A Review of Fish Passage at Culverts - with Potential Solutions for New Zealand Native Species

Publication Date



Prepared for Department of Conservation


culverts, design criteria, fish passage, New Zealand, salmonids, swimming, swimming ability, turbulent, turbulent flow, upstream, upstream passage, smelt, design, computer program, grayling, high velocity

Report number

Client Report DOC802.03/02

Publication place

Hamilton, New Zealand


National Institute of Water & Atmospheric Research Ltd


This report, commissioned by the Department of Conservation, reviews the literature on the effect of culverts on migrating fish. The applicability of passage solutions that have been devised overseas are discussed in terms of New Zealand conditions and species. Tests of swimming ability for inanga and smelt and limited trials of baffle designs which were judged suitable for small New Zealand species were undertaken and a culvert assessment computer program developed. Most New Zealand native fish species migrate upstream at a small size and are therefore poor swimmers in comparison to large salmonids, or even weak swimmers like Arctic grayling, which overseas culvert design criteria aim to protect. Due to their small size, New Zealand native species are also more easily confused by turbulent flows, and their upstream progress can be hindered by roughness elements that are often suggested to ease upstream passage. However, small fish need less water, so the width of the zone of suitable velocities is reduced and therefore easier to achieve. Furthermore, many New Zealand native species are good climbers and can negotiate very high velocity zones by progressing along the wetted margin. For these climbing species, it may not be necessary to provide a low velocity zone along the edge of the culvert, but ensuring the availability of a smooth, moist surface with no breaks or sharp angles is essential.

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