Intensification Breeds Infection

Salmon by Daryl Hunt CC BY-NC-ND 2.0 


Amoebic Gill Disease (AGD)

Staff and visitors to the CSIRO Marine Laboratory in Hobart may notice a regular ritual as they gaze out the big windows toward the sea. Often, boats pass up and down the river in the distance, towing salmon cages. To the unknowing, it may look like a delivery. But it's not. There's no farms up the river. They are towing nets, each packed with tens of thousands of fish, from the saltier waters where they grow in Storm Bay and D'Entrecasteaux Channel, into the fresher waters of the Derwent Estuary, to kill off their parasites, then back into saltier water, until the process repeats in a few weeks. These fish are the lucky ones; the unlucky ones either get vacuumed from their pens into bathing boats, or they simply die. They might die anyway from stress of the living fish being vacuumed out of their pens into a waiting ship to be cleaned, then vacuumed back again  at a rate of up to 3,300 salmon per minute [1]. 

The biggest threat to the salmon industry is gill disease. Indeed, it can kill up to 2% of fish per day if not treated [2]. Tassal, Tasmania’s largest salmon company, has admitted that it is standard to lose 17 percent of stock prior to harvest  [3] — rather alarming compared to the 3 to 5 percent mortality rate in intensive chicken farming! The industry spends a whopping 20% of its production costs on managing just this one problem [4]. The scale of these losses and costs would be incomprehensible in just about any other industry. 


What is Amoebic Gill Disease?

Under certain conditions, the salmon's gills become infected with amoeba, a microscopic protozoan parasite. The parasite in question is named Neoparamoeba perurans, and the illness it causes is called amoebic gill disease, or AGD. AGD can be precipitated by infectious agents such as parasites, bacteria, or viruses, or by injuries from microalgae or jellyfish stings [5]. 

Amoebic gill disease is a very serious threat to salmon farming. As the amoeba colonise the gills, mucousy swellings affect increasingly more of the gill surface, impairing oxygen exchange as well as control of salts, fluids, and ammonia that are regulated by the gills. Affected fish are unable to breathe properly, causing their growth rate to slow. If left untreated, the fish will die; many die regardless. Sublethal effects on the gills may nonetheless make fish more vulnerable during periods of low oxygen or when they they under higher stress due to higher temperatures, crowding, or transport [6]. AGD was first reported in salmon in Tasmania in the mid 1980s [7], shortly after salmon farming started here. Today, it is a much bigger problem here and is also now known from most salmon farming regions of the world [4]. 

Unfortunately, climate warming is a dream come true for this amoeba, both directly and indirectly. Firstly, the amoeba like warmer weather, and they thrive in these conditions. Secondly, Tasmanian temperatures are already too warm for salmon [2], so the fish are stressed out, making them even more vulnerable to disease, the same way humans are when we get stressed out and run down. We should fully expect that the disease burden of AGD will only get worse with climate change.

As if it's not bad enough that Tasmanian conditions are becoming less fish-friendly and more amoeba-friendly, it also appears that the amoeba species is adapting to the freshwater bathing. That is, our best line of defense is becoming less effective [2]. We are in an arms race with a pest that is having the greatest pest-fest it has ever known, and we are handing the conditions up to it on a silver platter. We are such a myopic species! 


The special case of Macquarie Harbour

The industry has demonstrated an apparent inability to perceive the needs of the habitat, even when it served them the best to exercise that perception. Macquarie Harbour is an excellent example of this myopia. 

Macquarie Harbour, or Mac Harbour as the locals call it, is six times the size of Sydney Harbour but unusually shallow. It has an average depth of just 15 metres, with a maximum of just 50. It has a natural sill at the mouth, or a raised bar, with the effect of rarely flushing the deeper waters. As such, it is basically a fjord. Because of this unusual geomorphology, the saltier water sinks to the bottom and a lighter lens of freshwater essentially floats on top. What's more, Mac Harbour water is dark with tannins from the forests surrounding it. This perpetually-dark water has the effect of seeming like the deep sea, even though it is in the shallows. It is a unique habitat, to say the least. One that is easily polluted and not easily (if ever) remedied. Fish should never have been farmed here in industrial quantities. 

To understand how radically if not suicidally myopic the salmon industry has been, consider the words of an article in the magazine FISH produced by the Fisheries Research & Development Corporation: “The freshwater layer is also what makes the harbour so attractive for fish farming. The potentially fatal amoebic gill disease is one of the most significant health issues for the industry in Tasmania. Bathing fish in fresh water cures the condition and, as a ‘self-treating’ environment, the harbour is free of the disease” [8]. Considering the high cost of managing amoebic gill disease outside Mac Harbour, one might think that it would be worth treating Macquarie Harbour as if the health of the ecosystem mattered. And yet, precisely the opposite occurred... on purpose! 


Adaptive management

Expanding more rapidly than the science that should have underpinned it, the Tasmanian salmon industry has chosen to adopt an operational strategy known as “adaptive management” [9], which basically means pushing the habitat into crisis, then backing off a little bit. On paper, it seems like a great experimental way to find the sweet-spot of productivity. But alas, things aren't always as straightforward as they seem on paper. 

Adaptive management would be a bad idea for any habitat that cannot easily recover. And Mac Harbour is just such a place. Because of the natural sill and low flushing rate, waste builds up for longer periods of time, making the Harbour especially prone to dead zones on the seabed under the cages. And the shallow nature of the Harbour means that these lethal conditions are frightfully close to the fish. The last thing in the world you would want to do, given these factors, would be to cram a lot of fish into pens over these shallow areas. But that’s what adaptive management means, you see how far you can push it. 

Initial warnings about Mac Harbour were ignored. Even when the oxygen readings plummeted, these were ignored. Even when tens of thousands of fish went belly up, warnings were still ignored. Even when one of the operators was practically apoplectic with outrage, warnings were still ignored. And then whammo, they found that place just past the sweet spot. By the time the dust settled, more than 1.35 million fish had died within six months [10]. That, friends, is how adaptive management works. 

Mind you, the Mac Harbour fish didn't die of adaptive management. Nor even did most of the fish die of amoebic gill disease. Most died of another disease (called Pilchard Orthomixovirus, or POMV). But everything is connected. The salmon companies pushed Mac Harbour to the point of collapse because the freshwater layer offered the promise of less expense dealing with AGD. But applying adaptive management in an unsuitable place was destined to end in tears. The fish died because of hubris: the promise of profit was bigger than the lives and suffering of more than a million fish. 

Perhaps not morally, but legally at least, the salmon companies have the right to kill the fish, and even, it seems, to make them suffer in the process. They even have the right to make foreseeably dumb decisions that cost their shareholders money — a concept known as Directors' Duties means that they don't legally have the right, but the wrath of investors didn't hold them responsible, so in practice it was okay. But here's something that is totally not okay: they have damaged a World Heritage Area and driven a native skate to the brink of extinction [11, 12]. Skates are related to sharks and rays. The Maugean Skate, whose only known home is Macquarie Harbour, has nowhere else to go to escape the pollution in its home. It probably won't survive. 



After all that, the salmon industry doesn't seem to have learned a thing. Mac Harbour was inevitable, given inadequate and unlistened-to science and bad governance, combined with lack of restraint. The industry is back in Mac Harbour, as well as expanding into Storm Bay (another calamity in waiting), Okehampton Bay (originally deemed unsuitable for salmon farming), and with plans now to farm on the northwest coast. It appears that the disaster that was Macquarie Harbour could easily happen again in other locations around the state, given that the structural issues with science and governance have not yet been addressed. 



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