Major league umpires strike on Opening Day, and amateur umpires are used as replacements. The arbiters, whose working agreement expired on December 31st, will settle and return to work the next day. Among the benefits won by the Major League Umpires Association is an increase in starting salaries from $41,000 to $60,000.

This Day In Baseball

On April 8, 1991 – Major league umpires strike on Opening Day, and amateur umpires are used as replacements. The arbiters, whose working agreement expired on December 31st, will settle and return to work the next day. Among the benefits won by the Major League Umpires Association is an increase in starting salaries from $41,000 to $60,000.

Source
Baseball Reference April 8