When the population of a port district that has three commissioners reaches five hundred thousand, in accordance with the latest United States regular or special census or with the official state population estimate, there shall be submitted to the voters of the district, at the next district general election or at a special port election called for that purpose, the proposition of increasing the number of commissioners to five.

At the next district general election following the election in which an increase in the number of port commissioners was authorized, candidates for the two additional port commissioner positions shall be elected as provided in RCW 53.12.130.