You’ve no doubt seen the coverage of mass salmon kills and rotting fish washing up on the shores of the Huon estuary in southern Tasmania. It’s apparent to me, having been a diver on various fish farms for a decade or so in the 90s, that opportunities are being missed by both journalists and activist groups because they don’t know the full story of how a fish farm works. So, I’ve written this explainer. I’ll add a caveat that I haven’t been on a fish farm in more than 20 years – then again, if you’ve seen one pile of rotting salmon, you’ve seen ‘em all.
Salmon are raised in freshwater hatcheries and transferred by truck to sea cages when they’re old and strong enough. The cages are attached with thick ropes to a network of heavy anchors on the sea floor. The fish are fed with pellets made partially from other fish (which is a topic for another day) by ‘spraying’ them onto the surface of the water; the fish go nuts, gobbling up the pellets in a frenzy. In the early days, the feed was delivered by a guy in a dinghy using a garden trowel and a bucket; a skilled feeder knew just when the fish had had enough. It’s now all automated.
Regardless of how efficient the food delivery process, some of the pellets end up being missed by the fish and fall through the bottom of the net onto the sea floor. Of course, the fish poop ends up there too. Pollution is therefore inevitable, as it is with any intensive animal farming operation.
This fallout of organic material has two outcomes.
- It causes ‘disturbance’ to the sediment on the sea floor and to the tiny invertebrates that live within: worms, tiny molluscs, and other bugs that you can barely see but play an important role in marine ecology. These sediment bugs are collectively known as the benthic community, or benthos. A diverse and healthy benthos, when covered in organic material, is replaced by an unhealthy one with lower and lower species diversity.
- The organic material breaks down. If there is a lot of material, the resulting sludge becomes anoxic. The lack of oxygen results in the release of toxic gases like hydrogen sulphide (rotten-egg gas). These toxic gases can bubble up and kill the fish.
The first outcome of fish poop fallout is of no real concern to the farmers. No one from the public can see how a healthy benthic community is replaced by an unhealthy one, nor in the scheme of things is it a big deal when the farm itself is small.
The second outcome – toxic gas production – is, however, a big deal, as an entire pen of fish can be wiped out if the event is sudden and severe enough.
Bear in mind that fish death is more likely in summer, as breakdown rates are faster and there’s less dissolved oxygen in the water for the fish to breathe already.
An underwater observer can see signs of problems early on. One is the formation of a white mat on top of the sediment formed by the activity of a bacteria called Beggiatoa. One of our jobs was regular monitoring of the sediment, when we’d collect samples and identify the bugs within, which is a reliable indicator of sediment health.
The need to keep an eye on sediment health is a good thing for the benthos: it’s in the farmer’s own interest not to let the organic material build up too much. They manage this by continually shifting their pens around, giving the sediment a chance to recover and avoiding any nasty rotten-gas-related surprises. This is kind of an aquatic form of crop rotation.
There’s a third component to the volume of organic material that accumulates in the vicinity of a farm: marine growth. Every cage ring, net, rope, anchor and cable under the sea provides a growing surface for marine plant life and invertebrates, and soon you have an entire mini-ecosystem covering everything in the water throughout the lease. (Fun fact: Tasmania’s early farmed mussel industry comprised of a guy called Dave on his hands and knees collecting mussels growing on fish pen rings.)
A 40-millimetre-diameter rope soon becomes a universe of weed and bugs, which is great for the weed and bugs but a real problem for the farmers. Ropes get heavy and unmanageable and put stress on the anchor system and cages. One job we had was to clean all these underwater nets, ropes and cables, consigning the accumulated growth to – you guessed it – the sea floor.
Growth on the cage nets in which the fish live blocks the holes of the net and restricts water flow. The fish begin to ‘gasp’, constantly flaring their gills as they struggle to breathe. Nets are therefore regularly changed (through a remarkably inventive switcheroo technique so as not to lose any fish; when that goes wrong, local fishers rejoice). The dirty net is taken ashore, where it’s cleaned in a huge washing machine. Other times, divers with high-pressure sprayers simply blast the nets clean. In either case, guess where the detritus goes.
