China’s fragile stock markets started weaker on Thursday after Wall Street struck its lowest levels since 2014, though the main indexes for Shanghai and Shenzhen bourses still clung to gains for the week. The currency also had a volatile start to 2016, but the People’s Bank of China (PBOC) has kept a steady course for the yuan’s daily midpoint fix, from which it can vary by up to 2 percent, for two weeks.