Retailers appear to be stocking up on merchandise.

Import cargo volume at the nation’s major retail container ports is expected to jump 16 percent during September compared to September 2009, according to the National Retail Federation’s Global Port Tracker report.

Economists look to cargo volume figures for October, the traditional peak month of the annual shipping season because retailers typically ship in merchandise for the holiday season then, but the big jumps in orders during summer months could indicate retailers are stocking up earlier to avoid shipping capacity problems.

U.S. ports handled 1.38 million, 20-toot equivalent units in July, the latest month in which actual figures are available. That was up 5 percent from June and 25 percent compared to July 2009, the eighth consecutive month to show a year-over-year improvement.

August was estimated at 1.35 million TEU, a 17 percent increase over last year.