State Department of Fish and Wildlife officials on Monday and Tuesday released 3 million juvenile salmon at the mouth of the American River in Sacramento.

The fall-run chinook salmon, produced at the Nimbus Hatchery, have historically been transported in trucks to San Francisco Bay to help the fish avoid predators. But research showed few found their way back to the river.

So in 2010, hatchery officials began releasing a major share of each year’s production into the river, 25 percent of them marked with a coded-wire tag.

The results have been encouraging. “It appears from the last couple years we’ve done it that the return to the river has increased, and we’re getting less straying from other hatcheries,” said Gary Novak, a manager at the hatchery.