The tragedy of the commons
The inevitable next step a small fish farm takes is to become a bigger fish farm. Why have only a cage of 60 metres diameter when you can have one that’s 120 metres? Why have only 12 pens a lease when there’s easily room for 24? As long as you keep an eye on the sediment, and keep expanding territory, what could go wrong?
Well, you get an event like that which occurred in Port Lincoln in South Australia in 1996. The tuna industry was in its infancy and the farmers hadn’t yet identified how much organic material buildup was safe for the fish. A single storm caused the sediment to erupt, spewing toxic gas into the water column and killing millions upon millions of dollars worth of fish. They called on us to help clean it up as their own divers didn’t yet have much experience with aquaculture diving. It took us three weeks to collect all the dead tuna, which were then dumped out at sea.
The reason fish farms are eager to expand their lease size isn’t just because of natural expansion.
It’s because they’ve literally shit in their own nest and need to continually move their cages to an area where the sediment isn’t dangerously disturbed.
Give the organic sludge a chance to wash away and be somebody else’s problem. Interestingly, Macquarie Harbour is rather enclosed so there’s little chance of flushing the rapidly accumulating organic material out to sea. In my opinion the whole harbour is irrevocably damaged already.
All about the dead fish you’re seeing
Unlike salmon farm locations in northern Europe, Tasmania enjoys water temperatures that, in summer, are on the upper limit of a salmon’s tolerance. Macquarie Harbour is an exception – it’s a bit colder than the waters of southern Tasmania, which is why west coast operators are so desperate to continue farming there for as long as they can.
In some months, in some years, on some days, it’s simply too warm for the salmon. Warm water holds less oxygen than cold water and the fish become stressed. Mortality rates are naturally higher. Furthermore, the summer months are conducive to salmon contracting conditions like amoebic gill disease (AGD) and bacterial infections. For AGD it’s common practice to ‘bathe’ the fish. A huge tarp is lowered into the pen, where it’s filled with fresh water from a dam dug nearby. Fish are shunted into the this temporary pool. The fresh water kills the amoeba but the salmon are unharmed. This is a labour-intensive practice long accepted by farmers, who knew from day one that Tassie waters were a little on the warm side for salmon.
Whatever killed the fish you’re seeing on TV is likely a combination of factors, each exacerbated by high water temperatures. With climate change and warming waters there’s no hope for anything but decline in conditions for salmon in the future.
Ghosties and the benefits of fish oil
Those white-and-red rotting monstrosities that you’re seeing floating on the water’s surface and washing up at beaches are salmon that have died and were left in the water too long. As a fish is dying it sinks to the bottom of the cage, where it can struggle and gasp for many hours before it’s over. Ninety-five per cent of the time the dead fish (called ‘morts’) are collected while they’re still relatively fresh. But if a mort is left in the water for more than a couple of days the breakdown and resulting gas production causes the carcass to float to the surface, where they’re exposed to the sun and the rotting process accelerates.
We called these fish ‘floaties’, or ‘ghosties’ owing to their colour and general unpleasantness. A dead fish becomes a ghostie very quickly in the summer months so it’s imperative to collect the morts at least every second day. That was our job.
If something goes badly wrong and a lot of fish die at once – particularly if it occurs when a mort dive isn’t scheduled for a few days – the first sign is typically a bunch of ghosties floating on the surface.
We often had major summer fish kills to clean up. A team of divers with catch bags would scoop up ghosties from the surface and the recently departed from the bottom of the cage, then hand them up to the crew in the boat. A major kill meant that our ‘mort bin’ soon filled up and we had no choice but to dump the morts straight on to the deck of the boat. It became a vile, oily mess of rotten fish, sometimes knee deep. The oil lingers on your skin, your wetsuit, all your equipment, for a long time after. The smell makes you retch and it’s virtually impossible to wash away. I certainly don’t recall experiencing any of the ‘fish oil is good for you’ benefits claimed in the media recently.
There’s a rule of thumb here: if you see a ghostie, there are five dead fish that you can’t see from the surface.
The photos we’re seeing probably represent only a fraction of the scale of the kill event they’re dealing with.
