Fishermen at the mouth of B.C.'s Fraser River are preparing for one of the biggest runs of sockeye salmon in nearly 100 years, but it's unclear what will happen to all the unexpected fish.

On Tuesday, the Pacific Salmon Commission announced it expects as many as 25 million fish will return to the Fraser this season.

That's the largest return since 1913 and more than double what was forecast just a few weeks ago.

On Tuesday night, fisherman were down on the docks preparing for what may prove to be an epic fishing trip. The tidal waters of the Fraser open to gillnet fishing at noon and many boats have been in place since Tuesday night.

Stewart McDonald said he does not plan to sleep for the entire 32-hour stretch.

"It's probably going to be the best fishing of our lives…. They're just coming in on hordes, it's amazing to see," said McDonald.

Steve Johansen is a salmon troller who's already been out fishing in other areas and says he's amazed by the number of fish he has seen around schooling around the river mouth waiting for a chance to spawn.

'Just amazing'

"The other day we were in a school of sockeye, trolling around, probably from Bowen Island right to the airport. It was just amazing, and that fish is still out there," he said.

Fishermen prepare to head out to fish on the Fraser River south of Vancouver on Wednesday morning. (CBC)

"The fish have adjusted. As you know, we've had a pretty warm summer, the Fraser's kind of warm, and they're actually waiting out front for some rain," he said.

Phil Eidsvik with the B.C. Fisheries Survival Coalition said it's great news for the industry that works the river mouth south of Vancouver.

"We're not surprised that the run size was increased. We've been seeing lots of fish in the river and very large test sets in Johnstone Strait and we're still hearing reports of sockeye being caught in the Queen Charlotte Islands, on the northern tip, so we expected it to be upgraded," said Eidsvik.

Last week, the Department of Fisheries and Oceans opened the fishery for 16 hours, but McDonald said he wants it to change the openings to 12 hours at a time, with days off in between, so he can get some rest.

Processors not ready

The Fraser River fishery is expected to be open for 32 hours starting at noon on Wednesday. DFO is allowing about 30 per cent to be harvested, or about seven million fish.

Fresh sockeye sit in a container on a Vancouver dock Wednesday. (CBC)

The news comes after three years of returns so low the federal government launched a public inquiry. Fish packing plants and processors are already reporting that, after three years of low harvests, they are no longer ready to handle such a large harvest.

Johansen, who works with Organic Ocean, said the industry has only about half the capacity to catch and process fish that it did a decade ago.

"Half the plants have shut down over the past 10 years and half the boats are no longer fishing," said Johansen.

Guy Dean, vice-president of Albion Fisheries Ltd, a wholesale supplier of seafood, said his company is ready to handle the increased supply, and consumers should be ready for deals.

"Prices are obviously going to come down with increased supply," said Dean.

Dean said he's not having to turn any fishermen away at this time, but he is hearing stories of woe from other processors unprepared for the spike in volume.

He's thrilled his company will be able to source most of its fish from Canadian waters this year for the first time in several years.

"We source fish from all over the world. In previous years, we've had to source fish from Russia as well as Alaska. And certainly our preference and our customers' preference is Canadian sockeye. So these early runs and large returns, it's great news for us," said Dean.

Fish may not be able to enter river

While fishermen and hungry consumers might be excited about the massive catches expected, UBC fisheries Prof. Carl Walters said he is also concerned not enough fish will be harvested.

The 30 per cent harvest rate could mean millions of salmon die before reaching their spawn grounds higher up the river, said Walters.

"They will be physically unable to enter the river. The river will be completely jammed with fish," he said.

"We could see five million dead fish laying in Shuswap Lake at the mouth of the Adams River. To me, this would be an utterly irresponsible catastrophe."

However, DFO area director Barry Rosenberger said sockeye are very well managed and the department has a proven history of adjusting harvest numbers as the projected returns change.

"I think the in-season management is the important part. The forecast is important in the scheme of things, but you have to have the management, the in-season tools, and I think that's the part that we've demonstrated. We are making the right kinds of decisions," said Rosenberger.

No ocean monitoring

In 2009, the return was so low the federal government called a public inquiry to examine the apparent collapse of the Fraser River sockeye fishery. Fewer than 1.5 million sockeye returned to the Fraser River, a fraction of the 11 million forecast.

The Cohen Commission began its hearing in Vancouver in June. Critics believe lice and other contaminants from fish farms are deadly to wild salmon, but the discussion paper said Fraser River sockeye experience a "suite of impacts" that must all be evaluated in order to understand the collapse.

Walters said it's not clear why the fish bounced back this season because there's no way to monitor the salmon when they're in the ocean, which is the time when survival rates are established.

"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, but we don't understand the mechanisms at all," said Walters.