Gabriel Plotkin, head of New York hedge fund Melvin Capital Management, has accumulated a $400 million short bet against the Japanese game maker, according to regulatory filings. The former star trader at SAC Capital Advisors accounted for as much as 7 percent of Nintendo’s daily volume in recent weeks, contributing to stock declines since May that have stunned analysts. Investors have been baffled by the sudden swoon and Plotkin’s position may add to their concerns.