Global container shipping is becoming increasingly concentrated among the top ocean carriers, according to a recent report from Drewry Maritime Research, with the top five carriers dominating almost half the market.

According to Drewry, the top five carriers—Maersk, Mediterranean Shipping, CMA CGM, Hapag-Lloyd and Evergreen—have seen their market share grow almost 30 percent from 37 percent in 2005 to 48 percent today. The top three container carriers have increased their share of the global container business by over 30 percent in the last decade, from 26 percent to 38 percent. The increases has been driven both new ship deliveries and by mergers and acquisitions.

The big three carriers—Maersk, Mediterranean Shipping, and CMA CGM—have also outperformed their counterparts in the container shipping business, enjoying an average annual growth rate of 12.5 percent since 2005, while the top 20 lines lagged by behind at 9.6 percent and the total world fleet grew by 8.7 percent.

The trend toward concentration of market share among the top carriers in the last ten years flagged only in 2009 in the aftermath of the global fiancial crisis.

“The concentration of power at the top isn’t at a critical point just yet,” Drewry said in a recent edition of Container Insight Weekly, but the market is in danger of evolving into an oligopoly if the top five carriers continue to increase their market share.

The top 20 carriers now control 87 percent of the world’s container capacity, up from 79 percent in 2005. The top 20 lines added over 10 million TEUs in the last decade. The total world fleet stands at around 17 million TEUs.

Mid-sized carriers are now defending themselves against takeoovers by ordering megaships. That move inflates their values and makes them less attractive to takeover. At the same time, Drewry noted, “it guarantees years of overcapacity that will depress freight rates and profitability for all.”