The Effects of Different Water Quality Parameters on Prawn (Macrobrachium Rosenbergii) Yield, Phytoplankton Abundance and Phytoplankton Diversity at New Zealand Prawns Limited, Wairakei, New Zealand

Aquaculture is the fastest growing industry in the food sector and demand for aquaculture products is continuing to grow as many wild stocks from capture fisheries continue to decline. It is imperative that water quality in an aquaculture system is closely controlled in order to maintain the health of the species under culture and maximize production. New Zealand Prawns Limited (NZPL) is an aquaculture facility in Wairakei, New Zealand that cultures the freshwater prawn Macrobrachium rosenbergii. Dramatically reduced yields of prawns have been observed in ponds following periodic blooms of benthic algae. In this study, water quality variables were measured in grow out ponds at 9-11 day intervals. I measured temperature, phytoplankton abundance, phytoplankton diversity, turbidity, and concentrations of ammonia, nitrate, orthophosphate, dissolved oxygen and chlorophyll a. This data was combined with information on pond depth and prawn yield in order to investigate what variables influence the abundance and diversity of phytoplankton, benthic algal blooms and prawn yield. The difficulty of combining scientific endeavour with commercial enterprises resulted in only a small data set being available for analysis but it appears that benthic algal blooms at NZPL may be due to excessive light penetration to the benthos due to shallow pond depths, and reduced shading of the benthos when phytoplankton abundance is low. Low phytoplankton abundance may possibly be a result of low orthophosphate. There was insufficient data to determine what impacts, if any, the variables investigated have on prawn yield or how water quality variables change with time. Future studies and experiments are recommended in order to increase knowledge of farming M. rosenbergii; a valuable crustacean that has been shown to have a lower social and environmental impact than many other more common aquaculture species.