After releasing 100,000 juvenile chinook in May, the Stillaguamish Tribe’s Brenner Creek hatchery is ramping up production to double that number.

The fall chinook are the offspring of juveniles that were captured in the South Fork Stillaguamish River and raised to spawning age in a hatchery. They are genetically distinct from the summer chinook that spawn primarily in the North Fork, which have been supplemented by the tribe’s Harvey Creek hatchery for 30 years.

Brenner Creek’s Hatchery and Genetic Management Plan, under review by the National Oceanic and Atmospheric Administration, has the eventual goal to release 200,000 fall chinook each year until enough adults return to spawn.

When the program began in 2007, fewer than 100 adult fall chinook were returning to the South Fork, too few to spawn at the hatchery.

“We relied on the few fall chinook adults we caught during summer broodstocking on the North Fork,” said Kip Killebrew, Stillaguamish fisheries enhancement biologist.

Since 2009, tribal natural resources staffers have been seining the South Fork for out-migrating juveniles. The fry live in small plastic compartments, called “fish condos,” while their DNA is tested.

“Once we know they are a fall fish, they get removed from the condo and placed in a trough,” said fisheries biologist Charlotte Scofield.

As the captive broodstock grow, hatchery staff separate them by size into circular tanks where they are held until they are mature enough to spawn. The first group of captive broodstock was spawned in 2013.

Like the fish in the Harvey Creek program, the Brenner Creek offspring are marked as hatchery fish by having their adipose fins clipped, and tiny coded-wire tags inserted in their snout.

“When the fish return as adults, the coded-wire tags give us important information about survival rate, migration patterns and harvest impacts on the population,” Killebrew said.

Photo: A captive juvenile chinook swims in a fish condo at the Stillaguamish Tribe’s Brenner Creek Hatchery.