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.
Fishways, also known as fish ladders or fish passes, are structures placed on
or around constructed barriers (such as dams or weirs) to give fish the
opportunity to migrate.
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, allowing fish to pass around the barrier by swimming through a
series of gaps or slots that control the velocity (speed) of water.
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, The species diversity and size of
the migrating fish community varies from site to site. 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. The
hydraulic conditions within a fishway need to provide both enough depth for
large fish whilst ensuring the velocity is suitable for smaller fish.
Types of fishways
Seven types of fishways have been used or considered in NSW
including
Pool-type
Denil
Lock
Trap and Transport
Rock Ramp
Bypass
Eel.
Pool-type fishway (includes vertical slot fishway)
Pool-type fishways were the first type to be developed and
consist of a series of interconnected pools bypassing an obstruction. Many
different types of pool fishways have been designed in Australia, however, the
vertical-slot design is the only one which has proved effective with native
fish.
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| Pool-type fishway (includes vertical slot fishway) |
The denil fishway was developed in 1909 by a Belgian
scientist, G. Denil. This style of fishway uses a series of symmetrical
close-spaced baffles in a channel to redirect the flow of water, allowing fish
to swim around the barrier.
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| Denil Fishway |
Lock Fishway
Lock fishways operate by attracting fish through an entrance
similar to that of a pool-type fishway, but instead of swimming up a channel
the fish accumulate in a holding area at the base of the lock. This holding
area is then sealed and filled with water to reach a level equal to the water
upstream of the barrier. Fish are then able to swim out of the lock. To
encourage fish to move through the various attraction and exit phases of the
lock cycle, a combination of attraction flows and crowding screens can be used.
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| Lock Fishway |
Trap and transport fishways
The trap-and-transport type of fishway involves attracting
and trapping fish below a barrier and then physically transporting them over
the barrier. The initial trapping is commonly done in a short section of
pool-type fishway, with the fish usually being transported by road, rail or
aerial car. The fishway at Tallowa Dam is the only example of a trap and
transport fishway in NSW.
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| The frame that supports the hopper transporting fish from downstream to upstream. |
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| The hopper (highlighted by the circle). The hopper is submerged on the downstream side of the wall. Fish are attraced into it by an enhanced water flow. A trapdoor closes and the hopper makes its way up and over the wall, transporting the fish to the upstream weir pool. |
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The structure that guides the hopper as it moves up and over
the wall.
|
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| On the upstream side of the wall. The orange circle shows the hopper. The yellow circle shows the fish protection structures. These structures provide protection from predators for the fish released from the hopper. |
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