Probably to direct more fish into the trap, which otherwise would've swam to the other side totally oblivious of the trap. Efficiency is increased by that middle fence.

Edited to add excerpt from the original paper.

>A leader set in a linear direction more or less perpendicular to the shore so that fish approaching the leader turn and are guided toward deeper water and the trap; a leader that bisects the trap’s entrance to allow fish to enter the enclosure from either side of the fence (i.e., on a rising or a falling tide); a pair of bilaterally-positioned wings that form a V-shaped funnel entry to concentrate and redirect fish toward the trap enclosure’s entrance; and an entrance which incorporates species specific design characteristics (i.e., the width of the entrance and non-return devices) to only allow certain types of schooling fish to easily enter the enclosure and, once inside, prevent their escape.