Catalysis of the reaction: all-cis-icosa-5,8,11,14,17-pentaenoate = (5Z,7E,9E,14Z,17Z)-icosapentaenoate.
(5Z,8Z,11Z,14Z,17Z)-eicosapentaenoate delta8,11-delta7,9-isomerase (trans-double-bond-forming) activity