Seasonal changes in C, N and P budgets of green-lipped mussels Perna viridis and removal of nutrients from fish farming in Hong Kong

Fish farming activities release substantial wastes, including uneaten fish feed, faecal pellets and dissolved excretory products, resulting in nutrient pollution and subsequent deterioration of water quality in surrounding waters. Owing to the nature of their high filtration rate and high population density, filter-feeding green-lipped mussels reared in fish culture waters can take up particulate matter with considerable efficiency; hence, the farming wastes are removed. The present study evaluated the feasibility and capacity of using the green-lipped mussel Perna viridis as a biofilter to remove farming wastes from fish rafts. Spats of green-lipped mussels were transplanted to artificial reefs deployed under fish farms in Hong Kong. After a 1 yr period of growth and acclimatization, the nutrient scope for growth (SFG) of the transplanted mussels was quantified in situ through a series of bimonthly measurements of carbon, nitrogen and phosphorus changes. The metabolic acquisition and expenditure of mussels exhibited considerable temporal fluctuation during the study period, owing to changes in exogenous environmental conditions such as food availability, water temperature and dissolved oxygen levels, and autogenous physiological status such as the reproductive cycle. For a standard Perna viridis of 70 rum shell length, the average SFG of carbon, nitrogen and phosphorus was 170.9, 18.6 and 4.3 mu g h(-1), respectively. As a result, the annual nutrient assimilation capacities of a 70 nun mussel for carbon, nitrogen and phosphorus were 1476.9, 160.3 and 36.7 mg, respectively. Based on the practical culture density of mussels in southeast Asia, the assimilation capacity has the potential to remove fish farm wastes at mariculture sites.