Cargo volume at the nation's major retail ports continued to drop last month, as retailers trimmed inventories and braced for a weaker holiday shopping season, the National Retail Federation reported this week.

Retail cargo in October was down by 5.7 percent from a year ago, dropping to 1.36 million twenty-foot-equivalent units, or TEUs, the Washington-based trade group said. With declines expected to continue over the few months, this year's total cargo volume is estimated to reach 15.3 million TEUs, compared to 16.5 million last year.

Retail forecasts are the lowest they've have been in at least five years, according to Jonathan Gold, the group's vice president for supply chain and customs policy.

"The balancing act between supply and demand is tougher than ever because retailers want to make sure they have enough merchandise on the shelves to satisfy customers and not be forced into unplanned markdowns to move excess inventory once the holidays are over," Gold said in a statement.

Last week, the Commerce Department reported a 0.4 percent drop in consumer spending for September, the first decline since the early 1990s.