NEW YORK 鈥 On three hours of sleep a night and with no time to eat, one New York鈥揵ased hedge fund manager overseeing billions in assets called last week the most stressful of his life.聽

鈥淣othing was making sense anymore,鈥 he said, as he watched the stock of GameStop Corp. soar, while he bet it would fall.聽

Wednesday was the worst. Fueled by chatter on Reddit and other social media to take on Wall Street, investors used venues like online trading app Robinhood to bid up the brick-and-mortar retailer. GameStop surged 135%, on its way to gaining more than 1,500% in January without any change in its business outlook. Hedge funds scrambled. By this point, trading had become so unpredictable across all sectors that funds were forced to dump their winning stocks to balance out their portfolios.聽

鈥淚t鈥檚 time to cut out,鈥 the manager said he told his team. 鈥淟et鈥檚 live for another day.鈥澛

The experience of this manager and a handful of others interviewed by Reuters over the last days, who all requested anonymity for fear of personal threats from retail investors, is a keyhole into a week of losses on Wall Street they say was far worse for them than last year鈥檚 historic pandemic sparked sell-off.聽

As a new week of trading begins, it remains to be seen how long the Reddit-fueled rally in traditionally unloved stocks will last.聽

More immediately, it could mean more market losses if hedge funds have to keep selling to meet redemptions or right their portfolios. Longer term, they may have to shift strategies to find their footing and turn the tide to their advantage.聽

鈥淲hat is not clear as yet is whether this disruption will evolve into a market accident,鈥 said Mohamed El-Erian, chief economic advisor of Allianz.聽

CROWDED TRADE
The extent of losses has exposed a big weakness on Wall Street. Analytics firm S3 said GameStop short sellers had mark-to-market losses of nearly $20 billion so far this year.聽

Several hedge fund managers said the idea to short GameStop had long been a favorite at exclusive 鈥渋dea dinners,鈥 where fund managers swap their best trades.聽

Managers also noted traders, many of whom work at multi-strategy funds that employ pods of portfolio managers, traders, and analysts, often know each other well and may compare notes.聽

Gabe Plotkin鈥檚 Melvin Capital, one of the funds gored most by GameStop鈥檚 gains, took a $2.75 billion bailout from his one-time mentor Steve Cohen and Citadel鈥檚 Ken Griffin. The funds involved have taken a dent: Mr. Cohen鈥檚 Point72 Asset Management lost roughly 15% in January partly because of its investment in Melvin. Melvin鈥檚 assets slid during the month from around $12.5 billion to $8 billion, a source familiar with the situation said.聽

Maplelane Capital, another fund that bet against GameStop, had lost roughly 45% in January, a person familiar with the fund鈥檚 returns said. Even Viking Global Investors, one of the world鈥檚 best-performing hedge funds, was off some 7%, people familiar with the returns said.聽

Representatives for Messrs. Plotkin, Cohen, and Griffin declined to comment.聽

鈥淏eing short consensus stocks is just bad business,鈥 said Dinakar Singh, a former Goldman Sachs trader who now runs hedge fund Axon Capital and was not short the stock. 鈥淚t is great while it is working but when it isn鈥檛 anymore one guy鈥檚 problem triggers everyone鈥檚 headache. It becomes a circular disaster.鈥澛

INVESTING 鈥業NSANITY鈥
Fund managers said the week鈥檚 experience is changing the way they do business. Citron鈥檚 Andrew Left, who ignited the retail trading frenzy by publicly detailing GameStop鈥檚 issues, acknowledged defeat by Friday. He had covered his positions, taken a loss, and said he will stop publishing short-seller reports, a strategy that earned him millions and made him a prominent name in the industry.聽

Hedge funds will also be much more cautious about having short positions going forward for fear of becoming a 鈥渕indless target,鈥 said one former hedge fund manager, adding that 鈥渒ind of defeats the purpose of hedge funds.鈥 Traditionally hedge funds promised investors a cushion against market declines by being able to sell short and make money when stocks drop.聽

Funds are also less likely to use trading strategies such as put options, a kind of derivative that has to be disclosed and that Melvin and Maplelane used, the managers said.聽

At the same time, though, one thing will not change: Wall Street thinking of new ways to make money again.聽

Already the tide started to turn somewhat late in the week for hedge funds as Robinhood and other online brokerages used by millions of retail investors restricted trading in the stock.聽

Many hedge funds still remain on the other side of the GameStop trade, with the number of shares shorted having dropped only 8% during the week as firms like Melvin covered positions.聽

One New York鈥揵ased banker said his clients were looking for other possible targets of retail trading frenzy and positioning themselves ahead of it to make money if the eye of Reddit turned that way. 鈥淧eople have spent all week running their screens on other situations that might fit this criteria,鈥 the banker said. 鈥淵ou鈥檙e going to see more of it.鈥澛

鈥淭his isn鈥檛 investing, this is insanity,鈥 said Kyle Bass, founder, Hayman Capital Management LP. 鈥淎 month, two months from now, I will be flabbergasted if GameStop isn鈥檛 below $100.鈥 鈥 Svea Herbst-Bayliss and Maiya Keidan/Reuters