An oil-focused hedge fund firm run by high-profile crude trader Andrew Hall suffered the second-largest monthly loss in its history in July as oil prices collapsed amid a broader commodity rout.

Hall's Astenbeck Capital Management posted a 16.6% loss for last month, according to investor documents reviewed by The Wall Street Journal. Assets managed by the firm, which makes bets in energy, metals and agricultural markets, fell roughly $500 million in the month, according to the documents.