Section 96. Sections ninety-three, ninety-four and ninety-five shall not impair the private rights of any person under any law passed before April twenty-fifth, eighteen hundred and sixty-six, or under any contract existing on said date, or authorize a town to enter upon or build canals or sluiceways into a pond which is private property. Whenever in section ninety-three or section ninety-four authority is granted to lease alewife fisheries, such leasing shall be made at public auction, held in such city or town, each of such fisheries to be leased separately, and no such lease shall authorize the operation of any fishery thereunder subsequent to June fifteenth in any year. Notices of any auction hereunder shall be published and posted as provided by law for notices of town meetings, and every such lease shall terminate on June fifteenth.