Stable isotope and fatty acid evidence for uptake of organic waste by green-lipped mussels Perna viridis in a polyculture fish farm system

To evaluate the feasibility and capability of using filter-feeding bivalves as biofilters for organic waste derived from fish faeces and feed wastage in marine fish culture activities, a polyculture system comprising fish and green-lipped mussels Perna viridis was developed by transplantation of mussels into fish cages. As a control, mussels from the same population were simultaneously transplanted to a distant reference site free of effects from fish farming activities. After 3 mo acclimation, samples of mussel tissue, particulate organic matter (POM), fish feed and fish faeces were collected for measurements of carbon and nitrogen isotopic ratios and fatty acid profiles. Enrichment of ¹³C and ¹⁵N in mussel tissue collected inside the fish cages as compared to those at the reference site indicated the uptake and assimilation of isotopically heavier fish feed and fish faeces. Compared with mussels from the reference site, the pattern of fatty acid profiles and single fatty acids of mussels in fish cages also tended to be closer to fatty acid profiles of fish feed from fish farms. Based on the concentration-weighted isotope mixing model, the proportions of mussel biomass assimilated from POM, fish feed and fish faeces to mussel dietary consumption were 68.3, 27.5 and 4.2%, respectively. The direct uptake of organic waste from fish farms by filter-feeding mussels is different to their consumption of phytoplanktonic biomass, because the nutrient flux is shifted between these 2 distinct pathways. © Inter-Research 2006.