Catalysis of the reaction: all-cis-icosa-5,8,11,14,17-pentaenoate = (5Z,7E,9E,14Z,17Z)-icosapentaenoate.
eicosapentaenoate cis-delta5,8,11,14,17-eicosapentaenoate cis-delta5-trans-delta7,9-cis-delta14,17 isomerase activity