Job growth at the smallest businesses has fallen to the lowest levels in nearly eight years as tiny companies struggle to attract and retain workers in the tightest U.S. job market in half a century.

The number of people employed by companies with fewer than 20 workers grew by less than 1 percent in both March and April, compared with the same months a year earlier, according to an analysis by Moody’s Analytics for The Wall Street Journal. Hiring at the smallest businesses hasn’t been this low since May 2011, when the economy was still recovering from the financial crisis.