Furthermore, if you are seeing rotten fish … err … I mean ‘fish oil’ on a beach at some distance from a cage, then it’s possible that the scale of the kill is greater than any I ever experienced in a decade of mort diving. I expect we’ll never know the true numbers of mortalities, however – commercial-in-confidence and all that.
Tip of the iceberg
To many, the sight of rows of black fish pens in Tasmania’s calm rivers and estuaries looks idyllic and low impact. Beneath the surface it’s another story.
We not only retrieved dead fish for a living, but built and maintained the mooring systems that anchor the pens to the sea floor, and also the giant seal fences of the early days (one we built was 600 metres in length and 25 metres ‘high’ in places).
To say that the underwater vista resembles a construction site is an understatement; think about a building site where no one can see if you drop unused ropes, nets, cables, plastic ties, wire … and then never have to clean it up. The sediment beneath a working farm isn’t just contaminated with organics, it’s a major dumping ground.
Sammy the seal
While it doesn’t have much to do with recent kill events, you might be interested in the seal problem.
Particularly in winter, for obvious reasons seals hang around the cages. To get a feed the seal charges a cage’s net, mouth open, hoping to take a chunk of salmon meat off its intended and very surprised victim. Of course it’s not an easy thing for a seal to do, having to bite through the net. Often the prey animal was badly bitten but not eaten; the seal would get enough of a taste to keep trying, however.
We would often get a call at 5.30 am on our off days from the night watch crew: ‘Sammy’s been in’, as it was necessary to immediately check for any damage to the nets and retrieve the wounded and dead fish.
I can’t talk of more recent practices, but in the 90s a seal seen hanging around the cages was eventually caught in a huge submerged cage like a possum trap. These animals were relocated on a trailer to somewhere in the north of the state. I believe that tracking data revealed that the record for a relocated seal swimming all the way home was four days.
So, most often the seal was shot with a shotgun kept in the workshop.
I have a question
Now you know the basics behind what we’re seeing in the news – but there’s one more issue that’s not being talked about.
Who will be responsible for decommissioning the farm infrastructure once a lease’s life is deemed to have ended?
The tonnes of concrete anchors, cables and old ropes?
I don’t buy the ‘it’s creating a reef for the fishies’ argument. Will there be a requirement for rehabilitating the sediment and its benthic communities? As with the hundred or so oil and gas rigs that have been abandoned throughout Australian waters by their operators, who walked away without penalty, decommissioning will likely not happen.
About me
I commenced a career as a contract diver on fish farms around southern Tasmania with my brother Mark in the early 1990s, predominantly as a mort diver, inspecting nets for holes and collecting any fish that had died, sewing up damaged nets and looking after cage mooring systems. I had recently graduated from the University of Tasmania with a science degree with a half-major in aquatic zoology and I had a working knowledge of the potential for ‘disturbance’ of the fauna from the effects of the fallout of organic material from salmon cages. I began running impact surveys on one of our client’s leases, devising experiments to assess the impact under a range of conditions.
My conclusion was that if the lease is small with a reasonable stocking density – as they all were back then – then the impacts are significant in the immediate vicinity of the pens but negligible or zero at distances of around 60 metres from the lease (depending on prevailing currents, size of the lease, storm events etc.) We were soon doing surveys for other leases as the state government began mandating routine assessments. But inevitably our work dried up as larger companies with ROVs (remote-operated vehicles) came along, negating the need for divers like us. By the end of the decade I had switched to an indoor job with no heavy lifting, though Mark and his crew continued the base business of farm diving for many years after.
Bruce Ransley was a diver in the early days of the fish farming industry, and has a unique insight into how the system is failing.
Don GIm
February 27, 2025 at 11:58
I was at Port Huon, and noticed that the stench there was unmistakable — it was from either the putrid smell of rotting fish, or something akin to a septic system being pumped into a machine.
Our environment is not just for us because it is actually a legacy for future generations. If we continue exploiting and damaging it beyond its natural capacity to recover, then we are not just harming the present — we are bankrupting the future.
Sustainable growth must strike a balance between industry and environmental stewardship, thereby ensuring that we do not leave behind a landscape of destruction for those who come after us.
