Gambusia spread after heavy rains

It has been a great growing season at Edgbaston Reserve and the wildlife is flourishing. But even moderately intense rainfall connects together creeks, clay pans and artesian springs in vast shallow lakes – perfect conditions for Gambusia (Gambusia holbrooki), a small feral pest fish, to move around and invade precious Red-finned Blue-eye habitat.

Gambusia have invaded about 30 artesian springs on Edgbaston Reserve and have excluded Red-finned Blue-eyes from at least seven of them. Red-finned Blue-eyes can't survive in springs with Gambusia.

Gambusia colonies in springs become persistent source points for further dispersal. Unlike Red-finned Blue-eyes, Gambusia are not restricted to springs. They can also persist and sometimes flourish in semi-permanent water holes, particularly within the margins of the normally dry Lake Mueller.

During the current wet period in western Queensland such waterholes have persisted in Lake Mueller since at least November 2015. By the end of 2016 there were countless Gambusia in widespread water bodies ready to spread out during the right conditions and invade critical Red-finned Blue-eye habitat.

As managers we've long wondered about Gambusia dispersal after rain. We know it happens, but how? The answers surely rely on monitoring as it happens. But the nature of the country poses considerable challenges to conducting surveys during or following rain.

Distance and access to the springs and waterholes are the biggest problems. Rain makes the roads and tracks muddy, and near impassable. Walking is fraught and the area is vast. An all-terrain buggy could feasibly negotiate the mud, but this must be weighed up against creating deep wheel ruts, causing damage to roads, or even worse, providing channels by which Gambusia could disperse even faster and more easily.

The heatwave conditions that impacted much of eastern Australia had a fortunate outcome for revealing the extent of the dispersal question.

An intense rainfall event dropped between 100mm and 150mm of rain over Edgbaston Reserve in one evening.

Volunteer ecologist Christina Kindermann and I followed the rain and arrived at Edgbaston as soon as we could safely access the reserve. And the heat came with us.

Phew, it was HOT. Daytime temperatures of more than 45 degrees combined with extremely high humidity and night time temperatures in the high 30s, made it 'a bit uncomfortable'. Sleep was hard won. But the extreme heat quickly dried the ground surface to a firm crust.

We had a short window to access most of the reserve while puddles of surface water remained over the landscape. It was a rare opportunity, so we thanked the sun and re-scheduled our plans to survey puddles for fish!

The results were alarming! We found Gambusia nearly everywhere we looked.

They were in small shallow puddles in washout gullies between the springs. They were left behind on rapidly drying clay pans. They were in water holes in creeks. They were even in wheel ruts on reserve tracks.

Surprisingly we found Gambusia in temporary waterholes in creeks many kilometres from the nearest known source populations. Clearly they have an incredible capacity to move over the landscape, even in arid country, when conditions suit them.

Bush Heritage has been successfully excluding Gambusia from springs that contain the critically endangered Red-finned Blue-eyes using exclusion fencing. The vital importance of this project is highlighted by this survey, and graphically demonstrated in the photographs where Gambusias (and native Spangled Perch) were found in a drying puddle alongside one of our exclusion fences.

This project has been supported by the Queensland Government Department of Environment and Heritage Protection Everyone’s Environment Grant Program and the Mohamed bin Zayed Species Conservation Fund.