With the intention of reducing the inputs of wastes from aquaculture to the environment, four main approaches have been developed. The first considers isolating an operation from the surrounding environment by developing enclosed systems at sea. These 'bag' techniques have been developed and tested over the recent decades, but costs and technological issues, especially in regions of strong tidal regimes, have prevented their development and adoption at commercial scale.
The second approach is to move facilities to land-based sites. This does not mean that waste issues disappear, as too often naively thought. Wastes, then channeled in pipes, still need to be treated. Pilot studies in Norway, Chile, South Africa, and Israel have demonstrated that land-based IMTA culture systems are technically feasible and in some cases economically profitable. In Israel, the integration of the cultivation of fish, seaweed, and abalone has been demonstrated, and the designs and criteria experimentally defined. The technology developed is generic and modular, adaptable to several fish/shrimp, shellfish, and seaweed culture combinations at any level of intensification. Another interesting example is taking place in South Africa. Collaboration between scientists from the University of Cape Town, Stockholm University (Sweden), and local abalone farmers is developing an IMTA system in which seaweeds are cultivated in wastewater from abalone tanks in a semiclosed recirculating land-based system. This supply of high-quality seaweeds is fed to the herbivorous abalone, lessening the harvest pressure on the ecologically important kelp forests along the South African coast. The economic benefits demonstrated by pilot-scale trials have encouraged farmers to implement the concept into commercial full-scale operations.
The third approach is the development of land-based recirculation systems using bacteria as biofilters. These closed systems have demonstrated a significant capacity for denitrification and transforming wastes. Preliminary results indicate that they can be economically profitable under certain conditions. However, they still remain a challenge at the commercial scale, one of the reasons being that transformation of wastes by bacteria does not generate other crops of commercial value like seaweeds and shellfish in IMTA systems.
As land-based systems are not yet widespread, and the cost of land in itself can be prohibitive, a fourth approach has been to increase the environmental and economic sustainability of marine open-water systems by developing IMTA at sea. This approach has been tested by incorporating cultures of extractive species, such as filter-feeders and seaweeds, to the culture of fed species. However, compared to land-based cultures, open-culture IMTA systems are more complex in achieving high efficiency. System design and methodology constraints, together with limited experience, still need to be handled. Previous trials indicate that the use of seaweeds for reduction of released dissolved fractions may be spatially limited due to light dependency. Filterfeeders, such as mussels and oysters, can capture waste particles, but within a limited-size bracket. Large particles will settle and accumulate within sediments rather fast, becoming unavailable to adjacent extractive organisms cultivated in the water column, unless other extractive organisms are introduced on the bottom. If oxygen levels do not fall below critical levels, the cultivation of scavenger organisms (e.g., crabs, lobsters, sea urchins, sea cucumbers, and polychaetes) could be an additional way to recycle wastes and remove them from the environment when these organisms are harvested. However, this possibility has not been much studied yet. With seafloor conditions under fish cages indicating low oxygen levels and high amounts of sulfides, bacterial components could also be key for further ecological engineering developments.
Establishing open IMTA systems requires a well-established knowledge of the different oceanographic features of a water body used by aquaculture operations to determine the real impact of the IMTA layout on reducing waste discharges. Water circulation modeling is, consequently, essential for the development of appropriate IMTA systems. Several suitable hydrodynamic models are available for water column and bottom physical description; however, few consider explicit links with biological components and processes that would allow modeling the services provided by IMTA. In addition, it is important to understand that no existing single model will likely serve all purposes and different complementary models will need to be coupled.
Modeling has helped the world's largest aquaculture producer, China, to develop more environmentally friendly production systems. A shellfish/seaweed IMTA system has been developed in coastal embayments by culturing the Chinese scallop Chlamys farreri, the Pacific oyster Crassostrea gigas, and the kelp Laminaria japónica. By integrating bay-scale ecological variables with individualbased modeling of scallop and oysters, it was possible to estimate the exploitation carrying capacity for scallops and oysters in the bay, the harvest potential, and the environmental consequences of different cultivation management strategies. The results demonstrate that it is possible to control phytoplankton abundance by manipulating the abundances of the extractive species. Other examples on the Zhangzidao Island in the Yellow Sea, and Zungo Bay in the Shandong Peninsula, indicate that the environmental benefits of having large-scale commercial IMTA farms are real.
The possibility of introducing IMTA approaches in open cultures in Western countries exists. Examples of such developments exist in Canada, Chile, and the USA, utilizing different technologies, developed for different environmental conditions and species. In Chile, salmon farming is the predominant aquaculture activity; however, the culturing of mussels and abalone are other economic activities growing rapidly in the same coastal areas (Figure 3). Furthermore, the cultivation of the giant kelp, Macrocystis pyrifera, for feeding abalone is also a growing activity (Figure 3). The cultivation of Macrocystis pyrifera should enhance the recycling process and provide commercial sustainability to the Chilean IMTA model. The IMTA components, knowledge, and technology are there to produce a full-scale development. However, actual environmental regulations do not allow IMTA planning for these organisms and require urgent modifications. Present regulations are often obsolete, or were not designed with IMTA in mind; they often are limitations to the introduction and implementation of ecological engineering approaches. Regulation needs to be changed through legislation and public education regarding the new aquaculture practices being put in place.
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