The Albanese Salmon Farming Decision Raises a Simple Question: What Happens in the Water?
Salmon farming is back in the news.
Reports say environmental advice raised concerns about oxygen levels in Macquarie Harbour.
Most public debate focuses on outcomes.
But the real question is simpler.
What actually happens in the water?
Date: 2026-03-04
What Are the Environmental Issues with Open Net Salmon Farming?
Over the past 6 to 12 months, salmon mortality and fish health have become topical issues around Australia. Public discussion has focused on outcomes, but many people are still asking a more basic question:
What are the environmental processes involved?
To help ground that discussion, we looked at two typical Tasmanian fish farming environments: Macquarie Harbour and Storm Bay. Macquarie Harbour is more enclosed and layered. Storm Bay is more open and mixed (High energy).
Macquarie Harbour currently operates under a biomass cap of about 9,500 tonnes. This is widely understood to represent the harbour’s carrying capacity under the present regulatory settings.
We compared two situations over the same 13 hour period across a 13 kilometer salmon lease corridor. (McQuarie Harbour)
The wild fish school weighs about 500 tonnes and swims through once. The fish farm lease corridor holds about 9,500 tonnes of fish in one place.
Five hundred tons is a reasonable size for a large wild fish school. The 13 hours comes from a school swimming about 1 kilometre per hour across 13 kilometres, ( McQuary Harbour Lease approximate length) allowing for tidal movement and normal travel speed.
The question was simple:
If both stopped after 13 hours, how long would nature take to return the water and seabed back to normal?
“Normal” means:
Oxygen back to usual levels.
Nitrogen back to usual levels.
Seabed waste broken down.
Respiration oxygen use
All fish “breathe”. Respiration consumes dissolved oxygen from the surrounding water.
In 13 hours:
The wild school uses about 1.6 tonnes of oxygen.
The farm uses about 30.9 tonnes of oxygen.
The farm load is 19 times larger because there are 19 times more fish.
If loading stops after 13 hours:
Oxygen recovery time
The wild school: about 1 to 3 hours
The farm: about 14 to 57 hours
Nitrogen recovery time
For both situations, nitrogen processing typically takes about 3 to 30 days.
Seabed recovery time
The farm produces uneaten feed and faeces that settle to the seabed beneath and around the lease area. Seabed recovery can take about 7 to 90 days depending on conditions.
The wild school moves through once and does not create a stationary deposition footprint in the same way.
Oxygen can recover relatively quickly. Full recovery is usually controlled by nitrogen processing and seabed breakdown.
The structural difference is clear.
The wild school is a one-off event. It passes through once and leaves. Recovery can begin after it has gone.
The farm is continuous loading. It remains in place. New 13 hour loading periods occur throughout the recovery time.
How this behaves depends on the water body.
Macquarie Harbour
Water moves slowly. Layers do not mix well. Nitrogen stays in the system for a long time. Oxygen in deeper water can fall because new oxygen does not arrive often.
Main issue: retention. The pollution stays and recycles locally.
Storm Bay
Water moves more. There is stronger mixing with ocean water. Nitrogen is carried away faster.
But carried away does not mean removed. It just spreads out into a larger area.
Main issue: throughput. The pollution keeps moving through the wider system.
The difference is not only the size of the biomass. It is one-off loading versus continuous loading.
About the author
The author has been analysing the salmon industry for the past three years. His background includes food research and development, program management, EPA pollution compliance for processing plants, internal audit, innovation, and solution architecture.
He has found that the Australian industry, like other global producers, is struggling with mortality, parasites, seabed impact, oxygen depletion, regulation and pollution.
Aquaculture is pivotal in food security.
The environment is pivotal in our survival.
This is where they meet.
We need to make it work for both to survive.
Includes this article, analysis papers and references