Beijing, April 29 (IANS) China's migrant workforce reached 277.5 million in 2015, an annual rise of 1.3 percent, but the year-on-year growth rate has been decreasing since 2011, according to the National Bureau of Statistics.

A report released by the bureau on Thursday said the growth rate dropped 0.6 percent in 2015, and the percentage of workers younger than 40 years dropped 1.3 percent to 55.2 percent. The average age is 38.6, about four months older than in 2014, the China Daily reported.

The slower growth of the migrant-worker population might be a result of the slower growth of income. According to the report, the average monthly salary reached 3,072 yuan ($472), while the annual growth rate dropped 2.6 points to 7.6 percent.

In the manufacturing sector, the growth rate dropped to 6.7 percent; the rate in the construction industry dropped to 4.4 percent.

In addition, 36.2 percent of migrant workers signed contracts with their employers last year, down from 38 percent in 2014, although the rate of signing short-term contracts-contracts for less than a year-grew by 0.3 percentage points.

One percent of migrant workers have unpaid wages owed them, 0.2 percent higher than last year. Unpaid migrant workers, on average had 9,788 yuan in wage arrears in 2015, with year-on-year growth of 2.9 percent.​