Exclusion fencing has become a modern land management technique. In a farming context it's done to achieve better production. Property owners or groups of property owners (called a cluster) build fences around their properties to exclude animals such as Dingoes and Kangaroos. In a conservation context it's done to exclude cats and foxes and to let native birds and mammals thrive. But how can this idea be transferred to the protection of an endangered fish? Can threats to the survival of the Red-fin Blue-eye be fenced out of the springs?
In the 1990s Edgbaston was still a working cattle and sheep property. One of the threats to Red-fin Blue-eyes was the use of the springs by the stock. The springs were watering points and provided green feed when the rest of the environment was dry.
Cattle typically would walk through the springs to get a drink and eat the plants growing there. Cattle would also defecate wherever they happened to be standing.
In spite of this, the Red-fin Blue-eye persisted in a number of springs.
The first exclusion fence built on Edgbaston was built around two springs with good populations of Red-fin Blue-eyes to exclude stock. This had the added benefit of excluding pigs, which were not farmed but present as a feral pest. Pigs also had a big impact on the springs by digging.
Looking at this fence now, we can learn from what's been done. In the time since this fence was built, the springs have drastically changed size. This fence (photographed) originally completely surrounded the spring and had a reasonable area around the spring for expansion. The spring now covers an area almost twice the area inside the fence.
Even in the nineties Gambusia were present and were posing a threat to Red-fin Blue-eyes. Gambusia are an introduced pest fish, and they can swim from waterholes in Pelican Creek to the springs when it rains.
Gambusia can swim long distances in as little as 3mm of water. They've travelled to 30 springs at Edgbaston.
Much discussion and thought went into how exclusion fencing could be used to keep Gambusia away from springs containing Red-fin Blue-eyes.
The first trial was black plastic garden edging. How big a fence do you really need to keep fish out? This fencing was commercially available, easy to put up, and relatively inexpensive.
The garden edging had problems – kangaroos knocked it over, crayfish dug under it, and it didn't withstand flood flows. This is the nature of Science; we learnt from this to develop the next style of fence.
Commercial silt fencing was the next version used. It was less likely to be damaged by kangaroos, could be buried into the ground to prevent crayfish burrowing under it, and was porous so could allow water to move through it. Unfortunately, it only has a two-year life span in the sun, so needs to be replaced regularly.
Kangaroos tend to lie up against it, maybe for shade, maybe for protection from the wind. It has very fine holes for water to flow through.
The unique water chemistry in the springs has produced a substance (soda) that blocks the holes in the mesh causing the fence to hold water at a higher level on the inside of the fence than the outside.
So what have we learnt from all of the fencing methods above? The fence needs to be:
- robust so kangaroos can't knock it over;
- built outside the wetted area of the spring,
- able to withstand flood flow, or have the water flow through while keeping Gambusia out,
- buried into the ground so crayfish and crabs can't dig under it, and
- have a long life span.
The new fence being trialled as a Gambusia exclusion fence meets all of these criteria. It's built as a normal fence, with corner posts and three wires, but it's low so kangaroos can still access the springs for water. It's built well away from the wetted area of the spring. The mesh is buried in the ground so crayfish and other animals can't bury underneath.
The holes in the mesh are quite large; water can flow through in flood events, but baby Gambusia will be excluded.
The mesh has a ten-year life span guaranteed by the manufacturer so the installation costs are spread over a longer time.
So what problems will this fence have? It can be damaged by fire. It may exclude other animals. It will limit the natural dispersion of spring biota including fish and invertebrates. This is an unfortunate consequence that needs to be managed until another method of Gambusia eradication can be found.
Our work to protect the Red-fin Blue-eye is supported by the Queensland Government’s Everyone’s Environment grants program.