(a) The joint district retirement board shall have charge and control of the district retirement fund of the district and of the payment of all retirement salaries and annuities payable from the fund. The joint district retirement board shall invest the funds in securities that are legal for the investment of funds of savings banks in this state and shall sell the securities and reinvest the proceeds in securities legal for investment of funds of savings banks when in the judgment of the joint district retirement board the sale and reinvestment is advisable. No investment in or sale of securities shall be made except upon authorization of the joint district retirement board at a meeting of the board.
(b) The board also shall collect the income from the securities and pay it into the joint district retirement fund.