TOKYO, Dec 24 (Reuters) - Japan's Government Pension Investment Fund, the world's largest pension fund, on Tuesday said Boston-based Wellington Management Company is the ninth manager of its $161 billion foreign stock portfolio. In September, the fund appointed eight managers for the foreign equity portion of its entire $1.2 trillion portfolio. It is targeting higher returns as payouts are increasing because of a rapidly ageing population. Wellington manages some $774 billion worth of assets. The MSCI Kokusai index will be the benchmark for the foreign stock portfolio. The pension fund occasionally replaces managers after reviewing performance and investment strategies. It is currently in the final stage of choosing new managers for its domestic stock portfolio, which it expects to announce by March-end. The fund allocates investments based on a model portfolio of 60 percent domestic bonds, 11 percent foreign bonds, and 12 percent each in domestic and foreign stocks. As of September-end, it had 56.28 percent in yen bonds and 9.82 percent in foreign bonds, plus 15.80 percent in domestic stocks and 13.09 percent in foreign stocks. It has invested the remaining 5 percent in short-term assets.