Integrated multi-trophic aquaculture (IMTA) aims to recapture portions of nutrient waste lost from 1 species as nutritional inputs for another. This process has the potential to augment growth of cocultured species while reducing the nutrient load from an aquaculture site. In several jurisdictions, open-water aquaculture is regulated through measures of benthic hydrogen sulfide concentrations, which is proportional to excess deposition of organic material such as feces and uneaten food. Interception, consumption, and digestion of organic portions from the fed trophic level (e.g., fish) by organic extractive species (e.g., shellfish, deposit feeders) results in “organic stripping,” with less organic material in resulting feces, thereby reducing the net organic load and benthic deposition potential. Shellfish are deployed beside fish cages in some openwater IMTA systems. In addition to the potential consumption of fish culture solids (i.e., feces, feed “fines”), natural particles (i.e., seston) will also be consumed. Consumption of seston portions by shellfish means that some suspended solids that would otherwise drift by fish cages now have the potential for redirection to the benthos as indigestible seston components, egested in shellfish feces. This raises the issue as to what dietary proportion of fish culture solids, consumed by extractive species, results in an increase or reduction of net organic load. As a scoping exercise, a simple mathematical model was created using known absorption efficiencies (fraction of organic material digested) of blue mussels (Mytilus edulis) on diets of seston, and Atlantic salmon (Salmo salar) culture solids, to estimate the resulting organic fecal load of blue mussels on a mixed diet. The organic load from the mussel feces was added to the unconsumed load of salmon culture organics to determine the net IMTA organic load. The dietary proportion threshold (DPT) is the percentage of fish culture solids in an extractive species diet that results in no change in net organic load at an IMTA site. The DPT of salmon culture solids that must be exceeded for mussel culture to reduce the net IMTA site organic load is 14.5% for salmon feces and high-quality seston, 19.6% for salmon feces and lowquality seston, 11.5% for salmon feed fines and high-quality seston, and 15.6% for salmon feed fines and low-quality seston. A net reduction of organic load occurs if the amount of salmon culture organics absorbed (digested) by mussels exceeds the organic fecal load produced by indigestible seston components. Changes in total particulate matter consumed by a mussel population or changes in the amount of salmon culture solids available did not change the DPT, although these factors did influence the magnitude of net organic load increases or decreases. Whether DPTs are achievable is a function of many biophysical parameters, and these avenues are not pursued in this study. However, knowledge of DPTs can be useful to frame expectations of nutrient mitigation and to assist in the interpretation of augmented growth and tracer data as a means to infer nutrient reduction potential.
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Vol. 32 • No. 2