Shipping rates extended gains to the highest since 2011 as Chinese steel mills replenish iron ore stockpiles amid record output.

The Baltic Dry Index, a benchmark for costs to ship iron ore, coal and grains, rose for an eighth session, adding 5.6 percent to 1,628, the highest since December 2011, according to the Baltic Exchange, the London-based publisher of freight rates. Earnings for Capesizes, the largest ships tracked by the gauge, led the gain, climbing 9.8 percent to $29,674 a day, data show.

The rally follows six months of Capesize rates averaging below operating costs. Earnings are still 87 percent below the 2008 record as owners ordered too many ships before the global recession. The glut of vessels may push rates down again after China’s import spurt subsides, said Nigel Prentis, head of consultancy at Hartland Shipping Services Ltd., a London-based shipbroker.

“Now that the steel mills are restocking iron ore, that’s all it takes,” Prentis said by phone. “It’s very easy to create a squeeze when you have a sustained import program and start to run out of ships in position. At some point the oversupply situation will is likely to come back to the fore.”

Chinese steel production reached a record 91.94 million metric tons in August, 17 percent higher than a year earlier, data compiled by Bloomberg show. Stockpiles of iron ore, the main ingredient, are 24 percent lower than a year ago, according to Beijing Antaike Information Development Co.

The Capesize fleet swelled 70 percent to 289 million deadweight tons since 2009, according to Clarkson Plc, the world’s largest shipbroker. Capacity will expand another 5.6 percent this year, the slowest pace since 2003, figures show.