Four days after vandals set loose 40,000 juvenile Chinook salmon, breaking the hearts of the high school students who raised them, the executive director of the Tiburon Salmon Institute was shocked Saturday morning when he discovered that net lines again had been cut overnight, releasing the remaining 20,000 fish into San Francisco Bay.

“It’s been a hell of a week and now this has happened,” said a dispirited Brooke Halsey, head of the institute. “There are nuts out there, and they did it the same way. They clipped the zip ties that hold the nets in place. We were trying to solicit funds for a security camera. But we couldn’t get a security camera put up quick enough, and they came back.”

The first act of vandalism, on Oct. 3, scotched a celebration set for the end of the month, when students at Casa Grande High School in Petaluma were to cheer on the fish they had raised from eggs as they were being set free into the bay, the finale of the institute’s annual program.

Although the remaining fish were raised by volunteers at a Feather River hatchery, the teens were still looking forward to attending an Oct. 30 open house, when those 20,000 juveniles were to be released from their floating pens at the Romberg Tiburon Center for Environmental Studies to begin their new lives beyond the Golden Gate.

Despite the latest disappointment, the institute has vowed to go ahead with the end-of-program party, even if it will be without fish.

“The kids are still obviously very upset now that this has happened a second time,” lamented Candice Guy, the director of education for the institute, whose motto is “empowering today’s youth to save tomorrow’s salmon.”

“We were planning on going forward with this release and the kids were going to take part in that,” she said. “Considering all the hard work that goes into this, it’s very disheartening to see this happen.”

Although no one has claimed responsibility for the vandalism, some animal rights activists have been critical of the way the institute has been holding the 8-to-10 inch salmon, called “smolts.”

Peter Young, in a “Voice of the Voiceless” journal for the Animal Liberation Movement, weighed in after the first episode of vandalism, calling the perpetrators “anonymous saboteurs.”

“If this was the act of animal liberators, it would be the largest recorded animal liberation ever in the U.S.,” he wrote, noting that the largest previous one was the release of 14,000 mink in an Animal Liberation Front raid on a fur farm in Iowa.

“Those who cut the nets may not have known the fish were slated to be released in the coming weeks,” he went on. “Or, they could have chosen to risk themselves anyway to give the fish a few extra weeks of freedom, sparing them the psychological suffering of being kept in intense confinement with approximately 40,000 others in a small net.”

In a People for the Ethical Treatment of Animals (PETA) blog post on Oct. 5, Heather Faraid Drennan, mistaking the Tiburon environmental education organization for a fish farm, decried the salmon being held in 16-by-25-by-8-foot pens.

“With 20,000 to a pen, this means that there were more than six 10-inch fish per cubic foot,” she wrote. “Fish kept in such crowded conditions often suffer from severe injuries, and in such filthy conditions they are also susceptible to parasites that can eat their faces down to the bone.”

The salmon institute’s director of education said there is no proof so far that the perpetrators are from an animal rights organization.

“But from what we’ve been reading on the Internet, there have been groups assuming we’re farming fish, which is not what we’re doing,” she said. “We are using these fish to educate the community on the declining salmon population, and also helping to repopulate the salmon here in the bay. “Whoever did this, they could have come in and at least talked to us about what we’re actually doing here. The fish aren’t suffering. We’re taking very good care of them.”

The institute is in partnership on this project with the San Francisco Tyee Club, a fishing and conservation group that claims to have released more than a million salmon into the bay over the past 30 years.

Halsey, the institute’s executive director, a former Sonoma County Deputy District Attorney, said the Marin County Sheriff’s Department is investigating the vandalism, and that he had a meeting planned with the FBI.

“This has put a damper on the whole project,” he said. “We’ve got to find out who did this to us and why. It’s so ludicrous.”