The acid-water fish of the wallum

Two fish live in the acid, tea-dark water of the wallum, in the swamps and the slow blackwater streams and the hollows between the sand ridges. You could cup either of them in one palm, and they swim almost nowhere else on Earth. One is the honey blue-eye (Pseudomugil mellis), a small, glinting amber-and-gold fish of the coastal sand country, its fins carrying the faint blue spark the family is named for. The other is the Oxleyan pygmy perch (Nannoperca oxleyana), a stubby, dark little fish of the same sour waters. Both are tied to the wallum blackwater, and both are now nationally threatened. They are the swimming counterpart to the wallum’s insect-eating plants and its acid frogs: specialists that make a home of a chemistry that reads, to almost everything else, as poison.

The water is punishing. Leached from the peaty, acidic sand, wallum blackwater runs sour — around the acidity of orange juice, well short of vinegar — and for most freshwater animals it kills, drawing the salts from the body and burning delicate tissue. So why would a fish evolve to live in it at all?

The answer has a twist in it. Water this sour is hard on almost every other fish: below about pH 5 it blocks the salt uptake at the gills and starts to damage them, and the recovery plan for the wallum’s frogs records that most of these acid breeding waters hold no fish at all. The fish that would otherwise be there, above all, is the eastern mosquitofish, Gambusia holbrooki, the pugnacious little import that has colonised so much of Australia’s fresh water, nipping fins and eating the eggs and young of native fish and frogs wherever it reaches. In the wallum’s sourest, most isolated pools it is mostly absent. So the blackwater that looks like a death trap is, for these two natives, something closer to a fortress — and the drawbridge, as far as anyone can tell, is chemistry. It is the same defence the acid frogs rely on, reached this time from the water rather than the land.

But the easy version gets one thing wrong. The acid makes a partial refuge, not a sealed wall. Nobody has shown that the sourness by itself bars the mosquitofish; what is on record is that the fish is largely missing from these waters and a serious predator wherever it does arrive, and that the recovery plan asks for every effort to keep it out of wallum swamps and lakes — which nobody would write if the sour water did the job by itself. Read the fortress as a moat, deep but fordable. It keeps most of the enemy out most of the time, and that margin is enough for the fish to hang on. A margin is all it is.

Which is why they are so fragile. The protection is chemical, and chemistry is easy to undo: run-off from a new estate, a dose of fertiliser, a drain that admits ordinary water. Any of it sweetens or dilutes the blackwater, joins isolated pools into permanent water, lowers the drawbridge, and lets the mosquitofish and the competitors through — joining the pools up is exactly what the recovery plan warns will bring the fish in. Anything that drops the water table can dry the breeding pools out altogether. So to keep the honey blue-eye and the Oxleyan pygmy perch you do not manage the fish at all. You manage the water, and the hungry wallum that brews it. You can lose them without ever laying a finger on one.