Applying IMTA to an Irish monoculture site for Salmon production

Integrated Multi Trophic Aquaculture (IMTA) is acknowledged as a promising solution for sustainable development of aquaculture. By farming multiple lower trophic species adjacent to traditional aquaculture enterprises, such as salmon monoculture, the wastes and by-products of one species become the feed, fertiliser and energy source for the others. This practice is shown to reduce waste materials from fed species and lower the nutrient load in the water (FAO,2018).
This paper aims to give an overview of applying IMTA to an Irish monoculture site for Salmon production with the introduction of the lower trophic species Alaria esculenta, Pecten maximus and Homarus gammarus. Utilising low trophic products such as invertebrates and seaweeds (Barbier, 2019) would maximise the use of licensed aquaculture areas and reduce the environmental impact on the monoculture activity. If successful, this method could be adapted by other monoculture sites to provide more eco-efficient practices, creating more goods and services while using fewer resources and generating less waste. This paper can help shift the current application of IMTA to a more commercial level.

In this trial the decision was made to cultivate winged kelp, Alaria esculenta, Scallop, Pecten maximus and European Lobster, Homarus gammarus within the current grid infrastructure traditionally established to hold salmon pens. The trial site is a pilot scale research site, with a moored grid infrastructure to support 6 pen structures.  Grid spacing is 50m between cushion buoys. Seaweed longlines were attached at the cushion buoys and orientated in varying directions to the prevailing current and at different levels of exposure on the site. Lobster units and scallop lanterns were suspended from the outer ring of the salmon pens on site.
Crop yield, biomass and condition were monitored throughout the growing cycle. This data was compared to abiotic data relating to current direction and to location on the site. Molecular analysis of the harvested seaweed was carried out to look for variation in composition based on location on the site. The costs to add the additional structures, and the operating procedures cost to maintain the additional species were also considered.

Utilising the existing grid structure successfully yielded a crop of seaweed. By utilising the existing grid structure, no additional moorings we required resulting in cost savings for the operator, less visual impact, and maximising use of the existing space. Waste from the salmon at the farm was remediated by the low trophic species added, through nutrient cycling in the kelp and the scallop, whilst the lobster also showed utilisation of salmon waste. 
Data on biomass accumulation and crop yields provide expected nutrient uptake rates and value of additional products to the site. Although yields were not at a commercial level, a significant tonnage was achieved, providing additional value from the added products. There were further ecosystem service benefits from co-culturing novel low trophic lobster species for potential release to the wild for restocking. This low visual impact approach and more optimal use of the limited space for aquaculture within the coastal zone proved successful.