In Tasmania, the majority of rocky reef environments are dominated by canopy-forming algae (e.g. kelp) which create a complex habitat for a wide variety of marine life (e.g. fish, octopus, crayfish, abalone etc.). While many of the basic functional components of temperate rocky reefs are quite similar around the coast of Tasmania, the ways in which we interact with and utilise these reefs differ; some reefs are highly important fishing grounds, others are significant for their value to the tourism and recreation sectors and many are just fringing reefs that bound our coastline. Some reefs are more prone to human interactions than others and all of these factors will influence their potential vulnerability and resilience.
It is well known that marine salmon farming practices result in the release of uneaten salmon feed, salmon faeces and other nutrient-rich organic matter (e.g. biofouling debris) into the water column. Some of this organic matter is dissolved in the water column while some of it forms particles which float in the water column and then sink to the seafloor at varying rates depending on their size and water current speeds. The release of these materials may interact with rocky reef environments in a number of ways:
- Nutrient enrichment
- Sedimentation
- Non-indigenous and problematic species
However, the magnitude of interactions depends on the location of the farm in relation to rocky reefs, the intensity of farming, the local hydrodynamics and the sensitivity of the rocky reef ecosystem. It is possible to manage these interactions and ensure the sustainability of both farming and the environment, but this requires an understanding of both the farming practices and the prevailing environmental conditions.Â
How can extra nutrient input interact with rocky reefs?
- In most instances, nutrients will be diluted and dissipated quite quickly.
- However, even when salmon farms are not located directly above or beside rocky reef habitats, it is possible for some of the dissolved nutrients to be transported to them by water currents. The effects of extra dissolved nutrients on rocky reef ecosystems depends on the amounts involved and the sensitivity of the rocky reef species present. Studies have shown that initial increases in nutrients can result in increased growth of opportunistic species (e.g. sea lettuce) and epiphytic species (species which grow on the surface of other marine plants). In the first instance, these species are the environment’s mechanism to adjust to an increase in nutrients and can be good early warning indicators of change, but if their growth is prolonged or sustained, they can change the dynamics of rocky reef ecosystems.
- On reefs where nutrient levels are naturally low, nutrient enrichment can enhance productivity. However in some cases, excess nutrient inputs may result in a reduction in reef productivity, and in extreme cases this may contribute to functional changes in the rocky reef environment. To date, such changes have not been observed in Tasmania as a result of salmon farming.
How can sedimentation interact with rocky reefs?
- Similar to nutrients, particulate waste from salmon farms can be transported to rocky reef habitats by water currents even where rocky reefs are not located directly below active salmon farms. The distances which particulates are likely to travel are however shorter than for dissolved nutrients. The effects of increased sedimentation (e.g. from discharged salmon farm waste) on rocky reef ecosystems depends on the amounts involved and the sensitivity of the rocky reef species present.
- Research into the effects of sedimentation on rocky reefs have shown that some species can be negatively impacted by increases in sedimentation while other species gain a competitive advantage. This has the potential to change the community structure of algae on rocky reefs, with a tendency to move away from canopy-forming species (e.g. kelp) toward opportunistic species (e.g. filamentous and turfing algae).
- Our research suggests that sedimentation rates decline rapidly further from salmon cages and there is no evidence of significant fish feed or faeces accumulation directly on reefs in the areas around fish farms.
For more detailed information on the interactions between salmon farming and rocky reef habitats and our research on this topic click here and visit our publications page to browse our research outputs.
