Notwithstanding any provision of law to the contrary, the board of directors of joint agencies or persons authorized to make investment decisions on behalf of joint agencies investing public funds are considered to be trustees and subject to the prudent investor standard. When investing, reinvesting, purchasing, acquiring, exchanging, selling, and managing public funds, a trustee shall act with care, skill, prudence, and diligence under the circumstances then prevailing, that a prudent person acting in a like capacity and familiarity with those matters would use in the conduct of funds of a like character. When investing, reinvesting, purchasing, acquiring, exchanging, selling, and managing funds of a joint agency, the primary objective of a trustee is to safeguard the principal of the funds under its control. The secondary objective is to meet the liquidity needs of the joint agency. The third objective is to achieve a return on the funds under its control. Monies of joint agencies not required for the immediate necessities of the joint agency may be invested and reinvested in securities and other investments as the board of directors of joint agencies shall determine in accordance with the objectives enumerated above. The securities and other investments which may be used by the trustee is to be listed in a formal, written investment policy approved by the board of directors of joint agencies.