Union Bank will lay off almost 500 Santa Barbara Bank & Trust employees, or nearly half the bank’s work force, the San Francisco-based lender announced on the eve of its acquisition of Pacific Capital Bancorp.

A spokesperson for Union Bank told the Business Times that 468 SBB&T positions will be eliminated over the next year. Union Bank will retain 570 Santa Barbara Bank & Trust employees, mostly customer-facing personnel, he said.

About 80 percent of the layoffs won’t occur until April 30, 2013. The laid off employees can apply for 175 open positions with Union Bank from Los Angeles to San Jose, the spokesperson said via email.

San Francisco-based Union Bancorp is acquiring Santa Barbara-based Pacific Capital Bancorp, parent of Santa Barbara Bank & Trust, for $1.5 billion. The deal is slated to close on Saturday, Dec. 1, meaning Pacific Capital Bancorp shares would stop trading on the Nasdaq at the market close on Friday.

The Union Bank spokesperson did not give a reason for the layoffs. The two banks do have some overlap in their branch networks in North Santa Barbara County, in San Luis Obispo and in Ventura County.

The merger will provide Union Bank with a coveted network of 45 branches between Westlake Village and the Monterey Peninsula, greatly elevating the Union Bank presence in a region that could be a rich source of deposits and loans to small and medium-sized businesses.

During 2013, Union is expected to rebrand all of the SBB&T operations to carry the Union banner, ending a 52-year-old Santa Barbara brand in the region.