What makes a good home for a fish in the desert?

Saving a ‘most endangered’ from a ‘most invasive’

Red finned blue-eyes in their natural habitat at Edgbaston Reserve. (Photo by May-Le Ng)

Red finned blue-eyes in their natural habitat at Edgbaston Reserve. (Photo by May-Le Ng)

Did you know that there are fish living in the Australian desert? Obviously, fish need water, and that’s usually scarce in the desert. However large parts of arid Australia sit over the Great Artesian Basin, which is an area where groundwater is under such pressure that if you dig a well, it’ll come to the surface all by itself – no pumping required! This is good news for fish, because weak points in the aquifer mean that water seeps to the surface and forms permanent springs in the desert.

Because these springs are so isolated from other sources of fish, unique species have evolved that live only in spring complexes. One of the best looking of these fish is the red-finned blue-eye, Scaturiginichthys vermeilipinnis, which lives only in springs on a single property in central-western Queensland. This property is called Edgbaston Reserve. It’s now owned by Bush Heritage Australia, and protects the outstanding biological values of the springs.

Red finned blue-eyes in their natural habitat at Edgbaston Reserve. A closer up shot of the fish in this image appears in the box on the left. (Photo by May-Le Ng)

Red finned blue-eyes in their natural habitat at Edgbaston Reserve. (Photo by May-Le Ng)

Searching for red-finned blue-eyes at dusk, Edgbaston reserve. (Photo by May-Le Ng)

Searching for red-finned blue-eyes at dusk, Edgbaston reserve. (Photo by May-Le Ng)

As you might imagine, life for a little fish living in the desert comes with many challenges (see the segment below, ‘the remarkable red-finned blue-eye’), but this resilient little creature has evolved to cope well with the constantly changing environment of the groundwater spring. What it has never had to deal with, however, is competition from an alien invader in the form of the mosquitofish Gambusia holbrooki. Unfortunately for the red-finned blue-eye, the mosquitofish has found its way to Edgbaston’s springs.

What makes a spring good habitat for red-finned blue-eye, and bad for mosquitofish? We’ve got a number of guesses.

Mosquitofish are an environmental pest species from North America, and one of the worst invasive fish in the world – it’s considered to be one of the world’s 100 most invasive species. We think they spread during floods, but however they got there, mosquitofish are bad news for the red-finned blue-eye. Wherever mosquitofish show up in a spring, red-finned blue-eye disappear, usually a short time later (though sometimes it takes a bit longer and there is at least one example where the species have co-existed in a spring for at least 15 years before the red-finned blue-eyes disappeared). The red-finned blue-eye has been placed on the IUCN list of the 100 most endangered species on the planet.

Scientists and conservationists are working to stop the spread of mosquitofish and protect the red-finned blue-eye, but since the red-finned blue-eye was only discovered in 1990 there’s much we don’t know about its biology.

Mosquitofish at Edgbaston Reserve  (Photo by Sam Nicol)

Mosquitofish at Edgbaston Reserve (Photo by Sam Nicol)

A typical net drag through a bore drain at Edgbaston: there’s no shortage of mosquitofish! (Photo by Sam Nicol)

A typical net drag through a bore drain at Edgbaston: there’s no shortage of mosquitofish! (Photo by Sam Nicol)

Occupancy

However, all is not lost for the red-finned blue-eye. As yet, the mosquitofish hasn’t gotten into all the springs at Edgbaston. What makes a spring good habitat for red-finned blue-eye, and bad for mosquitofish? We’ve got a number of guesses: perhaps the red-finned blue-eye survives in the higher altitude springs because they don’t flood as much? Perhaps the most isolated springs are the best habitat for the native fish because they’re harder for the mosquitofish to reach? Maybe the bigger springs are better for mosquitofish, or for red-finned blue-eye? There are a lot of hypotheses. We set out to test some of these with occupancy models (Nicol et al, 2015).

We found support for the hypothesis favoured by the fish biologists, which is that the high elevation springs are the best red-finned blue-eye habitat. Big springs were good for both species. Using our models we can estimate the probability that a spring will be occupied by each species of fish. This is good news for managers of the springs because it helps to decide where to re-introduce red-finned blue-eye and eradicate mosquitofish. However the physical factors that we tested explained only a part of the pattern in fish occupancy. We think that the biological characteristics of the springs (eg, water quality, vegetation composition and extent) probably explain a lot more of the variance. This data is currently being collected as part of a PhD project on endemic snails that inhabit the springs.

Extinction probability

We were also able to quantify the increase in red-finned blue-eye extinction probability imposed by mosquitofish. While ecologists have studied competition for a long time, there are few published examples that quantify the strength of the competition between species. In this case, the presence of mosquitofish increases extinction probability of the red-finned blue-eye by 8% on average across all springs.

So, what’s the future for the red-finned blue-eye? The main goal for managers is to prevent the spread of mosquitofish. At the moment this means erecting barriers around the springs that allow water to pass, but not fish.

Biologists are also working to eradicate mosquitofish from selected springs using a fish poison and continuing an ongoing program of translocation of red-finned blue-eye to increase the number of occupied springs.

Work is also being done to exclude stock and feral herbivores from the springs. This benefits not just red-finned blue-eyes, but also many of the other unique biota of these unique ecosystems.

We’re hopeful that, taken together, these efforts will protect the red-finned blue-eye so that this little desert wonder will be around for future generations of curious fish lovers who happen to pass through the desert.


The remarkable red-finned blue-eye

Red-finned blue-eyes have other interesting features beyond the striking red fins and iridescent blue eyes of the adult males (the females are silver and have translucent fins).

At just 3 cm long, it is Australia’s smallest freshwater fish and the only member of its genus. However perhaps most interesting is the fact that they can survive in the desert at all. While water conditions can be quite steady in some springs, in others they fluctuate wildly. Water temperature in the tail of one spring was recorded to change from 3 to 30˚C in a 24 hour period! Springs change size regularly, and may expand to be as much as four times larger in winter than in summer (one spring containing red-finned blue-eye dried up completely!). On top of these impediments to fish survival, most of the springs are only a few centimetres deep, have limited vegetation cover and no shade, so the existence of an endemic fish in this hostile environment is truly remarkable.

The red-finned blue-eye is listed as Endangered under both Australian (EPBC) and Queensland legislation and as Critically Endangered by the IUCN.


More info: Sam Nicol Sam.Nicol@csiro.au

Reference

Nicol S, TB Haynes, R Fensham & A Kerezsy (2015). Quantifying the impact of Gambusia holbrooki on the extinction risk of the critically endangered red-finned blue-eye. Ecosphere 6:41. DOI: dx.doi.org/10.1890/ES14-00412.1 available as open access: http://www.esajournals.org/doi/abs/10.1890/ES14-00412.1

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