Metacapital Management, the hedge-fund firm run by former Lehman Brothers Holdings mortgage-bond trader Deepak Narula, returned 28% in the first half of this year, boosted by government-backed debt that benefits from limited homeowner refinancing. Now the New York-based firm is seeking to protect against one of the “biggest” risks in the $5.2trn market for agency mortgage bonds, Narula wrote in a July 22 letter to investors.