PORTLAND, Ore. (TheStreet) -- All those roller coasters, spools of cotton candy and workers sweating it out in mascot costumes this summer aren't just U.S. roadside attractions, but vital pieces of the nation's nearly $15 billion amusement park industry.

The more than 400 parks and attractions that make up the U.S. amusement park circuit welcome roughly 300 million visitors through their gates each year, according to the International Association of Amusement Parks and Attractions in Alexandria, Va. The Top 20 amusement parks in North America that accounted for 131.5 million of those visitors last year have seen attendance rise 17% in the past decade, according to the Theme Entertainers Association amusement industry group in Burbank, Calif.

More importantly, they employ more than 100,000 year-round workers, plus upward of 500,000 seasonal workers each summer. It isn't exactly a bad gig, either, as a 2011 IAAPA survey found that 28% percent of Americans said they liked the idea of working for an amusement park company.

That's a quarter of the country that's willing to endure rides full of screaming patrons, extreme heat, musty costumes, interminable lines of disgruntled customer and lots of food made primarily with sugar or deep fryers just to get a little sun and a paycheck. Who can blame them? With few exceptions, it's fairly steady work. Roughly a quarter of Americans have visited an amusement park within the past year and 43% are planning to go within the next year.

Market research firm IBISWorld notes that even during the past five years since the recession, the U.S. amusement park industry has grown 4% each year. For the larger companies behind those rides, games, shows and other attractions, overseas growth during the U.S. downturn managed to minimize the damage while increasing their brand's global footprint. Worldwide, the Top 10 amusement park ownership groups alone welcomed 377 million visitors to their facilities, which was an increase of 5.4% from 2012.

A whole lot more are expected this year as parks add attractions and the economy picks up. The following five companies are not only behind most of the parks U.S. vacationers will be visiting this summer, but are some of the most successful entertainment companies in the world. They don't attract "guests"; they draw the population of small countries on an annual basis:

San Diego; Orlando, Fla.; San Antonio, Texas; Sesame Place in Langhorne, Pa.; Adventure Island in Tampa, Fla.; Water Country USA in Williamsburg, Va.
Number of visitors in 2013: 23.4 million

The bad news is that a whole lot of people saw the documentary Blackfish last year and have some serious questions about the treatment of orcas by SeaWorld and the ethics of animals as entertainment in general.

That helped drag down attendance at all SeaWorld parks by 4.1% in 2013 just after its parent company SeaWorld Entertainment went public. That includes a 5% drop at SeaWorld Orlando, the No. 19 amusement park in the world and 10th-largest in the U.S., with attendance of more than 5 million last year. That also hurt SeaWorld San Diego, which ranked No. 22 globally and No. 11 stateside in 2013 with 4.3 million visitors.

Worse for SeaWorld, it overshadowed what should have been a promising new penguin exhibit in Orlando and marred what was supposed to be its first year carrying its own weight after being sold off by Anheuser-Busch to the Blackstone Group in 2009. SeaWorld has gone on a public relations campaign refuting the documentary's claims and vouching for the safety of both its animals and their handlers, but is still waiting to see if its misfortune carries over to this summer.

Cedar Fair
California's Great America in Santa Clara, Calif.; Canada's Wonderland in Vaughn, Ont.; Carowinds in Charlotte, N.C.; Cedar Point in Sandusky, Ohio; Dorney Park and Wildwater Kingdom in Allentown, Pa.; Gilroy Gardens in Gilroy, Calif.; King's Dominion in Doswell, Va.; Kings Island in Mason, Ohio; Knott's Berry Farm in Buena Park, Calif.; Michigan's Adventure in Muskegon, Mich.; Valley Fair in Shankopee, Minn.; Worlds of Fun in Kansas City, Mo.
Number of visitors in 2013: 23.5 million

That downturn by SeaWorld allowed Cedar Fair and its enormous collection of North American amusement parks to leap into the No. 7 slot among amusement park companies worldwide. Its visitor base grew only 0.9% last year, but Cedar Fair will take that over a loss.

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And why shouldn't it? Cedar Fair has four of the Top 20 amusement parks in the North American market, with No. 13 Knott's Berry Farm (3.7 million visitors, 5% growth in 2013), No. 14 Canada's Wonderland (3.6 million, 2% decline), No. 15 Cedar Point (3.4 million, 5% growth) and No. 16 Kings Island (3.2 million, flat growth). It also has five parks ranked among the Top 6 in the world for most roller coasters. It has 74 roller coasters in Cedar Point, Canada's Wonderland, Kings Island, Kings Dominion and Carowinds alone.

Adding former Disney exec Matt Ouimet has helped update the company's infrastructure and get all the parks under one umbrella, but his predecessor Dick Kinzel's purchase of Paramount's theme parks from CBS in 2006 and his refusal to sell to Apollo Global Management in 2009 helped Cedar Point retain its identity and grow into the continental powerhouse it is today.

Six Flags
Locations: Great Escape in Queensbury, N.Y.; America in Mitchellville, Md.; Discovery Kingdom in Viejo, Calif.; Fiesta Texas in San Antonio; Great Adventure in Jackson, N.J.; Great America in Gurnee, Ill.; Magic Mountain in Valencia, Calif.; New England in Agawam, Mass.; Over Texas in Arlington, Texas; St. Louis in St. Louis, Mo.; Over Georgia in Austell, Ga.; Mexico in Mexico City; La Ronde in Montreal
Number of visitors in 2013: 26.1 million

Kind of hard to believe that this company was in bankruptcy as recently as 2009.

