Open water integrated multi-trophic aquaculture: constraints on the effectiveness of mussels as an organic extractive component

Mussels are currently the primary species employed in open water integrated multitrophic aquaculture (IMTA) systems to extract particulate organic fish waste (OFW) exiting fish net-pens. Information on mussel feeding and digestion physiology was used to identify constraints on the capacity of mussels to capture and absorb OFW under various open water IMTA scenarios. OFW capture by mussels is severely limited by the time available to intercept solid wastes contained in the horizontal particle flux. Intensive and spatially extensive mussel culture within IMTA systems could improve waste extraction, but maximum efficiency will ultimately be constrained by current velocity, available IMTA farm space, and any negative feedback effects on fish culture from flow reduction caused by mussel culture. The ability of mussels to absorb more of the ingested organic fish feces (OFF) than they egest as mussel feces (a waste biomitigation requirement) depends on the concentration of OFF available to mussels, relative to the ambient seston concentration and seston organic content. The biomitigation potential of mussels will be greatest where seston abundance is low and the organic content is high. Achieving maximum waste extraction by mussel co-culture entails depleting their particulate food supply to a level that may limit mussel production. Consequently, a constraint on waste extraction may be the desire to maximize profits from the extractive species. This study identified important constraints on the capacity of mussels to perform their intended role in IMTA systems that can only be partially addressed by optimizing system design.