Two trucks loaded with salmon smelts left Coleman National Fish Hatchery near Battle Creek River bound for San Pablo Bay.
By the time one of the trucks got to the ocean, three-quarters of the fish inside were dead.
Hatcher officials explained the loading process.
"Our fish are crowded to a pump, they're lifted up to a de-watering tower. The fish go into the truck, the water doesn't, and as this truck is loaded and is pulled away, we'll have about 80 to 90,000 fish in this truck right behind me," said project leader Scott Hamerberg.
Biologists are trying to figure out if it was too much oxygen, or not enoughthat killed the 75,000 salmon, about five per cent of the total.
Two more truckloads are scheduled to go out again on Tuesday morning.