Investment Bank Bear Stearns Cos. released some details of the heavy losses it suffered in the months before it nearly went bankrupt and agreed to sell itself to rival JPMorgan Chase & Co.

In a securities filing made late yesterday, the bank said that its main business -- which includes stock and fixed income trading -- since the end of November fell to well less than the 50% of activity levels in 2007 and in the first quarter of 2008.

The lower trading levels reflected the significant number of its prime brokerage clients who moved their accounts to other firms.

In the report, Bear Stearns said customer margin balances -- a figure indicating how much clients were borrowing for their transactions -- fell 23 percent in the same period from $86 billion to $66 billion.

Customer short positions in that time -- or investor bets that borrowed stocks from Bear would fall -- dropped 25 percent from $88 billion to $66 billion.