Horizon Lines Inc., the second-largest ocean shipper serving Hawaii, may file for bankruptcy as soon as next week after the shipping company agreed to pay a $45 million fine to resolve Justice Department price-fixing charges, according to three people familiar with the matter.

Horizon Lines failed to get bondholder approval to waive a default after the fine and may be forced to seek court protection if talks with creditors to restructure its debt fail, the Charlotte, N.C.-based company said in a regulatory filing on Monday. The company may seek to swap its debt for equity to avoid bankruptcy, said the people, who declined to be identified because the talks are private.

(Horizon accounts for approximately 36 percent of total U.S. marine container shipments from the mainland to Hawaii, Alaska and Puerto Rico, according to the filing.)

Jim Storey, a spokesman for Horizon Lines, did not return two calls and an email seeking comment. Stephen Fraser, interim chief executive officer, did not immediately return a message left on his voicemail.

Legg Mason Inc. and its Western Asset Management Co. unit, BlackRock Inc., Pioneer Investment Management Inc. and Angelo Gordon & Co. own about

85 percent of Horizon Lines’ $330 million in 4.25 percent convertible notes due August 2012, according to data compiled by Bloomberg.

The notes dropped to 80 cents on the dollar from 91.1 cents on Monday, according to Trace, the bond-price reporting system of the Financial Industry Regulatory Authority. The securities dropped from 91.125 the past two days, Trace data show.

Three years of mostly declining container volumes amid the worst recession since the Great Depression are being exacerbated by the Justice Department fine and a proposed $20 million settlement in a class action lawsuit.

Moody’s Investors Service on Tuesday downgraded Horizon Lines two levels to Caa3, its third-lowest rating.

Horizon expects to be in default on the terms of its convertible notes during the second quarter, the company said in the filing. It expects to violate the terms of its senior credit facility in the third quarter.

The company’s shares fell 35 cents, or 22 percent, to $1.27 yesterday in New York Stock Exchange trading. The shares dropped 61 percent this week and have fallen from as high as $36.55 a share in July 2007.