Imagine for a moment the incredible complexity of life that makes up a temperate reef: all the algae, seagrasses, weedy sea dragons and other fish, octopus, cuttlefish, crayfish, barnacles, abalone, snails, oysters, starfish, and even worms. Hundreds of species swimming, scurrying, darting, or laying in wait, just going about their lives. Now imagine every single last one of them suffocating and being covered over with sludge – a thick rain of excrement from above. Salmon farming creates what is known as 'eutrophic' conditions: too much excrement leads to low oxygen (also called 'hypoxia'). Those who can swim, scurry, or dart away, flee for their lives. But those who cannot, simply perish, their habitat no longer able to support life. This is what it looks like under salmon farms [1]. 

So, how much excrement, you ask? A lot.

Non-scientists may want to skip to the next paragraph, but for the geeks among us(!), let's walk through the numbers. In general, 10,000 tonnes of fish release 2,025 tonnes of poo. Currently the Tasmanian salmon industry produces 80,000 to 100,000 tonnes of fish annually [2], or around 20,000 tonnes of poo, or 20 million kilograms of poo. Ok, so that's a lot! But how much is it in comparison to other inputs, like people for example? As of 2021, 48 leases in Tasmania contain more than 500 pens combined; 28 of these leases and more than 300 of these pens are in the Huon and D'Entrecasteaux Channel, accounting for half of the state's production. Using figures from a submission to the Legislative Council FinFish Inquiry [3], 80,000 tonnes of fish produce around 4,500 tonnes of dissolved nitrogen, a whopping 6 times the amount of sewage produced by the entire human population of Tasmania!. 

As disgusting as it is to think about our food being raised in fetid conditions, and as brutal as it is to sacrifice all the native animals and plants in a habitat in the pursuit of profit, there are other far more frightening implications. 

 

Dead Zone Problem 1: Remobilisation of Heavy Metals

Tasmania has a shameful legacy, and you would definitely not want to wake this sleeping dragon. Decades of heavy metal discharges into the Derwent led to it being declared the world's most polluted estuary in the 1970s [4]. Mercury poisoning, or what's known as Minamata Disease, causes acute and chronic toxicity, neurological impairment, horrific birth defects, and death. Even decades later, fish and shellfish from certain parts of the Derwent and Storm Bay are still unsafe to eat. 

In a healthy ecosystem, heavy metal molecules bond with sediments, making them largely inert to us and to other organisms. So, over time, much of the Derwent and Storm Bay have become safe again. But due to an unfortunate feature of water chemistry, when water is low in oxygen, the chemical bonds between heavy metal molecules and sediments are broken, releasing the heavy metal molecules back into the water [5]. These remobilized heavy metals enter the food chain and accumulate in the bodies of predators – and in us! 

Salmon farming often leads to low-oxygen bottom water, creating this very condition! 

Wondering how heavy metals in the environment might get into the salmon when their food comes from manufactured pellets? As was explained in a Parliamentary submission [6], the excess nutrients (a polite word for excrement) of salmon farming act as high-octane fertilisers to stimulate overgrowth of phytoplankton (plant plankton). Much of the uptake of heavy metal molecules occurs with phytoplankton [7, 8]. Jellyfish-like organisms called salps gorge on phytoplankton. Perhaps not surprisingly, salps concentrate the heavy metals from their food into their body tissues [9]. And farmed salmon love to snack on salps [6, 10]... as do other animals like dolphins and penguins. 

The salmon industry in Tasmania is scheduled to double by the end of this decade. The impacts of remobilised heavy metals on shellfish aquaculture, recreational fishing, native species, and on the salmon themselves, is too alarming to contemplate. 

 

Dead Zone Problem 2: A New Normal of Jellyfish and Toxic Algae

One of the most conspicuous problems with dead zones is the phase shift of the ecosystem from a healthy, biodiverse habitat to one dominated by jellyfish and toxic algae. As explained above, toxic algae (phytoplankton) respond multiplicatively to nutrient overload, which is the inevitable result of salmon farming. Jellyfish, in particular, are a visible indicator that the ecosystem is out of balance [1].  