After selling off 10 properties between 2007 and 2010, selling its stake in Dick Clark Productions and cutting ties with licensed properties including The Terminator, Thomas The Tank Engine, The Wiggles and Tony Hawk, new Chief Executive Jim Reid-Anderson righted the ship and made the world's fifth-largest amusement park company fun again. Park attendance was up 1.4% last year, season passes sold at a brisk pace and new attractions including Magic Mountain's Full Throttle and Superman: Escape From Krypton brought in a flood of fresh faces.

Magic Mountain, the 18th-largest amusement park in North America, saw attendance increase 3.5%, to 2.9 million. No. 19 Great Adventure saw visitors increase 5% and push its total crowd to 2.8 million. Six Flags got some big help in Mexico as well, as Six Flags Mexico increased its number of visitors 1.7%, to 2.35 million thanks to its new Joker-themed ride.

Having three of the Top 20 water parks in North America during a rain-soaked summer didn't exactly hurt the cause, either. Still, despite all of its Warner Brothers and D.C. Comics tie-ins, Six Flags still has a long way to go if it wants to hang out in the top tier with the following chains ...

Universal Parks and Resorts
Locations: Universal Studios Hollywood in Los Angeles; Universal Studios Orlando and Islands Of Adventure in Orlando, Fla.; Universal Studios Japan in Osaka; Universal Studios Singapore
Number of visitors in 2013: 36.4 million

Comcast-owned Universal has four of the Top 20 amusement parks in the world and three of the Top 10 in the U.S., but neither Universal Studios in Orlando (7 million visitors and 14% growth in 2013), its nearby Islands Of Adventure (8.1 million, 2% growth) or Universal Studios Hollywood (6.1 million, 4% growth) are any match for Osaka.

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Universal Studios Japan is large beyond comprehension. It greeted 10.1 million visitors last year, increased foot traffic 2% from 2012 and was the only park in the global Top 10 not affiliated with a certain cartoon mouse. Orlando added a 3-D Transformers ride, tacked on an entire Springfield area dedicated to The Simpsons and grew a whopping 14% and still couldn't match Japan's enthusiasm. Osaka still hadn't received its version of Harry Potter's Wizarding World -- which just opened in Osaka this summer -- and it's still putting up huge numbers.

The scary part is that Universal isn't done. Its Singapore location just topped 3.6 million visitors and grew 5% with much of its most-awaited features still on the way. Meanwhile, Universal is planning locations in South Korea (2016), Beijing (2017) and Moscow (2018) and may yet revive a stalled project in Dubai. Considering two of the Top 15 amusement parks in the world are in South Korea (Lotte World and Everland, which took in 14.2 million visitors combined last year) and another two are in China (Ocean Park and Hong Kong Disneyland, with nearly 15 million visitors), this big-budget theme park chain is about to get even bigger.

Disney Parks
Locations: Disneyland and California Adventure in Anaheim, Calif.; Walt Disney World in Orlando, Fla.; Tokyo Disney Resort in Urayasu, Japan; Disneyland Paris in Marne-la-Vallee, France; Hong Kong Disneyland; Disney Cruise Lines
Number of visitors in 2013: 132.5 million

Just look at that attendance figure. It's not only about four times as many people as Universal welcomed to its parks last year, but it's enough to make Disney Parks the 10th largest nation in the world -- just behind Russia (146 million) and ahead of Japan (127 million).

That's roughly 2% of the global population, and it's about to get a lot larger. Shanghai Disney is on track to open in 2015 and will be the second Disney Park in mainland China. The first, in Hong Kong, drew 7.4 million visitors last year and saw demand skyrocket 10.4% from 2012. Only Walt Disney Studios in the Disneyland Paris complex drew fewer visitors with 4.4 million in 2013.

Not that Disney's really hurting anywhere on the globe. Attendance dropped 7% at Disneyland Paris last year and it still managed to be the No. 6 theme park in the world with 11.2 million visitors. The Magic Kingdom in Orlando's Disney World increased admissions 6%, to 18.6 million people last year. That's more than double what the biggest park in the U.S. not owned by Disney -- Universal's Islands of Adventure with 8.1 million visitors -- was able to do during the same span. Its least-popular U.S. park, Disneyland's California Adventure Park (8.5 million visitors), still managed to outdraw any other non-Disney park in the country.

Even its unpopular Studios in Paris would be the biggest amusement park in the Six Flags or Cedar Fair portfolios if either had the great fortune of adding it. How do you compete with a foe of that size? You don't. You just try to be the closer, less expensive option.

-- Written by Jason Notte in Portland, Ore.

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Jason Notte is a reporter for TheStreet. His writing has appeared in The New York Times, The Huffington Post, Esquire.com, Time Out New York, the Boston Herald, the Boston Phoenix, the Metro newspaper and the Colorado Springs Independent. He previously served as the political and global affairs editor for Metro U.S., layout editor for Boston Now, assistant news editor for the Herald News of West Paterson, N.J., editor of Go Out! Magazine in Hoboken, N.J., and copy editor and lifestyle editor at the Jersey Journal in Jersey City, N.J.