Science Advisory Report 2013/056
Review of the Organic Extractive Component of Integrated Multi-trophic Aquaculture (IMTA) in Southwest New Brunswick with Emphasis on the Blue Mussel
- There are a number of factors that will influence the ecological interactions and effectiveness of an Integrated Multi-Trophic Aquaculture (IMTA) operation, including: the configuration and design of the IMTA site; its species composition; culture scale; the efficiency of selected organisms to capture and assimilate specific wastes from fish net-pens; diet availability; site conditions, such as physical factors (e.g. depth, currents and temperature), background nutrient concentrations and substrate characteristics; and species interactions.
- Physiological model estimates suggest that salmon culture solids (i.e. faeces and waste feed) would have to comprise at least 10-20% of mussel total diet before they would begin to contribute to a net decrease in the site-wide net organic loading from an IMTA site. Based on available data, it is considered very unlikely that IMTA, using only fine particulate filter feeders (mussels) in SWNB, has significantly reduced loading of total particulate matter to the benthos at the sites. Information to-date is insufficient to evaluate the ability of IMTA, with a full complement of species, to reduce organic benthic loading directly and subsequent impacts to the benthos under the site as research is still currently underway.
- At the present scale of IMTA operations, mussels can be used to remove some fine particulates generated by salmon (i.e. small faecal particulates and “fines” from pellet fish feed) and kelp can extract some fraction of soluble inorganic nutrients resulting from salmon metabolic and respiratory processes. Some data suggest that mussels grown very close to the fish farms are capable of ingesting at least 20% of their diet from fish-derived sources. However, it has not been clearly demonstrated that there is a net ecological benefit from the shellfish component of IMTA for reducing benthic organic enrichment to the benthos under the site, as the bulk of the organic load is believed to be comprised of larger particles outside the selection range of the filter feeders. Present site configurations, combined with the lack of a benthic component of IMTA species, are not optimal for achieving such ecological benefits.
- The use of extractive deposit feeders, such as sea urchins and sea cucumbers, has the potential to be more effective at reducing benthic organic deposition beneath salmon aquaculture sites than mussels because they are capable of consuming larger particulates that will typically settle out close to a salmon farm. The design of any future IMTA aquaculture sites will have to evolve both structurally and in terms of complexity in order to accommodate additional species, as well as to ensure that the flow of water is sufficient for salmon respiration and to efficiently connect the various trophic levels.
- IMTA can both add and remove inorganic nutrients at local scales since animal IMTA species produce nitrogen while seaweeds absorb it. No data were presented on the effectiveness of seaweed to absorb inorganic nutrients, but there is no indication in SWNB that inorganic nutrient limitation or eutrophication occurs except at transitory local scales. No data were available to evaluate the potential near or far-field effects of inorganic nutrients from IMTA. The present scale of IMTA is likely to have a minimal effect on the overall ecosystem at the bay level.
- It is not expected that there will be significant changes to the phytoplankton populations on a broad scale due to IMTA, although there may be some local depletion (reduction) around the IMTA sites due to the activities of filter feeders. The increase in nitrogen levels through respiration of the IMTA species is not anticipated to cause any issues since primary production in the Bay of Fundy is thought to be light rather than nutrient limited.
- The effects on secondary productivity through interactions with intertidal species are less certain. Depending on the hydrographic conditions of the intertidal zone, it is possible there could be interactions with the increased nutrients and epiphytic algae that, under very specific conditions (e.g. an enclosed cove with limited flushing), could create negative consequences for intertidal organisms. This interaction is not limited to IMTA, but could occur with any form of aquaculture or other anthropogenic activity.
- It is possible that a significant proportion of the total mussel larval supply in the Passamaquoddy/Fundy Iles area could be generated from IMTA sources at projected maximum production. This may have impacts on fisheries activities at a small scale, but is not expected to be sufficient to overwhelm wild populations or change settlement patterns on beaches at a large scale. Information is insufficient to determine the actual likelihood of this occurring.
- More work on larger scale ecosystem effects is recommended.
This Science Advisory Report is from the October 3-5, 2012, Review of Integrated Multi-Trophic Aquaculture (IMTA) in Southwest New Brunswick. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.
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