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Research Document - 2014/026

Review of the Potential Near-and Far-Field Effects of the Organic Extractive Component of Integrated Multi-Trophic Aquaculture (IMTA) in Southwest New Brunswick with Emphasis on the Blue Mussel (Mytilus edulis)

By Shawn M.C. Robinson and Gregor K. Reid


Research on and development of Integrated Multi-Trophic Aquaculture (IMTA) has been ongoing in the Bay of Fundy over the last decade. The industrial development of this concept has resulted in some questions related to the impacts and benefits of IMTA, such as:

  1. What are the factors influencing the effects of IMTA
  2. Does IMTA help reduce the benthic organic loading at an aquaculture site?;
  3. Does the current scale of IMTA in Southwest New Brunswick reap any benefits? If not, at what operational scale (i.e. spatial scale), species mix, and biophysical attributes could result in a net environmental benefit? (e.g. a reduction in the rate of carbon deposition); and
  4. At what scale would IMTA have measurable impacts on other aspects of the ecosystem, such as on phytoplankton and on existing clam beaches?
There are several factors that determine the efficiency of open-water IMTA, including biological factors, such as species selection, the potential efficiency of the organism, species physiology, diet availability, and how efficiently the trophic linkages are established. The efficiency of those linkages strongly depends on the physical factors at the culture site, such as water depth, currents, and temperature. In addition, there will be a number of intentional and unintentional interactions within the ecosystem. These interactions can take the form of disease and parasitism at the small scale to predation events at the large scale. Understanding the scale and mechanisms of these ecological interactions will help to optimize outcomes and assist in the development of new management policies for IMTA. The more efficient the IMTA system, the less nutrients (energy) will be available for transfer to the outside environment.

Estimates from this study indicate that diets of extractive trophic level (ETL - blue mussels in this case) must consume 10 to 20% food from the fed trophic level (i.e. salmon culture solids) before there is a decrease in the net organic loading at an IMTA site. Whether an ETL can accommodate this amount of diet depends on the size classes of diet available and the ability of the ETL to exploit it. Several studies have shown that some open-water extractive species can utilize fish farm waste nutrients at a level of approximately 30% of their diet. To date, empirical field data on the blue mussel suggests these animals are capable of assimilating similar amounts of fish-derived nutrients in their diets as long as they are in contact with the farm derived food. The organic content of the waste from the ETL will be significantly lower compared to the original source, which makes a large difference to the loading of organic carbon to the bottom. Mussel faeces settle at a much slower rate and contain much less carbon than salmon faeces, depositing less carbon per unit area, and consequently resulting in a lower relative benthic impact. However, it is considered very unlikely that IMTA, using only fine particulate feeders (mussels) in Southwest New Brunswick, has significantly reduced loading of total particulate matter to the benthos at the sites.

Adopting IMTA can result in a number of costs and benefits. The present IMTA configurations are still in development, and, while their current configurations have likely not significantly lowered benthic loading at salmon farms, mussels have been used to remove fine particulates generated by salmon and kelp have extracted some fraction of soluble inorganic nutrients resulting from salmon metabolic and respiratory processes. Sea urchins and other species have the potential to be more effective at reducing benthic organic deposition beneath salmon farms. In the future, the design of the IMTA aquaculture sites will have to evolve both structurally and in complexity as the new sites will have to accommodate additional species and ensure that the flow of water is sufficient for the salmon, as well as efficiently connecting the various trophic levels. Calculations of the area available on existing aquaculture sites suggest that sufficient space is available on most existing leases to accommodate new IMTA modules. Projections on potential production suggest that significant quantities of organisms could be grown and are theoretically capable of assimilating much of the nutrient fish waste (assuming appropriate contact rates). This level of production will require hatchery production of juveniles due to the volumes required.

There is the potential for IMTA operations to contribute to ecosystem stressors via nutrient waste addition and, in some scenarios, localized species addition. However, detecting small changes due to the IMTA beyond that of salmon aquaculture will be difficult. Some IMTA trophic levels, such as shellfish, that consume natural particulates in addition to fish farm wastes, may add increased nutrients under some circumstances; however, there is no evidence in Southwest New Brunswick that there is any nutrient limitation or eutrophication due to dissolved nutrients occurring except at very local transitory levels due to the high level of tidal flushing. It is not expected that there will be many changes to the phytoplankton on a broad scale, although there may be some local depletion levels around the farms due to filter feeders. The effects on secondary productivity through interactions with intertidal species are less certain. Depending on the hydrographic conditions of the intertidal zone, there could be interactions with the increased nutrients and epiphytic algae that create negative consequences for intertidal organisms. More research on larger ecosystem-scale effects is warranted.

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