Is the craft beer craze going flat?

Craft brews have long been a bright spot for the struggling U.S. beer industry. But Americans' thirst for craft beer may be on the wane.

Citing Nielsen data, Trevor Stirling of Bernstein Research shows that craft beer sales are up only 6 percent this year, well under the 19 percent increase seen only two years ago. "The consensus is that craft beer has slowed down significantly this year," he said. "The scale of the slowdown has come as a surprise to everyone."

One potential explanation for the slowdown is the increasing fragmentation of the beer market, with more small-scale local breweries opening in recent years. Over the last six years, marketshare for the top 10 craft brewers has slipped to 39 percent, from 50 percent.

The Brewer's Association, the trade group representing the craft brewing industry, isn't concerned about the decline.

"Certainly, there are challenges for particular companies out there," said Bart Watson, chief economist for the Brewer's Association, asserting that some of the drop-off in growth is the result of changes in how market researchers gathered their data. "It's hard to know what the exact number is out there. If it's 6 percent or 10 percent, that's a number that many industries in the U.S. economy would kill for."

Boston Beer (SAM), the largest U.S. craft brewer, has been hurt by the decline in demand. In its most recent quarter, the Boston-based maker of Sam Adams beer and Angry Orchard cider reported that sales were far below its expectations. The company also reported weaker-than-expected earnings and slashed its outlook, citing increased competition from smaller brewers, among other reasons.

The company's stock price has dropped about 20 percent this year.

A spokeswoman for Boston Beer did not immediately respond to a request for comment.

Other large craft brewers are also hurting, with five of the top 10 companies facing shrinking sales. Constellation Brands' (STZ) Ballast Point and Lagunitas, which is partially owned by Heineken, are among the exceptions.

Anheuser Busch-InBev (BUD), the world's largest brewer, has been expanding its footprint in the craft beer market for years, adding Goose Island in 2011. The Belgian-based company has added six brewers in the past two years and four in the last six months.

The Brewers Association estimates there were nearly 4,300 breweries in the U.S. as of 2015, up 14 percent from the previous year and from only 93 such breweries in 1983. Craft beer accounts for about 21 percent of total U.S. beer sales. A craft brewery is defined as one that produces no more than 6 million barrels of beer per year and whose beer gets its taste from traditional brewing ingredients, among other criteria.

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