Where fish are present, the proper choice of a structure, and its correct installation, will reduce the impact on habitat and ensure fish passage. Over half of New Zealand’s 35 (or so) indigenous fish species migrate up rivers when they are small. Juvenile fish may lack the ability to swim upstream through a long steep culvert. While many are good climbers, they cannot enter projecting culverts. Not all culverts require fish passage. Some locations do not have fish, or the likely presence of them. This could be due to large natural barriers downstream, the distance from the coast, or their altitude.
The Department of Conservation (DOC) and regional councils have specific responsibilities to manage fish passage in New Zealand waterways under the Freshwater Fisheries Regulations 1983 (FFR83) and Resource Management Act 1991 (RMA91), respectively. The FFR83 regulations came into force on 1 January 1984, so they generally apply to all structures built after 1 January 1984. However, regulation 42(2), which is the requirement for culverts and fords to be maintained to prevent the development of fish passage barriers, applies to all culverts or fords built before and after 1984.
Poor culvert design restricts fish passageThe NES-PF have specific permitted activity conditions for fish passage at river crossings, and the discharges, disturbances, and diversions that can impact on fish spawning. The Ministry of Primary Industry (MPI) has produced the fish spawning indicator, which is a tool to help councils and foresters plan forestry operations. See here. It shows where and when fish that are sensitive to bed disturbance are spawning. For permitted activity status, river crossings must provide for the upstream and downstream passage of fish, except where a statutory fisheries manager advises that fish passage would have an adverse effect on the fish population upstream. For example, access for trout to predate on isolated galaxiids. The river crossings must provide for passage by maintaining river bed material in any structure that would be in place of the river bed.
The following are principles of good fish passage design, while also taking into account other design factors including cost and hydraulic flow requirements:
- Maintain continuity of in-river habitat within the culvert. The NES-PF requires that the invert be embedded by 20% of the culvert height
- Minimise alterations to river alignment
- Minimise alterations to river gradient
- Maintain water velocities and depths within a range equivalent to adjacent river reaches
- Minimise constraints on bankfull channel capacity resulting from the structure
- Avoid vertical drops
- Provide an uninterrupted pathway along the bed of the structure.
Bridges, temporary bridges and large open bottom culverts provide excellent unhindered passage. Culverts aligned with the river, of an appropriate size, with similar gradient and with culvert inverts set well below the river bed, are also fit for purpose. Fords are the least preferred crossing type, as they do not prevent vehicles or animals from entering the waterway. Natural fords provide uninterrupted fish passage; however, concrete pad fords need to be carefully constructed so they do not restrict fish passage, especially in periods of low river flow.
Culverts can be retrofitted to improve fish passage. The NZ Fish Passage reference, listed at the end of this section, provides several options. Mussel spat ropes are the most cost-effective solution for forestry, but they need maintenance since they can be swept away in storm flows. Mussel spat ropes can be used on culverts where the diameter is less than 1.2 m. When installing them it is recommended to:
- Use a minimum of two rope lines for a 0.5 m diameter culvert. For larger culverts, more ropes may be necessary
- Install ropes so that they are tight and flush with the base of the culvert through the entire length of the culvert, and not loose at one end or out of the water
- Set ropes apart to provide ‘swimming lanes’ between the ropes
- Tie knots (half hitches) along the sections of rope in the culvert barrel to break up the flow, and potentially create additional rest areas for fish
- Use non-loop rope types to reduce the likelihood of debris snagging on the ropes
- Anchor ropes to shackles attached to waratah sections upstream of the culvert
- Drive anchors below river bed level or on the river banks
- Seek specialist assistance and view online resources.
A good reference document is the New Zealand Fish Passage Guidelines for Structures up to 4 Metres (DOC 2018). It provides useful and comprehensive information on new structures, and on how to retrofit non-compliant crossings. However, the designs are more suitable to highways rather than forestry applications. See here.