WASHINGTON — The Cosco Busan was the only big ship to get under way the morning it sideswiped the San Francisco Bay Bridge, as at least four other large vessels waited out the heavy fog, the Coast Guard said Thursday.

The decision of the Cosco Busan’s crew and pilot to get moving despite the weather is a focus of the Coast Guard’s investigation into the Nov. 7 collision that spilled 53,000 gallons of oil into the Bay’s fragile ecosystem.

With visibility less than a quarter-mile, pilots and crew on the other four ships “deemed it prudent not to get under way until visibility improved,” Rear Adm. Craig Bone, the Coast Guard commander for California, told reporters Thursday in a preliminary assessment of the accident.

“These human decisions are critical elements to causal factors of this casualty,” Bone said.

Bone also disclosed that the Coast Guard has instituted a policy in the Port of San Francisco prohibiting ships larger than 1,600 gross tons from traveling in certain areas of the port when visibility is less than a half-mile. The Cosco Busan was more than 65,000 gross tons.

The ongoing Coast Guard investigation is one of several. Bone disclosed some information learned and preliminary steps are being taken, but declined to reveal other details or assign blame for the decision to start the ship that morning.

The Cosco Busan’s pilot, Capt. John Cota, has been charged with criminal negligence and breaking environmental laws and pleaded not guilty earlier this month. He’s the only person charged so far in the incident. In citing human error, Bone noted that Cota wasn’t the only one aboard the ship that morning.

The Coast Guard also played, for the first time, audio of the communications between Cota and the Coast Guard’s Vessel Traffic Service in the moments before and after the crash. Written transcripts of the communications already had been made public.

On the audio the traffic service can be heard questioning the cargo ship’s heading as it appears off-course for the opening in the Bay Bridge called the Delta-Echo span. Cota tries to correct course but it’s too late.

“Traffic, we just touched the Delta span,” he can be heard to say after the vessel hits the bridge.

A report from the inspector general of the Homeland Security Department is due out next month and the National Transportation Safety Board is also holding a hearing next month. A Justice Department investigation is ongoing, along with congressional probes.

On the Net:

Cosco Busan weather conditions: http://cgvi.uscg.mil/media/main.php?g2—itemId244267

http://cgvi.uscg.mil/media/main.php?g2—itemId244264

Cosco Busan: http://cgvi.uscg.mil/media/main.php?g2(underscore)itemId205279