Keep the weir

By: Chris Murphy

Auckland Council’s wish to remove Warkworth’s historic Wilson Weir was the subject of a Town Hall Talk hosted by One Warkworth and Mahurangi Action on February 13. One Warkworth’s manager, Murray Chapman, facilitated presentations by Council’s senior freshwater ecologist Matt Bloxham; whitebait authority, Paul Decker, from Premium Marine Technology and Mahurangi Technical Institute; and planner Shane Hartley.  

I put value on the historic weir and attended to better understand the reason for the proposed removal. In isolation, Matt gave a reasonable argument for removing the weir. In short, it is all about whitebait, with generic examples used to assert that the weir is the primary reason whitebait struggle to reach further up the Mahurangi River for spawning. He talked about five native fish species and the limited ability of inanga, which makes up more than 90 per cent of the whitebait population, to climb over structures like the weir. I was beginning to think removing the weir was justified. Then, supported by data and evidence gathered over 20 years by the National Institute of Water and Atmosphere, Paul Decker told us:

•    Only two of the five fish species live in the Mahurangi River. The absence of the other three is not because of the weir.

•    The pH level (acidity/alkaline content) of the river is a significant factor. New Zealand rivers usually have a pH level of 6-8.5 with whitebait preferring low pH levels. The Mahurangi River flows over limestone and has a pH level of 8.5 – generally too high for whitebait survival, though some obviously do survive.

•    High pH levels are ideal for native eels, so there is an exceptionally large and healthy eel population in the river. Eels eat whitebait, so this is a likely factor in low whitebait numbers.

•    Water temperature is another factor. Whitebait like water temperatures to be about 18C. Most whitebait won’t survive in 27C. The mean temperature of the Mahurangi River (according to a 1980s University of Auckland study) is 24C. On the day of the meeting, the river was 26C, so arguably global warming has and will continue to contribute to lower whitebait numbers.

•    Waterfalls a short distance up from the weir represent a greater barrier to whitebait, so if the weir was removed it would only provide a small additional stretch of river for spawning.

•    Whitebait return to where they hatch their eggs. There are whitebait above the weir that make their way down river each year in sufficient numbers to lay eggs and breed. This has been the case ever since the weir was built.

•    If the weir does represent a significant barrier to whitebait (strong evidence suggests other factors play a bigger part), fish ladders or stairs are an option.

Paul and Shane both called for a decision not to remove the weir in the foreseeable future while additional evidence and data specific to Mahurangi River is gathered. Objectively considered, no other course of action is logical. There seems no reason to support removing the weir at this time, which is the strong position of One Warkworth.


Chris Murphy, One Warkworth Chair
www.onewarkworth.co.nz

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