Buoyed by explosive asset growth at Highbridge Capital Management, JPMorgan Asset Management has solidified its position as the largest hedge fund manager in the world.

JPMorgan, which bought a majority stake in Highbridge two years ago, saw the assets under management of its new acquisition more than double during the first half of 2007, adding $20 million to reach $37 million.

Overall, JPMorgan manages $56.2 billion in hedge fund assets, according to new figures from Absolute Return magazine, a 65.3% increase over the first six months of the year.

Goldman Sachs remains in second place on the list of top U.S. managers, in spite of its widely-reported troubles. Goldman Sachs Asset Management now manages some $40 billion, up 22% from Jan. 1.

D.E. Shaw Group leapfrogged Bridgewater Associates for third place, with assets rising 29.3% to $34 billion. Bridgewater, for its part, added just 6.3% in the first half, taking fourth on the list with $32.1 billion.

Och-Ziff, in seventh place on Jan. 1, jumped into fifth as assets soared 39% to $29.2 billion.

Of the other top-ten firms, only San Francisco’s Farallon Capital Management did not see its asset base rise.