A staggering number of Chinook salmon are returning to a California river that hasn’t sustained salmon for decades due to agricultural and urban demands, giving biologists hope that threatened fish are finally spawning in their native grounds without human help.

Officials working on a restoration program announced Tuesday that they have counted a record number of spring-run Chinook salmon fish nests (redds) so far this fall on a stretch of the San Joaquin River near Fresno. Program staff has discovered over 160 redds with several weeks to go, toppling the total of 40 recorded in 2018.

Not only have the number of redds increased, biologists say many of them appear to have been fashioned by fish that weren’t hatchery raised or part of the billion-dollar program – meaning salmon were able to swim from the Pacific Ocean and through dams on their own.

“The volume of returns is a complete surprise,” said Pat Ferguson in a statement, a program fish biologist with the California Department of Fish & Wildlife.

Biologists say the quadrupled number of redds is exciting considering they have only released 37 adult female salmon this year to breed in the river below Friant Dam. There are other signs that natural or “volitionally passed” salmon have returned to the river: Biologists have found untagged spring-run carcasses in recent weeks.

“The majority of the fish that we’re seeing in the river spawning right now don’t appear to have tags,” said Lori Smith, a program fish biologist with the US Fish & Wildlife Service.

Smith said it is possible that the salmon lost their tags during their 370-mile journey back to the river, but genetic testing will ultimately confirm if they were hatchery bred or not.

Tuesday’s announcement is the second major milestone for the restoration program this year, as in April spring-run Chinook adults returned to the river for the first time in 65 years. The hatchery salmon returned from the ocean on their own in the spring but had to be transported by researchers to bypass a series of dams and diversion canals.

Biologists believe an abnormally wet rainy season may have helped some of the fish return to their ancient spawning grounds on their own.

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