Jellyfish are evolutionarily simple creatures, and yet they have a highly effective way of seizing control of an ecosystem and then not letting go. In a healthy ecosystem, jellyfish can't really compete with fish, so they mind their P's and Q's and stay well behaved. But in a disturbed ecosystem – any disturbance that affects fish numbers – jellyfish thrive. Dead zones are just such a condition. 

Jellyfish eat the eggs and larvae of other species, as well as the planktonic food that these larvae would eat. This 'double whammy' of predation and competition allows jellyfish to take over the role of top predator. Jellyfish controlling fish may seem as absurd as the grass controlling cows, but it happens. In fact, in disturbed ecosystems, it is the new normal. Scientists call this phenomenon "phase shifting", and the end result is a grossly simplified ecosystem, devoid of all but a few fish and shellfish, no penguins, no dolphins, no fishing. 

 

Dead Zone Problem 3: Threats to Endangered Species 

Tasmania is home to a wealth of endemic species, that is to say, found nowhere else. Because of their restricted distribution, they are highly vulnerable. Some, in fact, are already severely impacted and formally recognised with Endangered Species listing. 

Handfish are adorable little fellows. These fish have fins modified into legs of sorts – they look like they are walking on their hands, similar to how seals and walruses do. But cuteness isn't enough to save you when your habitat becomes overgrown with stringy, slimy algae, stimulated by the same excrement from salmon farms that causes dead zones. 

Weedy sea dragons. Abalone. Octopus. Snails. Crayfish. Crabs. Kelp. All sorts of fish, invertebrates, even worms – dead zones do not discriminate, they just kill everything. Even the cute ones. Especially the cute ones. 

It's hard to say which one will vanish first, the critically endangered Red Handfish or the critically endangered Maugean Skate. Both appear to be in great peril because of salmon farming. The handfish is down to two small reefs about the size of a tennis court [11], and the skate, which has been called the "thylacine of the sea" [12], lives in the God-forsaken habitat of Macquarie Harbour, badly impacted by low oxygen and disease from the salmon industry [13]. Sadly, we will hear when the last remaining individuals of Red Handfish are snuffed out because they are well-studied, but even more tragically, IMAS appears to have stopped monitoring the Maugean Skate. 
 

Dead Zone Problem 4: Memory

As if the dead flora and fauna aren't bad enough, another unfortunate aspect of dead zones is that they have memory, that is to say, they cause profound changes to the water chemistry and the way habitats process nutrients, making the ecosystem more prone to 'snapping back' into a dead zone state. 

Interesting research from Europe found that hypoxic conditions on the seafloor cause the sediments to release dissolved phosphorus [14]. This stimulates a positive feedback loop that drives phytoplankton blooms, which drive more dead zones, which drive more plankton, and so on. Hypoxia also lowers the rate of off-gassing of nitrogen, which in turn acts as another positive feedback loop for continued blooms of phytoplankton. Dead zones therefore enhance the volume of available nutrients that stimulate phytoplankton growth, which in turn enhances the dead zone. Moreover, seafloor communities are made up of large, slow-growing species like sponges and corals, as well as echinoderms and worms that continually bioturbate and bioirrigate the sediments, speeding up the cycling of nutrients. When these animals suffocate in a dead zone, they are replaced by smaller, faster-growing rapidly-colonising species that don't mix or dig into the sediments, so the sediments remain anoxic. Finally, it also appears that the interplay between nutrient and sediment processes may act in a threshold-like manner, where after reaching a tipping point, these processes act to stabilise the ecosystem at its 'new normal', resisting any shift back to its original, well-oxygenated state [1]. The take-home message is clear: look out for your sediments, because once you've created a dead zone, it can be very difficult to undo. 

 

REFERENCES

[Click here to go to Dead Zone References]

Image Redondo Fish Kill by seadigs CC BY-NC 2.0