The number of cruise passengers sourced from the U.S. isn’t growing as fast as it once was, leaving U.S. travel agents with a market that may contract if trends continue.

Figures from the 2014 Economic Impact study recently released by CLIA show that growth has declined in each of the past four years.

While the absolute number of U.S. passengers has grown since 2010, from 10.1 million to 10.7 million last year, the relative share sourced from the U.S. has dropped from 68% to 61%.

Growth was a barely positive 0.3% in 2013, following increases of 2.2% in 2012, 3.5% in 2011 and 6.2% in 2010.

Separately, the number of passengers embarking on a cruise at a U.S. port last year fell 1.3%.

What accounts for the slowdown?

Andrew Moody, one of the authors of the study and president of Business Research and Economic Advisors of Exton, Pa., cited several factors that could be diminishing growth in the U.S.

One is that cruise lines have put the brakes on ship construction in recent years, to try to allow prices to rise. That has slowed growth in the supply of cabins available for sale.

At the same time, the industry has been devoting ships both new and old to distant markets and has increased its capacity to sell cruises on those ships in Europe, Asia and elsewhere.

“Marketing is becoming global,” Moody said. “As more and more capacity comes on, they’re going to move it to some of these markets. Certainly Australia has been a singular market [that has] benefited from all that.”

The Carnival Spirit and Royal Caribbean International’s Rhapsody of the Seas are among the ships operated by North American lines that have been shifted full time to Australia in recent years, helping to make it the fastest-growing cruise market.

In contrast to the Australian economy, which never really lost its footing, the U.S. economy has been in recovery mode since 2008, Moody said, another factor slowing growth.

And while the U.S. economy is on the rebound this year, growth in the cruise sector has been uneven. That shows up in the count of passengers embarking from a U.S. port, which fell in 2013 to less than 10 million, vs. nearly 10.1 million in 2012.

The number of passengers sourced from the U.S. includes residents who went abroad to catch their cruise, be it to Barcelona or Vancouver. The number embarking from U.S. ports could include foreign customers who fly to the U.S. to board a cruise but is mostly Americans.

That 1.3% decline Moody attributed mainly to a slump in the Western states.

“Some of that decline has to do with the Mexico/West market,” Moody said. “It still remains very weak, and that has an effect on the California ports.”

On the East Coast, Florida ports are holding their own, and New York has seen an increase in embarkations, Moody said. “The California declines offset a lot of the growth elsewhere in the country,” he said.

In a separate study of the impact of the cruise industry on the global economy, Business Research and Economic Advisors said the top 10 sources of cruise passengers worldwide, in descending order, were the U.S., U.K., Germany, Italy, Australia, Canada, Brazil, China, France and Spain.

Beyond the slowdown in U.S. passenger growth, the impact study estimated that for the first time, direct spending by North American cruise lines, passengers and crew in the U.S. topped $20 billion.

The global study pegged direct spending by cruise lines, passengers and crew worldwide at $52.3 billion last year.