NEW YORK ( TheStreet) -- The nation's major retail ports are expected to see container traffic spike by 16% in July, but it will likely mark the end of a string of double-digit increases as retailers play it safe with inventory levels heading into the holiday season.According to the National Retail Federation's monthly global port tracker report released on Thursday, key U.S. ports handled 1.3 million containers in May, representing a 20% increase from year-ago volumes and a 10% increase over April 2010 volumes. It was also the sixth consecutive month of year-over-year increases. Estimates for June volumes are pegged at 1.24 million containers, representing a 22 increase, while July is anticipated to post a 16% gain to 1.3 million containers. Despite the positive momentum, retailers are expected to pull back on orders from their overseas suppliers as they head into the late second half of the year. October is traditionally a peak-volume month for the nation's ports as retailers bring goods into the country for the holiday season. The NRF's report estimates October container volumes will come in at 1.24 million containers, representing an increase of only 4%. "Retailers are being cautious as they look at numbers for employment, housing and the availability of credit," said Jonathan Gold, the NRF's vice president of supply chain and customs policy. "There clearly can't be consistent growth in consumer spending when customers don't have jobs. That means retailers are going to have to manage their inventories more carefully as the year progresses." The news comes on the heels of lackluster same-store sales results for June and a report from IBISWorld released on Friday dubbing retail as one of the riskiest sectors of the U.S. economy. -- Reported by Ross Tucker in New York. Get more stock ideas and investing advice on our sister site, Stockpickr.com. Follow Ross Tucker on Twitter.