Christie’s International, the world’s leading auction house by revenue, reported a 5 percent decline in annual sales following five straight years of growth, a sign that the art market may be slowing amid stock price volatility and greater selectivity by wealthy buyers.

Sales of art and collectibles fell to 4.8 billion pounds in 2015, or $7.4 billion using a sales weighted currency exchange rate, Christie’s said in a statement, from about 5.1 billion pounds in 2014. Sales of postwar and contemporary art, Old Masters, 19th century and Russian art declined.

Roiling equity and bond markets, plummeting oil prices and economic weakness in China are crimping billionaires’ appetite for buying art. Despite records for individual masterpieces in 2015, sales in three key categories have contracted at the major auction houses and the outlook for 2016 is less optimistic, according to a survey by researcher ArtTactic published Monday.

“The froth is starting to come off,” Anders Petterson, founder and managing director of ArtTactic, said in a telephone interview. “Looking back, in terms of auction sales, 2014 was the peak of the market.”

Christie’s, which is privately owned by French billionaire Francois Pinault, doesn’t report profit or loss.

‘Exponential Growth’

Sales at Sotheby’s totaled $6.7 billion in 2015, unchanged from 2014, spokesman Dan Abernethy said in an e-mail.

Christie’s saw declines in several categories. Its postwar and contemporary auctions fell 14 percent to 1.5 billion pounds. Auctions of Old Masters, 19th century and Russian art declined 37 percent to 154.9 million. Private sales were down 39 percent to 554.9 million pounds.

Postwar and contemporary art “has seen an exponential growth in the past 10 years,” Stephen Brooks, Christie’s deputy chief executive officer, said in a telephone interview. “It won’t come as a surprise to me if we see some moderation in that arena.”

Impressionist and modern art countered the decline in contemporary art, rising 57 percent to 1.3 billion pounds. Christie’s boosted the segment by organizing additional themed, cross-category auctions, led by 20th century masterpieces. The industry’s top three results of 2015 were achieved during these auctions in May and November in New York.

Picasso Sale

On May 11, Pablo Picasso’s painting “Les Femmes d’Alger (Version O)" fetched $179.4 million, the highest auction price for any artwork. In the same auction, Alberto Giacometti’s bronze sculpture of a pointing man sold for $141.3 million. Amedeo Modigliani’s “Nu Couche" fetched $170.4 million on Nov. 9.

“Despite these high-profile, media-grabbing, marquee sales, the truth is that the art market has softened in 2015,” said Todd Levin, director of Levin Art Group, who advises collectors.

Publicly traded Sotheby’s will report its 2015 earnings in late February. Sotheby’s auctions of Impressionist and modern art rose 21 percent to about $1.7 billion and its contemporary auctions climbed 8 percent to $1.8 billion, Abernethy said. Private sales increased 7.7 percent to $673 million, the company said in a regulatory filing on Jan. 22.

Sales Strategy

Christie’s attributed its decline in private sales to a change in strategy that promoted special auctions focused on masterpieces. The three top artworks were consigned to auctions after Christie’s advised the sellers that they may get “a breakout price” for the works, Brooks said.

“In the past, they may have gone down as private sales,” he said.

Such breakout prices are often backed by undisclosed guarantees the auction houses offer owners regardless of a sale’s outcome. While they support escalating prices for top works, the guarantees also can erode profitability.

Sotheby’s issued the largest single guarantee in auction history to win the collection of its former chairman A. Alfred Taubman, forgoing any commission on sold works until it reached $515 million and promising to cover any shortfall. The company will lose at least $12 million on the guarantee and expenses, according to a filing.

‘Unsustainable’ Guarantees

“The auction houses realize that this guarantee race is unsustainable and not profitable,” Petterson said. “They need to reduce that kind of exposure.”

There are signs that the auction houses are already bracing for a potential slowdown by slimming down their sales of Impressionist and modern art in London next month, the market’s first test of 2016.

At Sotheby’s, the mid-point estimates are down 16 percent from last year’s mid-point estimates, Chief Executive Officer Tad Smith said during a call with investors last week. Christie’s middle estimates for the same auctions are 4 percent lower than a year ago, according to data provided by a spokeswoman.

Still, said Brooks, “If 2015 saw a teeny-weeny reduction, I don’t see it as a weak market. I see it as absolutely solid.”