Shares of Western Asset Mortgage Capital Corp. fell in their first day of trading on Thursday, after the real estate investment trust initial raised $160 million in its initial public offering.

The Pasadena company, which specializes in a type of residential mortgage-backed security called Agency RMBS, priced its offering of 8 million shares at $20 a share late Wednesday. The stock opened 6 percent lower at $18.80 a share on the New York Stock Exchange, and fell about 1 percent during the day to close at $18.55.

Western Asset Mortgage, a subsidiary of Legg Mason Inc., gave underwriters that include Deutsche Bank Securities and J.P. Morgan the option to buy up to 1.2 million additional shares to cover any over allotments. The stock trades under the ticker symbol “WMC.”