an autodidact meets a dilettante…

Canto: So here’s a question. Why are so many fish dying in our rivers? I believe it has something to do with oxygen, but that raises a whole heap of questions, like why do fish need oxygen, how does this relate to fish physiology, what’s the difference between freshwater and saltwater fish (is all fish physiology basically the same), and is climate change a factor in all this – or rather, how is it a factor?

Jacinta: Okay so that’s a good focussed question, or set of questions, a bit easier to deal with than the management of our river systems, which would involve inter(and intra)-state politics and the rights or wrongs of irrigators, farmers, industry and the like. I’ve heard all that is rather complicated.

Canto: Right, so we’re just going to focus on the internal and external environment of freshwater fish.

Jacinta: Well, maybe. News reports have claimed up to a million fish deaths in the Darling River, with critically endangered Murray cod being among the victims. Algal blooms ‘which can be caused by agricultural chemicals’ (sky news) are being cited as the proximal cause, but the ultimate cause, according to Menindee Lakes residents, is government mismanagement.

Canto: I would wonder whether there are so many fish in the river to begin with – and that million figure is grossly exaggerated according to various sources. The figure appears to be something between 100,000 and 200,000, still a staggering number, and I wonder about the deaths in proportion to the population, and if some species are dying more than others. We need some science here.

Jacinta: And my preliminary enquiries into the science show that it’s complex and unsettled. The most recent mass death occurred in the Menindee Lakes, south of Broken Hill, a series of lakes connected to the Darling. We know that there was very little flow-through at the time, the water levels were low and water temperature had risen. As a result, there was a large bloom of cyanobacteria, aka blue-green algae. But apparently according to an ABC Science report, the cyanobacteria weren’t exactly the problem, it was their death, caused by a cold front, and the sudden explosion of other bacteria feeding on the dead and dying cyanobacteria, and in the process depleting the water of oxygen, that caused the fish to drown.

Canto: So what we call drowning is really loss of oxygen, which fish have evolved to capture from water but we can’t. How do they do that?

Jacinta: Gills. Fish breathe through their mouths like us. And also like us they need oxygen to function and they breathe out carbon dioxide. Gills – and we could expend pages and pages on their origin, structure and function – are those organs found on each side of the pharynx or throat, and they’re made up of protein structures called filaments. Each filament has a tiny network of blood vessels, providing a large surface area for the exchange of gases. So fish suck in water, with its oxygen, and then pump it out through the gills, where all the essential gas exchange occurs. But if the water isn’t sufficiently oxygenated, then it’s goodnight, sweet fish.

Canto: So the proximal cause isn’t the blue-green algae, it’s the oxygen-consuming bacteria that feed on the algae.

Jacinta: But if the algae weren’t there in the first place, the bacteria that feed on them wouldn’t be there.

Canto: Yes, but that’s only because the algae were dying. What if there hadn’t been this sudden drop in water temperature? Do the algae themselves affect the oxygen levels?

Jacinta: Well, actually, very much so. Cyanobacteria were the first photosynthesising organisms – we wouldn’t be here without them, and they’re now incorporated, in the form of chloroplasts, in all the plants around us. They were the principal means of oxygenating the biosphere.

Canto: So does that mean it’s good to have blue-green algae in our waterways? I’m confused.

Jacinta: The answer is yes and no. The Murray Darling Basin Authority (MDBA), currently under fire from all and sundry, have a useful factsheet about what we’ll henceforth call cyanobacteria. They’re a natural part of the system, and there are a number of species, the two most prominent being Dolichospermum and Microcystis. Under certain optimal conditions for growth, they produce ‘blooms’, which can be toxic at high levels. Mostly though, they don’t affect irrigation, recreational use of the river, or drinking water, if properly treated. It’s the decay of these blooms that causes most problems…

Canto: I note that these fish deaths occurred in the Menindee Lakes, and may have been a result of low water levels, which in turn were due to drought. Lower water levels means a lower volume of water, so that the environmental temperature would more rapidly affect the water temperature than if the volume was greater – no doubt there’s an equation to account for that – which would more quickly affect the decay of the bloom and the growth of the oxygen-depleting bacteria feeding on the bloom. So it seems to me that the ultimate cause is drought, which creates a less stable environment for the fish, and other organisms. How’s that?

Jacinta: Well, it’s the beginning of an explanation, but it’s too simple. It isn’t just drought that’s affecting water levels, it’s the fact that water is drawn from the system. And that involves politics, which we were hoping to steer clear of – oh well. By the way, the fish that have died include Murray cod, golden perch, silver perch, and bony herring. I don’t have relative numbers though. The Menindee Lakes region, which is at the centre, not only of this fish death controversy, but of the entire Murray-Darwin Basin management controversy, appears to be at crisis point, and the locals aren’t happy. Here are some quotes from The Guardian on the issue:

Since the 1960s, the original Menindee Lakes have been significantly altered to serve as a major storage for water for the Murray-Darling Basin as well as the water supply for Broken Hill. The lakes are also a major fish breeding area for native fish, and critical to maintaining stocks of fish throughout the river system.

However, the NSW government has proposed shrinking the lakes and altering the way it manages the water storage, in order to reduce evaporation. It is currently building a $500m pipeline from the Murray to Broken Hill in order to provide the inland city with an alternative water supply.
But the plan is highly controversial because it will mean the government has less reason to keep the lakes full and will likely see the Lower Darling run dry more often.
Local graziers and the towns of Wilcannia and Pooncarie are up in arms about the state of the river, accusing the NSW government of sacrificing their 500km stretch of the Darling in order to benefit upstream cotton growers.
They say the current crisis is due to Water NSW’s decision to run the lakes dry despite forecasts of drought.

Canto: The MDBA, which is a federal body, presents their reasons for the fish deaths in two concise points:

• the lack of water flowing into the northern rivers
• the impact of 100 years of over-allocation of precious water resources throughout the entire Basin.

Drought and the removal of water from the system, precisely your point, Jacinta. The MDBA of course avoids blame, and says nothing about possible current over-allocation. It does, however, say, in the same information page, that ‘the Menindee Lakes are currently under the sole control of New South Wales and have been carefully managed since December 2017’, which appears to court some controversy.

Jacinta: And finally, something important to watch out for as we seek an ultimate answer to our question. An independent panel of six science worthies has been appointed by the Federal government to enquire into these deaths. Fields of expertise include catchment hydrology, fluvial geomorphology, freshwater ecology, plant ecotoxicology, aquatic ecosystem health and much much more. The team will provide a preliminary report to the Feds by February 20, and a final report should be completed by March 31. We’ll look out for it – presumably it will be released to the public.