Kevin Ellard
February 28, 2025 at 12:42
As a vet and a diver who worked in the Tasmanian salmon industry for many years, I can only support Bruce’s comments.
Freshly dead fish sink, but rotting ones float and quickly disintegrate while releasing disease-causing organisms as they do so. This is a very significant event that industry and government appear to be struggling to manage. It is certainly the biggest mortality event in the history of Tasmanian salmon of which I am aware. What we see on the surface is only a small portion of what is going on, and how it is impacting the surrounding environment.
My support and best wishes go out to farm workers and managers trying to deal with this situation, but it does beg the question “Has the Tasmanian salmon industry reached it limit for economic, social and environmental sustainability?” This happened in Chile where rapid salmon production growth led to poor biosecurity and outbreaks of ISA, RLO, and algal blooms.
Tasmania cannot continue to grow this sector using the current production model where companies are located cheek by jowl in common inshore waterways, and where the poor practices of one company impact on their neighbors thereby leading to disincentive for bio secure and sustainable farming practices.
There has long been discussion about separating the individual companies and restricting them to specific regions so that each can develop and implement effective long-term farming practices. This is the approach in Norway, and it has largely served them well. Unfortunately, the development of Macquarie Harbour and Storm Bay has further undermined this single-company-per-region approach.
Scott Salarno
March 3, 2025 at 14:27
Bruce’s Layman’s Guide is a guide to what salmon farming was like 25 years ago. A lot has changed since then.
Almost no feed is wasted due to AI-powered automatic camera-based feeding. Feed conversion ratios are down to 1.1:1 which means 1.1kg of feed is used to produce 1kg of growth. As it is not possible to produce 1kg of growth on less than 1kg of feed, due to feed used for energy to swim and breathe etc, the implication is that there is essentially no waste. There is a financial incentive not to waste feed too, as it costs around $4,000 a tonne.
Farmers want a healthy benthic community because it is that which breaks down the waste and keeps the environment healthy for the salmon as well. New leases farmed since the 1990s are mainly in areas with strong tidal flows with Macquarie Harbour being the exception.
Yes, nets, like any other structure left in the ocean, become covered with marine life. No, “Dave” was not the first mussel farmer. There were mussel farms before the first salmon farm. Nets are now cleaned by automatic robotic in-situ cleaners which allow for much more frequent cleaning, thereby resulting in much less weed growth, and hence much less “detritus.”
Some pens are now 240m in circumference. There has never been an event in Tasmania like the one Bruce says happens in Port Lincoln. There was an event in 2019 in Macquarie Harbour where storm surge pushed the naturally anoxic bottom layer up into some pens owned by Petuna.
The optimal growing temperature for Atlantic salmon is 16.5 degrees. Tasmanian water rarely gets above that, but sometimes it gets to the top of their preferred range of 20 degrees. They stop feeding and can get stressed. But generally salmon grow best in Tasmanian waters because our water is relatively warm.
Macquarie Harbour was not used because the water was colder. In 2021 the coldest water in summer was actually at Okehampton Bay on the east coast. Macquarie Harbour was chosen because of the layer of freshwater on the surface so the fish could naturally bathe their gills when they became irritated by amoeba. Amoebic gill disease now occurs year round, and even in some Northern Hemisphere countries. Fish are bathed in fresh water, but these days it is in the large “well-boat” ships, not by tarps in pens. Huon had the first well-boat back in 2016.
Morts are not collected by divers any more but by automated suction devices discharging them onto barges. The recent bacterial disease outbreak got to the stage in a few pens where the suction mort retrieval systems couldn’t keep up, thus leading to the “floaties.”
In the 1990s the seals charged the nets and bit numerous fish. Today, and for the last 10 years, pens have external seal fences built into them, which is one of the reasons why the pens seem more visible, namely because of the high fences around each of them. Seals rarely get into the pens, but when they do they actually cause less harm than they used to. They eat a few whole fish rather than damaging hundreds, and then cheekily haul themselves up onto the pen edge while knowing that they will be let out. They are very smart. They are no longer shot with a shotgun as Bruce suggested he used to do.