Most salmon farms in Tasmania are located over soft sediment ecosystems. The main ways in which salmon aquaculture interacts with these ecosystems are via:
Typically, these interactions are greatest immediately below actively farmed salmon cages, decreasing with distance from the farm. The magnitude of interactions depends on the intensity and management practices of farming, environmental conditions at the farm site (e.g. depth of water below cages, water movement in the area) and the sensitivity of the soft sediment ecosystem.
Some organic matter released by salmon farms dissolves in the water column (dissolved waste), while some is released in the form of particles (particulate waste) which float in the water column, sinking to the seafloor at varying rates depending on particle size and water current speeds.
Animals in the sediments break down this excess organic matter and consume oxygen in the process. Organic enrichment can cause community changes in the fauna, with small amounts of enrichment stimulating biodiversity and abundance, but sustained organic inputs will favour opportunistic species that can rapidly breakdown organic material. If the ability of these animals to break down this matter is exceeded, oxygen levels may decline, there will be a loss of infauna from the sediments, and hydrogen sulphide gas may be released as organic matter break-down is left to microbes. Signs of an enriched soft sediment ecosystem can include:
Metals (e.g. copper and zinc) from farm equipment and additives in fish feed can accumulate in sediments. If these metals reach elevated levels, they can become toxic to marine life.
Antibiotics used to treat fish can also end up in sediments, leading to concerns around long-term environmental persistence and antibiotic resistance.
Our research points to several key strategies for managing aquaculture interactions with soft sediment environments, including:
Soft sediment ecosystems impacted by aquaculture interactions can recover, however rates of recovery depend on the severity of the impact and local environmental conditions.
Research and ongoing monitoring are vital for understanding and managing the interactions of aquaculture with soft sediment ecosystems. It is important to understand how the responses of soft sediment ecosystems to aquaculture vary depending on farm location, so that regulators can put in place the right monitoring and management practices for each aquaculture farming region. This can ensure that soft sediment ecosystems are able to effectively assimilate and process aquaculture waste without damaging their functioning.
