ANSWER 1

Yes, oysters (and mussels) can function as natural biofilters, but they should not be presented as a simple or complete mitigation solution for offshore salmon farming.

Shellfish are very effective at filtering particles from the water column. They can remove phytoplankton, suspended solids, and some particulate nitrogen and phosphorus. In integrated multi-trophic aquaculture (IMTA) systems, bivalves can recover a portion of the nutrients released from finfish farming, particularly particulate organic material. However, much of the nutrient waste from salmon farms is dissolved nitrogen, especially ammonium. While shellfish can contribute indirectly to nutrient cycling and removal, they do not directly assimilate dissolved nutrients as efficiently as seaweeds.

A second qualifier is frequently overlooked: filtration is not the same as removal. Bivalves reject and egest a substantial fraction of what they filter as faeces and pseudofaeces, which sink to the seabed. Co-locating dense shellfish culture around a finfish farm may therefore concentrate organic matter beneath the farm rather than disperse it. Nutrients are only permanently removed from the system when shellfish biomass is harvested and taken out; filtration alone redistributes nutrients within the environment. Most IMTA studies indicate that only a modest proportion of farm-derived nutrients can be recovered through shellfish production alone, meaning that very large shellfish production areas are generally required to offset a meaningful share of nutrient outputs.

For these reasons, shellfish filtration around offshore salmon farms is biologically plausible but not automatically practical or sufficient. It would need to be designed as a site-specific IMTA system supported by hydrodynamic and nutrient-budget modelling, environmental monitoring, and regulatory approval. Importantly, each site would need to be assessed to determine whether shellfish culture reduces overall environmental impacts or simply redistributes them. Shellfish IMTA should therefore be viewed as a complementary tool, not a substitute for feed efficiency, stocking controls, fallowing, benthic monitoring, waste reduction, and strong environmental regulation.

On reuse of the harvested shellfish (Question 2), the pathways exist in principle but the economics are the real constraint. Shells can be processed into lime, soil amendments, construction aggregate, or calcium carbonate products. Soft tissue may be suitable for fertiliser, pet food ingredients, low-value aquafeed inputs, or anaerobic digestion for biogas production. The biofuel-by-combustion pathway is generally less attractive because dewatering wet biomass is energy-intensive and the energy return is relatively low.

Any feed or fertiliser use would require testing for contaminants, pathogens, and residues, together with compliance with relevant biosecurity and waste regulations. The more important economic consideration, however, is that shellfish grown on or adjacent to finfish leases may not be suitable for premium food markets. The comparison is therefore not simply between processing costs and by-product revenues; it is between lower-value industrial uses and the foregone value of a marketable food crop. On that basis, the costs of leasing, infrastructure, labour, monitoring, processing, and compliance will often exceed the recoverable value of the shellfish and shells.

None of this means shellfish IMTA should be dismissed. Rather, it highlights that its value is likely to be greatest where nutrient recovery, additional seafood production, and ecosystem-service outcomes can be achieved simultaneously, rather than as a stand-alone pollution treatment system.

Shellfish and seaweed production can generate measurable public environmental benefits through the removal of nitrogen and phosphorus from coastal waters, and potentially carbon through seaweed cultivation and other blue-carbon pathways. However, these ecosystem services are generally not recognised or rewarded by existing seafood markets.

On why this is not widely adopted in Tasmania specifically (Question 3), there are several practical barriers beyond simple economics.
Food-safety classification. Shellfish harvest areas are managed under the Tasmanian Shellfish Quality Assurance Program. Locating shellfish adjacent to a source of organic and faecal loading may create challenges for classification and market access, potentially limiting access to premium food markets.
Biosecurity. Tasmania’s oyster industry has experienced significant impacts from Pacific Oyster Mortality Syndrome (POMS). Co-locating oysters near another high-value aquaculture sector introduces additional disease-management and biosecurity considerations that would need careful assessment.
Site hydrodynamics. Many newer salmon farming areas are located in higher-energy, more offshore environments where waste plumes disperse rapidly. While this can reduce localised impacts, it may also reduce the efficiency with which shellfish can capture farm-derived nutrients.
Planning and leasing. Existing marine farming zones, lease arrangements, and approval processes were not designed around IMTA systems, creating regulatory and administrative hurdles to implementation.
Macquarie Harbour and social licence. Macquarie Harbour’s principal environmental challenge is dissolved oxygen depletion associated with organic loading. In a low-flushing and highly stratified system, the net effect of shellfish IMTA on oxygen dynamics would require careful assessment, as increased biodeposition could offset some of the potential water-column benefits. This illustrates the broader point that IMTA performance is highly site-specific and should not be assumed to deliver environmental benefits in all locations.

This raises an important policy question: if society benefits from nutrient removal and improved water quality, should mechanisms exist to recognise and reward those services? Internationally, nutrient trading schemes and ecosystem-service markets have emerged in some jurisdictions to incentivise nutrient reduction activities. While Australia does not currently have an established nutrient-removal market, there may be an opportunity to develop a credible framework capable of recognising and rewarding verified nutrient removal delivered through restorative aquaculture and integrated multi-trophic aquaculture systems.

Such an approach would require robust measurement, verification, governance, and regulatory oversight, but it could potentially improve the economics of shellfish and seaweed production while supporting broader environmental outcomes.