HOW DO FISH SWIM?
To understand fish passage requirements through waterway crossings, it is important to first examine how fish swim. Fish use the following swimming modes to negotiate waterways (Cotterell,
Burst speed - Fish can swim at high speeds for only short periods of time (seconds). This speed is normally used to negotiate high flow conditions. Fish must rest between such bursts of speed. This speed may be used by fish to try and negotiate barriers to move upstream. The majority of native fish do not reach speeds that enable them to "jump" barriers.
Sustained speed - Fish can swim at moderately high speeds for longer periods (minutes). This speed can be used to negotiate medium flow conditions associated with flowing streams. However, fish will also need to rest between periods of sustained speed.
Cruising speed - Fish can swim at their cruising speed continuously with little effort (days). This speed is used in low flow or no flow conditions, such as in pools in a waterway. Cruising speed is generally the speed used when fish rest.
HOW DO WATERWAY CROSSINGS AFFECT FISH PASSAGE?
Waterway crossings provide access for road vehicles, rail, pedestrians and stock movement. Just as waterways vary in their size and shape, so too do waterway crossings. Not surprisingly, the potential impact of these crossings on fish passage can vary as much as the structures themselves.
BRIDGES AND ARCH STRUCTURES
Bridges and arch structures generally have the least impact on fish passage as they normally involve limited disturbance to the flow or the aquatic habitat of a waterway. Possible impacts include:
large scale turbulence resulting from bridge piers.
increased flood flow velocities.
changes to in-stream and bank vegetation affecting water shading, habitat values and water velocities.
blockage of fish passage along floodplains caused by elevated approach roads.
limited light penetration under the bridge deck creating a non- physical barrier for some fish species that may avoid dark areas during daylight hours
A culvert uses a pipe or box shaped cell to allow water to pass underneath a roadway. Flow conditions can be significantly modified both within and immediately adjacent to these crossings resulting in reduced opportunities for fish passage over a wide range of flow conditions. At worst culverts can cause a complete blockage to fish passage for all flow conditions.
The most common fish passage problems associated with both pipe and box culverts include:
fish passage requirements for waterways crossings