Safety regulations limit the number of passengers per ferry, based on the estimated weight of an average adult.
Under new Coast Guard vessel stability rules, which took effect December 1, the estimated weight of an adult passenger was raised to 185 pounds from the previous 160 pounds, the Associated Press reported. The new average weight was based on population data from the Atlanta-based Centers for Disease Control and Prevention.
The revised rules mean that Washington state's ferry service is carrying fewer passengers per vessel.
"That has effectively reduced the amount of passengers by about 250 passengers or so depending on the particular ferry," Coast Guard Lt. Eric Young told the AP. "They generally carry about 2 000, so it's down to 1 750 now."
Canadian newspaper the Victoria Times Colonist called Washington state's vessels "the fat ferries."
"The US Coast Guard certifies our vessels in terms of stability. It's considered a safety issue. And so, as Americans increase in weight, if we have too many passengers on them, that would affect the stability of the vessels," Marta Coursey, spokeswoman for Washington State Ferries, told the Times Colonist.
The AP noted that "the ferries themselves could be contributing to passenger girth," with the galleys selling fast food such as hamburgers, hot dogs and chicken strips.
The Washington ferry system is the biggest in the United States and the most widely used in the world, carrying more than 22 million passengers a year.
According to the CDC, a third of American adults are obese. The southern US has the highest obesity prevalence, at 29 percent, followed by the midwest and northeast of the country.