Centro i-mar & CeBiB

Reducing negative environmental impacts from aquaculture activities is a key issue for ensuring long-term sustainability of the industry. This study examines the major findings and methodology aspects from 28 peer-reviewed studies on marine aquaculture systems integrating fed and extractive organisms. All studies include seaweeds as extractive organisms. The main objective was to analyse the degree of relevance these findings have for large-scale implementation of integrated mariculture practices, and to identify necessary research areas for a future research agenda.The following directions for future research were identified: (1) understand in detail the important biological/biochemical processes in closed recirculating and open seaweed culture systems; (2) conduct research into these advanced aquaculture technologies at scales relevant to commercial implementation or suitable for extrapolation; (3) broaden the focus to include factors affecting seaweed growth and uptake capacity; (4) improve experimental design for statistical calculations; (5) attain a detailed understanding of the temporal variability in seaweed-filtered mariculture systems; (6) define numerical design parameters critical for engineers in designing commercial recirculation systems with seaweed filters; (7) study the influences of location-specific parameters, such as latitude, climate and local seaweed strains/species, on seaweed filter performance; (8) include economic components, considering the added value of seaweeds, and feasibility aspects; (9) analyse the role and function of integrated aquaculture practices for improved environmental, economic, and social acceptability within the broader perspective of integrated coastal management initiatives; and (10) develop educational, training and financial incentive approaches to transfer these novel and somewhat complex technologies of integrated mariculture from the scientists to the industry.

In Chile the integration of Gracilaria chilensis with salmon culture has shown high potential. Seaweed integrated aquaculture is of great interest as it allows waste recycling within fed cage aquaculture. The development of economically feasible suspended methods of seaweed cultivation is therefore of high importance. Hence, production and performance of two suspended Gracilaria cultivation methods, spore inoculated ropes and ropes with twined field collected seaweed, were studied in open water. The production from spore-seeded ropes was comparable to that of twined ropes for the first month of culture. Thereafter, the twined ropes had a significantly higher productivity. Fish farm wastes had no significant fertilizing effect upon Gracilaria growth rate. In addition, spore-originated thalli and field collected thalli were compared under laboratory conditions and in suspended culture using the same cultivation method. Spore-originated thalli had a 50% lower growth rate than the field collected thalli under laboratory conditions; however, no significant differences were detected in the field. Also, the occurrence of spore coalescence growth enhancement was not significant on the spore-seeded ropes. It was concluded that spore-originated cultivation techniques could be of interest for an integrated open seawater aquaculture system due to the high levels of Gracilaria polymorphism. This would result in greater adaptability to environmental variations, and a continuous supply of restocking material.

Fish farms produce large amounts of wastes, including dissolved inorganic phosphorus and nitrogen. To exploit these nutrients as a resource input, and at the same time reduce the risk for eutrophication of the environment, Gracilaria chilensis on ropes was co-cultivated with a salmon cage farm in southern Chile during two summer month. Gracilaria cultivated at 10 m distance from the cages had up to 40% higher growth rate (specific growth rate; 7% day−1) compared to growth at 150 m and 1 km distance. The algae nutrient content was also higher close to the cages (1.9–2.1 mmol N g−1 dw−1 and 0.28–0.34 mmol P g−1 dw−1) compared to the other stations. Yield of agar per unit biomass varied between 17–23% of dry weight and was lowest close to the farm, but due to higher growth rate, the accumulated agar production was highest close to the fish cages. The degree of epiphytes and bryozoa coverage was overall low. An extrapolation of the results showed that a 1 ha cultivation of the algae, close to the fish cages, had the potential to remove at least 5% of dissolved inorganic nitrogen released from the fish farm and 27% of released dissolved phosphorous. Such a cultivation would give an annual harvest of 34 dry t of Gracilaria, worth 34000 US$. We conclude that both economical and environmental advantages can be achieved by integrating the cultivation of algae with fish farming in open sea systems. However, further studies focusing on full scale cultivation and functions during different seasons are needed.

Rapid scale growth of intensive mariculture systems can often lead to adverse impacts on the environment. Intensive fish and shrimp farming, being defined as throughput-based systems, have a continuous or pulse release of nutrients that adds to coastal eutrophication. As an alternative treatment solution, seaweeds can be used to clean the dissolved part of this effluent. Two examples of successfully using seaweeds as biofilters in intensive mariculture systems are discussed in this paper. The first example shows that Gracilaria co-cultivated with salmon in a tank system reached production rates as high as 48.9 kg m−2 a−1, and could remove 50% of the dissolved ammonium released by the fish in winter, increasing to 90–95% in spring. In the second example, Gracilaria cultivated on ropes near a 22-t fish cage farm, had up to 40% higher growth rate (specific growth rate of 7% d−1) compared to controls. Extrapolation of the results showed that a 1 ha Gracilaria culture gave an annual harvest of 34 t (d. wt), and assimilated 6.5% of the released dissolved nitrogen. This production and assimilation was more than twice that of a Gracilaria monoculture. By integrating seaweeds with fish farming the nutrient assimilating capacity of an area increases. With increased carrying capacity it will be possible to increase salmon cage densities before risking negative environmental effects like eutrophication and toxic algal blooms sometimes associated with the release of dissolved nutrients. The potential for using mangroves and/or seaweeds as filters for wastes from intensive shrimp pond farming is also discussed. It is concluded that such techniques, based on ecological engineering, seems promising for mitigating environmental impacts from intensive mariculture; however, continued research on this type of solution is required.

The marine aquaculture sector is growing rapidly. Offshore aquaculture installations have been drawing increasing attention from researchers, industry and policy makers as a promising opportunity for large-scale expansion of the aquaculture industry. Simultaneously, there has also been increased interest in both land-based and nearshore aquaculture systems which combine fed aquaculture species (e.g. finfish), with inorganic extractive aquaculture species (e.g. seaweeds) and organic extractive species (e.g. suspension- and deposit-feeders) cultivated in proximity. Such systems, described as integrated multi-trophic aquaculture (IMTA), should increase significantly the sustainability of aquaculture, based on a number of potential economic, societal and environmental benefits, including the recycling of waste nutrients from higher trophic-level species into production of lower trophic-level crops of commercial value. Several of the challenges facing IMTA in nearshore environments, are also relevant for offshore aquaculture; moreover, the exposed nature of the open ocean adds a number of technical and economic challenges. A variety of technologies have been developed to deal with these constraints in offshore environments, but there remains a number of challenges in designing farm sites that will allow extractive species (e.g. seaweeds and shellfish) to be integrated in fed aquaculture systems and be able to withstand the strong drag forces of open oceans. The development of offshore IMTA requires the identification of environmental and economic risks and benefits of such large-scale systems, compared with similarly-scaled monocultures of high trophic-level finfish in offshore systems. The internalizing of economic, societal and environmental costs of finfish monoculture production by the bioremediative services of extractive species in IMTA offshore systems should also be examined and analyzed. The results of such investigations will help determine the practical value of adopting the IMTA approach as a strategy for the development of offshore aquaculture.