Lower production at Hawai'i's largest oil refinery contributed to a first-quarter loss at Tesoro Corp., the San Antonio refiner that operates seven refineries nationwide.

Tesoro said it swung to a first-quarter loss of $82 million from a $116 million profit a year earlier as gross refining margins fell and product prices failed to keep up with swelling crude oil prices.

The company reported a net loss of 60 cents a share, or more than analysts projected, compared with a profit of 84 cents a share a year earlier.

Tesoro, which also operates gasoline stations in Hawai'i, had lower production at its Campbell Industrial Park refinery as it undertook projects to give it more flexibility in processing crude. In February the company said the refinery pulled down profitability and that it would upgrade the plant.

At the time, Tesoro President and CEO Bruce Smith was questioned on whether it made sense to upgrade the local plant since it had been one of the company's least profitable. He responded by saying an analysis had shown profits could be improved.

Yesterday, the company reported it was making progress with several profitability initiatives in Hawai'i, including reducing reliance on light sweet crude from Asia and getting better prices for finished products. It also is working on better reliability with modernized controls and has installed a new electronic substation.

During the quarter the refinery reduced more expensive light sweet crude as a percent of total throughput to 19 percent from 30 percent a year earlier.

The Hawai'i refinery's output fell to 69,000 barrels a day of gasoline, jet fuel, diesel and other petroleum products from 85,000 a year earlier as it worked on improvements and suffered a temporary outage in February from a power failure and fire.

Tesoro spokeswoman Natalie Silva did not immediately return calls seeking comment.

The company has scheduled a conference call today to discuss the results.