Cargo volume at the ports of Los Angeles and Long Beach slowed in September, a normally busy shipping month, indicating the economy remains weak, according to figures released Friday.

At the Port of Los Angeles, 583,557 cargo container units, or TEUs moved last month, a decline of 16 percent compared to the 692,889 containers handled in September 2008.

At Long Beach, 440,364 TEUs moved through the port, a 21 percent decline compared to the 554,837 TEUs handled a year ago.

The September numbers at both ports are down from August, marking a slowdown in shipping just as international trade should be heating up in anticipation of the holiday shopping season.

Both ports had seen their cargo volume peak in August after hitting a low point in February. In Los Angeles, cargo volume was 612,581 TEUS in August; in Long Beach it hit 493,339 TEUs.

“September’s numbers show that retailers are holding tight until they see more consumer confidence for sales this holiday season,” said Jack Kyser, chief economist with the Los Angeles County Economic Development Corp. “And if they do order more last minute, most of it will likely come through air freight instead of by ship, which doesn’t bode well for the ports.”