Students in Petaluma's renowned fish hatchery and creek-restoration program are heartsick that someone vandalized holding pens in Tiburon and released about 40,000 young salmon into San Francisco Bay.

The six- to 10-inch Chinook salmon were set to be released at a public event on Oct. 30 at the Romberg Tiburon Center for Environmental Studies.

The vandalism, discovered Tuesday morning, deprives the Casa Grande High students, who painstakingly raised about half of those fish from eggs, of the opportunity to celebrate and take part in their release.

Senior Greg Gillis, president of the United Anglers of Casa Grande, said he'd spoken Tuesday afternoon and evening to many of his 47 classmates in the school's award-winning restoration program.

"They just asked the same question, &‘Why?'"

"They were like our babies," said Gillis, 17. "For someone to release them without us saying good-bye just set everyone off."

Dan Hubacker, director-advisor of Casa Grande's 29-year-old salmon hatchery and Adobe Creek restoration program, said he believes someone approached the Romberg center by boat and came up along the three large, netted pens that held about 60,000 juvenile salmon.

One of those pens contained the 40,000 individually tagged fish that Casa students raised at their on-campus hatchery from eggs harvested from 11 adult salmon they caught in the Petaluma River. In the other two pens were more than 40,000 salmon that the Tiburon Salmon Institute brought to the bay from the state's Feather River Hatchery.

Somebody cut the hefty zip ties that held in place the netting on the pen that held the Petaluma-raised fish and on one of the other pens. The third pen was not damaged, so about 20,000 of the Feather River fish remain.

Casa students, Marin County Sheriff's deputies and officials of the Romberg center and Tiburon Salmon Institute wondering why someone would release 40,000 of the salmon and leave the others in place.

Brooke Halsey, a former Sonoma County Deputy District Attorney who directs the salmon institute, theorized that fishermen might have set the small salmon free to attract larger game fish.

The incident will not halt the Oct. 30 open house and fish release that Casa Grande students have been anticipating as the culmination of the many hours they dedicated to raising and tagging their 2010 fall-run chinook salmon.

Gillis, the United Anglers president, said he was glad there was no apparent harm done to the salmon.

"They released them and they're fine," he said. "But it's the fact that they are our fish."