The need to travel

Many native fish need to move between estuarine and freshwater habitats at different times of year, for several reasons:

  • Migrate to and from breeding and spawning grounds
  • Find deep waterholes and wetlands for refuge in the dry season
  • Access waterways after drought
  • In response to changing water quality such as dissolved oxygen levels
  • Find new areas for feeding
  • Evade predators

Some iconic fish that spawn in marine habitat and migrate to freshwater are barramundi, mangrove jack, sea mullet, freshwater eels, jungle perch and tarpon.

What is a fish barrier?

Barriers are structures or conditions that prevent fish from moving between different habitats. This can mean there is less overall habitat available for fish to live in, which can lead to reduced fish populations. Barriers also impact fish diversity and disrupt life-cycles.

Invasive fish species like tilapia and gambusia can survive and reproduce in degraded and polluted waterways much more successfully than even our hardiest native fish species. Enhancing waterway connectivity and health by removing barriers to fish movement is one way we can support native fish.

Common fish barriers

CAUSEWAYS: Causeways and crossings can effectively turn into small dams in low flow conditions, and only reconnect when high rainfall and flow events occur.

Causway creating a fish barrier

DROPS: Vertical drops downstream of road crossings and culverts are enough to form barriers for many fish, particularly juvenile and small bodied species. Even drops of several centimetres can be too high for them to get past.

Drop creating a fish barrier

HIGH VELOCITY: In slow flowing waterways fast water speeds can occur through pipes and culverts when they restrict the area of flow too much. Unless pipes and culverts are nice and low, and bank-to-bank, the water flow in them is often too fast for small fish to swim against. Woody debris can build up on the upstream side and create further velocity issues. The length of the pipe is also important as the fish may be able to only swim short distances against fast flowing water.

High velocity water creating a fish barrier

WEED CHOKES: When aquatic weeds build up, they can create a physical choke as well as a chemical barrier when there’s simply not enough dissolved oxygen left for fish to survive.

LACK OF LIGHT: Some fish don’t like low light conditions and may avoid swimming through culverts and pipes that are excessively long and dark, or under very low bridges.

OBSTRUCTIONS: Flood gates that stop the upstream flow of the tides also stop the fish moving upstream.

Barramundi Life Cycle

Illustration of a barramundi life cycle

Thumbnail image on our blog home page that links to this story is courtesy of Thomas Ragh.

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