Fish Passage at Culverts: A review, with possible solutions for New Zealand indigenous species

Publication Date



baffles, barriers, culverts, design criteria, fish passage, flow requirements, habitat, high velocity, migration, salmonids, smelt, swimming ability, turbulent, turbulent flow, upstream, upstream passage

Publication place

Wellington, New Zealand


Department of Conservation


None supplied. From executive summary: This joint NIWA/Department of Conservation publication reviews the literature on the effect of culverts on migrating indigenous and exotic freshwater fish. The applicability of passage solutions that have been devised elsewhere is discussed in terms of New Zealand conditions and species. Tests establishing the swimming ability of inanga and smelt, and limited trials of baffle designs potentially suitable for small New Zealand species, were undertaken. A culvert passage requirement checklist, construction checklist, evaluation procedure, and computer assessment program were also developed. All users - whether they be scientists, industry, consultants, or consenting bodies - are encouraged to submit comments to the authors for incorporation into any future review and updates. Over half of New Zealand's indigenous fish species migrate up stream at a small size. They are therefore poor swimmers in comparison to large salmonids, which most traditional culvert design criteria aim to protect. Because of their small size, New Zealand indigenous species are also more easily confused by turbulent flows, and their upstream progress can be hindered by roughness elements such as baffles that are often suggested to ease upstream passage. However, small fish need less water, so the width of the zone containing suitable velocities for fish passage can be reduced and therefore more easily achieved. Furthermore, many New Zealand indigenous 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 without breaks or sharp angles is essential. When assessing whether fish passage is required at culverts, the following factors need to be considered: presence of other migration barriers both upstream and downstream of the culvert, the composition and distribution of fish within the catchment, size and type of habitat available upstream, timing of fish migrations, duration, and flow requirements of the species concerned, altitude, and distance from the sea.

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