Environmental officials believe the amount of diesel fuel that flowed into Rich Passage late Wednesday and Thursday was less than 100 gallons after a small but hot fire destroyed a shed and an air compressor at the fish-rearing pens off Bainbridge Island’s Beans Point.

Seattle-Bremerton ferry personnel reported the fire on the pier containing the outer salmon pen to the U.S. Guard, which then alerted the Bainbridge Island Fire Department.

“It was an accidental fire,” said BIPD Assistant Chief Jared Moravec. “We found a structure (10-by-20 feet) on fire that housed an air compressor used to aerate the fish pen. It was some kind of malfunction.”

Moravec said the diesel fuel tank, which he estimated holds about 400 gallons, was not involved in the fire, “but a fuel line ran from the tank to where the air compressor was along the floating walkways.”

Coast Guard and state Department of Ecology officials were working Thursday with the owner, American Gold Seafoods, to mitigate the spill.

“Once we got there we determined the tank was three-fourths full, but we don’t know how much fuel was originally in the tank.,” Moravec said. “A conservative guess is we had a spill of less than 100 gallons, and probably less than that.”

Devon Blankenship, manger of the pier, said the Coast Guard put down oil booms upon arrival.

“It was an intense fire but it was easy to put out,” he said, “It didn’t extend beyond the shed and the air compressor. It singed some of the nets, but no fish were lost.”