Thousands of sockeye will end their journey up the Cedar River to spawn this fall almost under the bridge that carries Interstate 405 over the river.

It’s there that using a weir and trap that crews with Seattle Public Utilities and the state Department of Fish and Wildlife will capture sockeye salmon, perhaps as many as 10,000, and then truck them to a sockeye hatchery at Landsburg.

When the female sockeye are “ripe” or ready to spawn, they will be killed and stripped of their eggs, which then will be fertilized by sperm from the male sockeye trapped with them.

The idea is to produce a generation of sockeye that will return in roughly four years to start the cycle again. There’s a sense of urgency this year, because of a record-low return of sockeye through the Ballard Locks in Seattle.

“Every fish counts,” said Frank Urabeck, a sports fisherman and self-described long-term advocate for Lake Washington sockeye. “We need to run scared.”

And, he said, those who harvest salmon, including the Muckleshoots who gill-net for coho salmon in the lake, need to do so “conservatively.”

The goal of the collection is to produce 17 million young sockeye at the three-building hatchery built on the edge of the Cedar River Watershed.

But a key SPU official doesn’t expect to collect that many sockeye, given the small number of sockeye that have entered Lake Washington – just under 34,000 – through the Ballard Locks.

That’s the smallest number of sockeye to return through the locks since counting began in 1972, according to Gary Sprague, Landsburg mitigation manager for Seattle Public Utilities.

The Landsburg hatchery is one of the mitigation measures required for Seattle to divert water from the Cedar River.

That water (and water from the Tolt River) supplies Seattle residents and Seattle’s wholesale customers in the suburbs. Renton has its own municipal water supply.

Typically, about 7 percent of the sockeye run is collected and then used as broodstock for the Landsburg hatchery, according to Sprague.

Last year, just 2,000 sockeye were collected, producing just under 3 million eggs, he said.

Two years ago, enough sockeye returned to have a rare fishery on the lake. A likely reason for this year’s small run is poor conditions for sockeye in the Pacific Ocean; smaller-than-normal sockeye runs are being reported in Canadian rivers, too.

Eventually, once a new sockeye hatchery is built at Landsburg – construction is expected to begin in 2010 – the goal is to produce 34 million eggs, double today’s goal.

The sockeye run is beginning just now, after the fish waited for a time in Lake Washington. A survey done Tuesday of the river from Landsburg to its mouth showed 257 sockeye in the river and 26 Chinook salmon. A handful of sockeye were milling around in a shallow area near Carco Theater. The run will last through November.

Much of the sockeye was likely waiting for some rain and higher flows to begin the swim up the Cedar. About 10 percent of the run also migrates up the northern tributaries that flow into the lake.

The weir – basically a fence across the river – directs all fish in the river to a “tunnel” in the weir that leads to a fish trap. There, the fish are sorted, with such species as Chinook and coho salmon returned to the river, along with the bulk of the returning sockeye.

The weir was constructed earlier this week and the actual trap will be installed next week, when the collecting is expected to begin, Sprague said. The trap, a 6-foot by 12-foot metal box, is just upriver from the weir.

The weir and trap replace ones that had been in place at Cavanaugh Pond upriver since 1991. The new location will allow Seattle to collect sockeye that spawn farther downriver in order to get a better genetic representation of the sockeye.

Seattle and the state will collect fish Monday through Friday, during the daytime, and probably one to three times a day, depending on each day’s run, according to Sprague. On the days the city isn’t collecting salmon, such as the weekends, all fish will move freely up the river.

The sockeye will go into two holding pens and then rubber tubes, which are put into tank trucks for delivery to Landsburg.

Next year, Seattle will build a paved access road directly to the river so that the truck can back up right to the river. Until then, the fish will be carried to the Cedar River Trail.

The trapping system should provide some prime viewing of returning sockeye, not only from the side of the river, but also from the pedestrian bridge under the freeway.

“The optimum time is in three or four weeks,” he said.

Sprague points out it’s illegal to harass fish, such as throwing rocks at them. Fishing also is prohibited on the river.

Moving the trapping system to Renton is a cooperative venture by the Seattle Public Utilities, the City of Renton and the Washington state Department of Fish and Wildlife.

Renton issued permits for the project because it is utilizing city parkland.

The effort, according to Urabeck, wasn’t easy.

He wasn’t sure the weir would go in place this year.

“It took a Hail Mary pass for all of it to come together,” he said.

As part of the agreement, Seattle will install an interpretive kiosk that talks about the collection project and the region’s efforts to preserve salmon, according to Leslie Betlach, the city’s parks director.

Swimming already isn’t allowed in the area of the weir at Cedar River Park and boaters will have a clearly designated chute through the weir (between two flashing amber lights). Boaters also could portage around the weir.