Section 73. The provisions of sections seventy-one and seventy-two in respect to the open and close season, and in respect to the number of scallops that may be taken, may be temporarily modified if, on petition of the aldermen, city council or selectmen to the director, the director after investigation, determines that, owing to unusual circumstances, such modification is expedient. In that case, he may authorize, for a prescribed period, the aldermen, city council or selectmen to issue permits to inhabitants of their respective cities or towns to take scallops in such quantities and at such times as he deems expedient.