5In section 9(5) of the Salmon and Freshwater Fisheries Act 1923 (which provides that certain activities authorised by that section shall not constitute an offence under provisions which include section 2(1)(a) of the Rivers (Prevention of Pollution) Act 1951) for the words " section 2(1)(a) of the Rivers (Prevention of Pollution) Act 1951" there shall be substituted the words " and section 31(1)(a) of the Control of Pollution Act 1974 ".