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DUBROVNIK, Croatia — It's morning here and a human tide is pouring through the Pile Gate, the massive fortifications that have protected this ancient port city from invaders for half a millennium.
Hordes armed with credit cards and camcorders now breach Dubrovnik's medieval walls every day, part of an ever-growing army of tourists surfing a global upsurge in cruise ship vacations.
The boom is bringing much-needed cash to coastal communities around the world. In Europe alone, the cruise industry says it generates jobs for 326,000 people and contributes more than $50 billion a year to the economy.
As flotillas of Titanic-dwarfing mega-cruisers loom over historical harbors, however, residents of port cities from the Arctic to the Mediterranean are asking if the daily onslaught of passengers is causing irreparable damage to their fragile historical districts.
"We have a problem because the numbers of cruise passengers have moved beyond capacity," said Sinisa Horak of the Croatian Institute for Tourism.
"Dubrovnik is a beautiful city and we would like to be open to everyone who wants to come see that beauty, but we must take care with sustainability," he added in an interview from the capital Zagreb.
Dubrovnik is expected to receive over 1 million cruise passengers this year for the first time. That's a four-fold increase in barely a decade, which puts the Croatian port among the most popular Mediterranean destinations of Barcelona and Venice.
With the average cruise passenger spending almost $50 a day in the city, the income is more than welcome in Croatia, which is struggling with recession. It joined the European Union in July as the bloc's third-poorest member.
But critics point out that land-based tourists — who don't have all-inclusive packages keeping them eating and sleeping on board — spend around three times more per day and stay for longer.
They fear those bigger-spending landlubber visitors are being scared off by the cruise-based mobs disgorged each morning to swarm over the city walls and clog the narrow cobbled alleys of the UNESCO-protected old city.
"Dubrovnik is maybe already at the point where cruise tourism is beginning to displace land-based tourism," says Ross Klein, an expert on the cruise industry at the Memorial University of Newfoundland.
"The crowds, the congestion, all these issues that cruises draw with them mean that others stop coming,” he adds. “And the folks coming on land have a much larger economic impact."
Dubrovnik is far from alone, Klein says. The development of giant liners capable of carrying more than 6,000 passengers has cut the cost of cruising, triggering a three-decade rise in global demand.
Industry figures show worldwide passenger numbers doubling over the past 10 years, meaning almost 21 million vacationers took a cruise last year.
Such figures have left coastal cities around the world facing the same dilemma: how to balance the quick bucks brought by the rising tide of cruise passengers with the potential negative impact on local quality of life, heritage, the environment and the risk of alienating other tourists.
"It's really a big issue," says Erica Avrami, research and education director of the World Monuments Fund.
"It's a very difficult type of tourism to manage," she said from the fund's New York headquarters. “It's different from simply a lot of tourists, it's the entry of all of them at one time, that really becomes a bit problematic."
The WMF, which works to preserve architectural and cultural heritage sites around the world, organized a major international conference on the problems facing historical port cities last February in Charleston, South Carolina.
Residents associations there are locked in a dispute with the port authority over plans for a new cruise terminal, which they say will degrade the colonial-era skyline of the city's historical district, on top of increasing pollution and congestion. Supporters counter that the terminal will generate $37 million for the local economy.
Other battles to halt planned cruise-terminal construction or limit numbers of passenger disembarkations are raging from Australia to Alaska, Norway's fjords to Belize's Caribbean beaches.
The highest profile fight is in Venice. The ancient Italian city has seen cruise visits increase by 400 percent over the past five years. In 2012, 1.78 million cruise passengers tramped through Venice's historical heart, outnumbering its local inhabitants by 30 to 1.
The sight of cruise liners 18-stories high passing several times a day by the entrance to the Grand Canal and tower over the architectural splendors of St. Mark's Square tower has alarmed local residents.
Cruise opponents claim the city can’t cope with the numbers of visitors they bring in. There are also concerns that the giant ships are disturbing the underwater environment beneath Venice lagoon, undermining the island city's delicate foundations of the island city, and adding to the risk of flooding or subsidence.
Last year’s sinking of the Costa Concordia off Italy's west coast heightened concerns about environmental disaster.
"Many heritage professionals believe that the advent in the last decade of large-scale cruise-ship tourism is pushing Venice to an environmental tipping point and undermining the quality of life for its citizens," the World Monument Fund said last month as it announced it was placing the city on its "watch list" of threatened world heritage sites.
Faced with local protest and international pressure, the Italian authorities earlier this month approved plans to ban mega-ships over 96,000 tons from Venice's canals and cut the number of smaller vessels passing St. Mark's by 20 percent.
The restrictions have been condemned by the cruise industry, which claimed it could put 5,000 local jobs at risk. "This is an irrational limit devoid of any scientific foundation that will eventually bring down the port of Venice," Massimo Bernardo, president of the lobby group Cruise Venice Committee, told Italian media.
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Some 350 miles down the coast from Venice, Dubrovnik has also introduced limits on the number of cruise liners. They are supposed to keep the number of passengers in the old town to a maximum of 5,000 at a time — double the number of residents living in the walled heart of the city.
However, Croatian tourism expert Horak says the numbers aren’t respected. He says there were at least 20 days during the peak summer season when more than 10,000 cruise passengers squeezed into old city's warren of cobbled streets.
Dubrovnik's mayor has expressed concern that Venice’s new restrictions could also lead to a reduction in cruise visits to his city since fewer ships will be sailing up the Adriatic Sea.
Until then, however, residents will continue to look forward to the end of the day when they can reclaim the streets after the cruise crowds head back through the Pile Gate.