Introducing Regional IMTA (RIMTA)
An ecosystem-based approach to IMTA
Integrated Multitrophic Aquaculture (IMTA) has emerged as an attractive solution to mitigate the negative impact of additional inputs of nutrients and organic waste from fed aquaculture. More recently, the co-culture of seaweeds and bivalve mollusks alongside the production of higher trophic finfish species has received a lot of attention and publicity; with the concept being touted as a win-win solution. But there is a significant mismatch between the results of laboratory studies and in situ experiments, concluding that seaweed and bivalve co-culture IMTA does not work as expected in open coastal systems (Sanz-Lazaro & Sanchez-Jerez, 2020).
The conventional approach to IMTA constitutes the co-culture of lower trophic level species in close proximity to higher trophic level species and fails to account for this mismatch. Reasons for the ineffectiveness of seaweed and bivalve IMTA in open coastal systems include (Sanz-Lazaro & Sanchez-Jerez, 2020):
- Dissolved nutrient dispersion in open systems is rapid due to farms being commonly located in areas with high ocean hydrodynamism and water renewal (a lot of ocean currents and water movement). The concentration of dissolved nutrient and organic waste in such areas is low.
- Bivalve molluscs usually have a diet preference, preferring phytoplankton (“primary production”) over dead organic matter. Bivalves are also unable to target the waste (and associated primary production) from aquaculture farms specifically, but consume what is available in the water column.
- Waste from aquaculture farms constitute a minimal source of direct nutrition for bivalves and seaweeds.
The apparent limited viability of IMTA using bivalves and seaweed is constrained by its implementation at the farm-level. The persistence of dissolved waste in open systems is so limited, that bivalves and seaweeds can not assimilate the nutrient outputs of the aquaculture farm sufficiently to constitute meaningful impact (Sanz-Lazaro & Sanchez-Jerez, 2020).
“…. In the case of dissolved waste, cultured macroalgae and bivalve mollusks can do their job even when these species are not in the close vicinity of the facility, as long as they are located within their area of dispersion…” - (Sanz-Lazaro & Sanchez-Jerez, 2020)
Solutions are contextual, and the conventional IMTA approach is valid when considering systems with low hydrodynamisim and high sedimentation, where deposit feeders (such as crabs and sea cucumbers) can be effective when placed in close proximity to higher-trophic level species, in mitigating the total organic waste input to the environment.
Enter Regional IMTA…
“…locating low trophic level cultures in close vicinity to high trophic level cultures, aiming to sequester dissolved waste in an open system, is comparable to planting trees close to factories that emit CO2, aiming to reduce their carbon footprint.” - (Sanz-Lazaro & Sanchez-Jerez, 2020)
The localised model of IMTA for dissolved waste has not been revised, despite the evidence to suggest it being necessary. IMTA is regional and does not have to be confined to the close vicinity of the high-trophic level species, as the dissolved nutrients and organic waste is not confined to the immediate location (low retention time due to ocean and current dynamics). Rather, the dissolved waste generated by the high-trophic species can be mitigated or bioremediated when low-trophic species are set up in the region of nutrient dispersion.
“…IMTA should focus on the scale at which low trophic level species are able to sequester the waste generated by fish farming, which in the case of dissolved waste corresponds to the water body where it is located.” - (Sanz-Lazaro & Sanchez-Jerez, 2020)
IMTA in open systems should be managed in terms of different ecosystem services and functionalities rather than absolute distances considering the scale and reach of aquaculture waste products (Sanz-Lazaro & Sanchez-Jerez, 2020).
For the purposes of Marine Spatial Planning (MSP), IMTA should assume a regional approach such that low trophic and high trophic species are cultured in the same regional environment (such as in the same bay or geographical location). This is also an excellent concept/model for the implementation of regenerative aquaculture on a larger scale, as the appropriate management of “farmed ecosystem services” increases the sustainability of the region/area, while producing food and increasing biodiversity.
From Sanz-Lazaro & Sanchez-Jerez, 2020.
From Sanz-Lazaro & Sanchez-Jerez, 2020.
It is imperative to define the boundaries of a water body or region, such that aquaculture leases sites are connected (such as through ocean currents and tides) despite being spatially segregated. Tracer studies or Langrangian ocean analysis are recommended for the site selection of lower trophic level aquaculture (such as seaweeds and bivalves), but arguably can only be set up once higher trophic-level aquaculture has been established in an area and its waste flows can subsequently be defined.
With this RIMTA model, specific types of aquaculture can be incentivised as needed (through financing or tax incentives), and their subsequent establishment can thus enhance the sustainability of the entire “region” by reducing the environmental impact of added nutrients and increasing primary productivity (as opposed to smaller areas where IMTA is confined to a single farming location within that region).
However, caution must be taken when areas where low-trophic species occur naturally, such as oyster or mussel beds, when they could be leveraged to incentivise higher trophic level aquaculture. While these environments (whether managed or left to themselves) can mitigate and reduce some of the impact of newly established high-trophic level aquaculture in a region, environmental accounting may need to be used to establish whether the additional nutrient load can be remediated as well as the “naturally occurring” nutrient load (i.e., not from aquaculture).
Maintaining the balance between these systems such that the nutrient fluxes are larger than the capacity of the bioremediation or nutrient uptake, will be key to reinforce the sustainability of the region. Nutrient input in coastal environments is not isolated to only aquaculture, and can come from other sources (anthropogenic or otherwise).
“Irrespective of the origin of the nutrients (from high trophic level aquaculture or from another anthropic source) that the low trophic cultures sequester, the low trophic culture will reduce the regional nutrient input or the derived primary production to which high trophic level aquaculture is contributing.” - (Sanz-Lazaro & Sanchez-Jerez, 2020)
The integration of rigorous environmental accounting for regional nutrient loads will be the key to facilitating sustainability, and the implementation of such a regional approach to IMTA.
However (as we shall see in next week’s newsletter), the balance of trade-offs may not be enough to incentivise the widespread implementation of IMTA(…yet).
Caio for now.
The Cultured Carp
References:
Sanz-Lazaro, C., & Sanchez-Jerez, P. (2020). Regional Integrated Multi-Trophic Aquaculture (RIMTA): Spatially separated, ecologically linked. Journal of Environmental Management, 271, 110921. https://doi.org/10.1016/j.jenvman.2020.110921