The unstoppable fish

By Tim Low 19 September 2016
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They’re aggressive, have been seen crossing paddocks, found in remote puddles and can handle water saltier than the sea.

WHO SAYS FISH need water? The spangled perch will ‘swim’ upstream without enough liquid even to cover its face. Thanks to a zest for travel after flooding rains, it occupies more rivers in Australia than any other fish. Spangled perch have been seen crossing paddocks, and swimming 16km over six hours along a flooded wheel rut, until the water seeped away.

The perch shown in the image above were heading upstream in 2010 during a flow over the Stuart Creek causeway on the Oodnadatta Track, South Australia, just a few kilometres upstream from Lake Eyre South.

During storms fish are sometimes alleged to fall from the sky, but in Australia those found in remote puddles are nearly always spangled perch, inviting suspicions that they arrived over wet ground. Fish have never been found in Australia on roofs or in tanks, suggesting that ‘rains of fishes’ are a myth. Meteorologists say that fish drawn up by a whirlwind would freeze in the air then plunge to Earth at a deadly speed.

Spangled perch are super hardy, surviving temperatures of 44°C and water saltier then the sea. Because they are compulsive travellers, they can be found in most waterbodies in the northern two-thirds of Australia. They show next to no genetic variation across the continent, indicating recent spread, and justifying talk about a single perch population.

People are now spreading them further afield by placing them in streams and dams, often for no good reason, since they are too small to be worth eating. I have seen them in in my local stream in Brisbane, outside their natural range. The Western Australian government wants them kept out of streams in south-western Australia, where they rate as an unwelcome invasive species. They are very aggressive towards other fish.

The spangled perch has two close relatives: the Kimberley spangled perch, living in two rivers in the Kimberley, and the Fortescue grunter, confined to the Pilbara region. Compared to their dynamic relative, these fish look like losers, even though they must share most of its DNA. The spangled perch has acquired a drive to succeed that is something to marvel at.