Environmental Infrastructure

This four-metre fish ladder was designed by Cato Bolam on an online pond at Westgate Town Centre to accommodate challenges around existing downstream constraints to retain fish passage. The design is a collaborative effort between our engineers and in-house ecologists along with specialist input from NIWA. Construction was undertaken by Cameron Civil Limited.

The Inanga successfully climb the ladder as proven through a significant testing and refinement regime to achieve optimum performance. The blue vertical pipe within the video is discharging the majority of the base flow which limits the head on the inlet to the fish ladder, reducing the velocities and flows entering the ladder.

With the pond providing extended detention, the design solution manages as much of the low flow as possible through the ladder, which provides an inlet to the fish ladder large enough to allow eels to pass but keeping flows in the ladder low enough for the Inanga (2-4cm fish) to be able to climb.

The ladder channel contains regular resting pools, spat ropes and hand placed epoxied rocks to mitigate any areas identified with higher velocities. Fish migration is seasonal, so the project completion and testing program were programmed to align.

The successful installation of this fish ladder has helped contribute towards the major ecological success in the area.

“As a representative of the client (Auckland Council) Cato Bolam’s fish ladder design enabled us to mitigate our effects on the environment through allowing fish passage over such an incredible height difference. There was serious doubt as to the feasibility of creating a fish passage of such height.  To our great relief it worked and ultimately allowed us to unlock a significant amount of employment land upstream of the device.  It was just one example of the numerous innovations that Cato Bolam came up with over the life of the large Westgate project.” James Copley