Restoring fish passage
Many juvenile native fish (eels, galaxiids, torrentfish, smelt, lamprey and bullies) develop at sea and migrate to river habitats, where they grow to adulthood. These movements are often hindered by barriers to fish passage: dams, weirs, falls, tide gates, culverts, screens and high water velocities.
Juvenile eels and some galaxiids (giant, banded, shortjaw kokopu and koaro) are good climbers and can ascend near vertical wet rock faces and falls of up to 15m high. Consequently, passage for these species is only blocked by high dams, perched culverts and small weirs or dams with an overhanging lip.
Redfin bullies and torrent fish have a moderate climbing ability and can surpass small falls, but smelt, inanga and mullet are comparatively poor climbers and their upstream passage is easily blocked by falls, high water velocities and culverts.
What to do:
The New Zealand Fish Passage Guidelines sets out recommended practice for the design of instream infrastructure to provide for fish passage. These guidelines have been developed to assist infrastructure designers and managers, waterway managers, environmental officers, iwi and local communities with understanding and promoting better management of fish passage requirements in New Zealand.
Gauging weirs can be retrofitted with a sloping ramp studded with baffles to allow smelt and inanga to move upstream.
Creating fish passage over weirs (W&A, 10(1), 2002)
A more natural ramp can be constructed using cobbles embedded in a flat concrete matrix, or large boulders can be cemented together to create a solid but variable surface.
Fish ramps may require maintenance (especially after floods) along with the creation of a pool using rocks below the ramp. This will ensure water levels remain over the lower end of the ramp. In general, if inanga can move upstream so can other native fish.
If the barrier is a culvert, redesign it to be more fish-friendly. Build large, flat-bottomed culverts that act as bridges over a stream (top right photo), or retrofit baffles to provide a low-velocity route throughout its length. Addition of coarse mesh material to the lip of culverts can improve fish passage.
Tide and floodgates can restrict fish passage and degrade habitat. Reduce these impacts by retrofitting ‘fish-friendly’ gate openers, or by replacing them with gates specifically designed to reduce these problems if possible. Some regional councils in New Zealand are currently trialling various designs.
If fish passage cannot be restored due to the presence of large dams downstream, repeated stocking via a trap and transfer programme is the only restoration option.