determine what happened. The Cohen Commission began its hearing in Vancouver June.