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Fish passage barriers may result from the actual physical blockage of the waterway by a dam, weir, floodgate or even debris blocking a culvert. Alternatively the blockage may result from an alteration to the natural flow conditions within the waterway caused by the construction of a waterway crossing. These barriers can exist all the way along a waterway and can include off-stream barriers such as those associated with adjacent wetlands and floodplains.

A recent audit of barriers within the tidal reaches of NSW waterways recorded 4,308 barriers to fish passage, including poorly designed waterway crossings. Similarly, just under 4,000 dams and weirs have been documented throughout NSW waters. In the South Australian River Murray valley, a survey of floodplain wetland complexes identified over 400 potential barriers to flow and fish passage.

The cumulative effect of all of these barriers has been identified as one of the major threats to the continuing survival of native fish in Australia.


Barriers to fish passage can effectively stop many fish species from breeding and re-populating waterways by restricting their ability to access breeding partners and spawning grounds. Fish attempting to negotiate barriers are forced to use up precious energy reserves. If this occurs during a breeding event, fish may actually reabsorb their eggs and sperm to replenish their energy reserves, effectively losing a breeding season with possible long- term flow on effects to the size and sustainability of the population.

Local extinction is likely to occur where barriers have stopped fish undertaking migrations. For example, the golden perch has become locally extinct above some waterway structures.

Similarly, barramundi in the Fitzroy River in central Queensland, and freshwater mullet in the Burnett River in south-eastern Queensland are considered locally extinct upstream of the tidal barrages constructed on these rivers. Australian grayling and Australian bass have disappeared completely from some coastal rivers, silver perch has declined by over 90 per cent over the last 50 years, and current levels of native fish populations within the Murray-Darling Basin have declined to about 10 per cent of their original size prior to pre- European settlement. Of all the native fish species within the Basin just under half are listed as threatened under various pieces of State legislation.

Some barriers can also create excellent habitat for pest species to proliferate, such as European carp and mosquito fish. Barriers create still-water pools that are favoured by these species, allowing them to out-compete native fish for food and shelter.

fish passage requirements for waterways crossings

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