Import cargo volume at major US retail container ports is beginning to pick up again, ending a summer-long downturn as retailers prepare for the holiday season.

According to the monthly Global Port Tracker report released by the National Retail Federation, US ports handled 1.32m 20-foot equivalent units (TEU) in July - up 6% from June but down 4% on July last year. One TEU is one 20-foot cargo container or its equivalent.

July was the second month in a row to show a year-over-year decline, and August was flat compared with last year, at an estimated 1.42m TEU.

Rather than indicating an economic downturn, however, the numbers are a skewed comparison against higher-than-normal numbers last summer, when fears of shortages in shipping capacity caused many retailers to bring holiday merchandise into the country earlier than usual.

"With the most crucial spending period of the year just weeks away, retailers have made careful decisions on the amount of merchandise they need to properly stock their stores during the holidays," said Jonathan Gold, NRF vice president for supply chain and customs policy.

"This year, retailers have the luxury of importing holiday goods later than last year, which better ensures their inventory levels will accurately meet consumer demand."

Year-on-year growth is beginning to resume in September, which is forecast to be up 11.8% from last year at 1.5m TEU. October is seen at 1.48m TEU, up 9.5%; November at 1.33m TEU, up 8%; and December at 1.2m TEU, up 4.5%. January 2012 is forecast at 1.19m TEU, down 1% from January 2011.

The total for 2011 is forecast at 15.4m TEU, up 4.3% from 2010. Imports during 2010 totalled 14.7m TEU, a 16% increase over unusually low numbers in 2009.

However, the researchers also warn that the relatively strong import volumes of August and September are due the low levels of inventory that needed to be raised to meet the return-to-school and post-Thanksgiving sales.

While growth is expected in the third quarter, decreases are forecast for the following two to three quarters.