Fish undertake migrations for a number of reasons including to spawn, feed and seek refuge. These migrations are also essential to ensure the dispersal of species and maintain genetic fitness within fish communities.
Unlike Northern hemisphere species, most Australian native fish can't swim up and over the artificial drop that weirs and dams create in waterways. This can lead to fish becoming stranded at the base of a weir or dam and not able to move upstream. One way to get around this is to install ladders, otherwise known as fishways, so that fish can swim up and over the barriers.
The first written reports of fishways date to 17th century France, where bundles of branches were used to create steps in steep channels, allowing fish to bypass obstructions. Today, most fishways follow a similar basic concept, ensuring fish can pass around the barrier by navigating a series of smaller changes in water levels and swimming through a series of gaps or slots that control the velocity (speed) and depth of water. These structures are often made of rock, steel or concrete, and sometimes mimic the shape of a natural channel.
Each weir or dam on a river that is targeted for fishway construction represents a unique situation. There are many aspects that need to be considered within the design of a fishway including the species diversity and size of the migrating fish community, structure characteristics (in particular how high the barrier is), where the barrier is in the waterway (is it near the estuary, or far upstream), and stream characteristics.
Stream characteristics include how much the upstream and downstream water levels change over time and how they change with respect to each other, as well as the seasonality and extent of flows.
Fishways are designed to cater for the physical characteristics and swimming abilities of the prevailing fish community. Typically, the smaller species of fish are weaker swimmers and are unable to negotiate the faster flows in a fishway that larger fish can. For coastal catchments, juvenile fish will likely be more prevalent closer to the estuary, so a gentler slope will be required here.
The hydraulic conditions within a fishway need to provide both enough depth for large fish whilst ensuring the velocity is suitable for smaller fish and turbulence will not prevent large or small fish from becoming confused and unable to find a way forward
Fishways in NSW
The first fishway in NSW was constructed at Audley Weir on the Hacking River near Sydney1. Unfortunately, due to poor knowledge of the swimming abilities of native Australian fish at the time, its design - and that for other fishways built up to 1985 – were based on northern hemisphere salmonid species such as salmon and trout that are much better swimmers. This meant that they were mostly ineffective in passing Australian native fish.
Fishway designs employed in NSW are vertical slot and denil (used on weirs up to 6 m in height), rock ramp, trapezoidal or cone fishways (used on low weirs) and "trap and transport" or fish locks (used on high dams). JFCS fishways are a new design untested in the field.
Follow the links below to find out more about the different fishway designs and watch our animations showing how vertical slot fishways and rock ramp fishways work.
Reference
1 NSW Department of Fisheries (1913) Annual Report. NSW Department of Fisheries.
