European market rules introduced after the financial crisis require hedge funds to report their short positions — effectively bets that a company's shares will fall in value — to regulators. But only those that are larger than 0.5 per cent of a company's outstanding shares are supposed to be made public.

An error by the AFM, the Netherlands' watchdog, meant that details of hedge fund short trades going back to 2012 were published on its website on Tuesday evening before being quickly removed.

The Dutch market regulator mistakenly published the details of hundreds of previously private short selling trades by international hedge funds, including bets against Dutch banks by George Soros and the positions of Renaissance Technologies' enigmatic Medallion fund.

The regulator instead published details of all short trades reported in thew Netherlands since the new rules came into force, meaning hundreds of previously non-public trades were briefly released into the public domain.



A spokesman for the AFM said it had made a mistake and moved quickly to correct it.



Hedge funds whose trades were included in the release were angry. "When a bank or hedge fund makes a mistake they are fined — who will fine the Dutch regulator?" asked one manager.



Hedge funds typically jealously guard the details of their short trades. Some deliberately keep their positions just below the 0.5 per cent threshold to avoid them being made public. Some fund managers do not want to tip off rival funds, while others prefer not to show their hands to the management teams of the companies they are betting against.



A list of the trades seen by the Financial Times included a number of previously unreported short positions including George Soros' family office betting against the Dutch bank ING, a trade that chimes with its existing public bet against Germany's Deutsche Bank. The position against ING was opened in June and was increased to as much as 0.3 per cent of the lender's shares before being scaled back a month later.

Other previously private trades in the list include bets by the computer-driven Medallion fund operated by the US hedge fund Renaissance Technologies. The fund, which was launched in the early 1980s by mathematician and code breaker James Simons, has long been closed to outside investors and only manages the fortunes of the Renaissance partners.



As a private vehicle, it is not required to disclose its positions unless they are so large they are caught by general market rules. Any details of Medallion's trading strategy, no matter how small, are closely scrutinised by investors.



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The Dutch regulator's list shows that the Medallion fund has taken out multiple bets against small Amsterdam-listed companies since 2012. Those that are still outstanding include the specialty metals company Advanced Metallurgical Group, Ordina, an IT company with a market value of under €200m, and the closed-ended real estate company Wereldhave N.V.



Short positions are often used to hedge against for other trades meaning it is not always possible to tell their full purpose without having information about a hedge fund's whole portfolio.

