July 7, 2014 6:45 p.m. ET

Speaking up is paying off.

Activists are once again at the top of the hedge-fund heap, after a profitable stretch of clashes with companies around the world.

Activist managers gained 6.5% in the first half of the year, almost double the total for the average hedge fund, according to data to be released this week by research firm eVestment. Activist investing, in which managers buy stakes in companies and then agitate for changes in the form of buybacks, divestitures or management shakeups, was also the top-performing strategy among hedge funds in 2013.

The fund managers could earn millions for themselves—and billions for their investors—if the gains stick through the end of the year.

But with deal making near its historical peak, some investors and analysts wonder if the activist rally could start to sputter.

William Ackman, founder of Pershing Square Capital Management LP, is likely the biggest winner among the larger firms this year. His main fund posted a 25% gain in the first half, investor documents show, likely earning his firm fees estimated at nearly $1 billion so far in 2014.

Mr. Ackman has profited from his unusual backing of Valeant Pharmaceuticals International Inc.’s bid for Botox maker Allergan Inc., as well as a recent decline in the price of Herbalife Ltd., the nutritional-supplements maker he is betting against. Allergan traded at $116 a share before Pershing Square began rapidly building its stake in April. Shares closed Monday at $165.85, or 51% higher. Herbalife, which closed Monday at $66.18, is down 16% this year.

Mr. Ackman has long been known for his large, concentrated public bets. But this latest rise of shareholder activism, which was earlier known by less-flattering terms like corporate raiding, is creating new standouts.

Keith Meister spent most of his career as a little-known deputy of Carl Icahn before striking out on his own three years ago to start Corvex Management LP in New York. Since the start of last year, Corvex has more than tripled in size to manage more than $7 billion.

Its main fund is up nearly 11% this year, a person familiar with it said, helped by the resolution of Corvex's battle against CommonWealth REIT and the purchase of jeweler Zale Corp. by Signet Jewelers Ltd., a large Corvex holding.

Despite the strong first half from activists, they still trailed the broader stock market: The S&P 500 rose 7.1% in the first half, including dividends.

Hedge funds as a whole gained about 3.1% in the first half, though most managers profess not to measure themselves against the broader stock market, because they aspire to do well in both rising and falling markets, and because they also invest in areas outside equities.

Bolstered by wealthy investors eager to profit from corporate changes, activist managers continue to engorge with cash—they attracted some $4 billion of new allocations in May alone, eVestment said.

"It's not just the money that's gone in, but the world is more supportive of what they are doing," said Justin Sheperd, chief investment officer at Chicago-based Aurora Investment Management LLC, which puts more than $9 billion into hedge funds. "It's a different environment today."

Activists also are increasingly busy. According to FactSet SharkWatch, there were 148 activist campaigns launched in the first half of this year, the most since the financial crisis.

The tide could yet turn if stock markets stumble. Many activists, as well as others who bet on company changes, lost money in the second half of 2011, when economic uncertainty put the brakes on mergers activity.

Mr. Sheperd said he was optimistic for the back half of the year, as long as interest rates remain low, encouraging mergers and acquisitions. However, he said if markets go "haywire," then activists could do the same.

There are some indications that the great swelling of many activist firms, which has given them broader heft to agitate for corporate changes, may be on the ebb. Jana Partners LLC, a New York activist firm that rose past $10 billion under management this year, closed its largest fund to new investment in the spring so it could better manage the growing size, according to investor communications.

Jana, which gained 8% in that fund in the first half, also is weighing shutting off fundraising at its other fund in the near future, a person familiar with the firm said.

Several investors cited Jana founder Barry Rosenstein's reported purchase this spring of the most expensive home in the U.S., a $147 million beachfront mansion in the Hamptons, as a reminder of past market peaks.

Mr. Ackman, whose Pershing Square manages more now than it ever has before, has actually seen more money leave than come in this year, documents show, as some investors take money off the table.

Investors have pulled more than $400 million from Pershing Square over the past six months, though that sum represents less than 3% of the firm's capital.

Other well-known activists also posted solid starts in 2014, as Daniel Loeb's $15 billion Third Point LLC and Trian Partners LP, part of the $9 billion firm co-founded by Nelson Peltz, each returned about 6%.

It also has been a long wait for one big-name hedge fund that has increasingly dabbled in activism in recent years.

Paul Singer's Elliott Management Corp. is sitting on defaulted Argentine bonds and related claims valued at as much as $2.5 billion, or 10% of the firm's total assets under management.

As Mr. Singer and Argentine officials tussle publicly over the repayment of the bonds, however, the New York-based firm's Elliott International Ltd. fund is lagging behind its peers. It is up just 4.1% in the first half of 2014, an investor update shows.

Write to Rob Copeland at rob.copeland@wsj.com