DESIGN, CONSTRUCTION AND OPERATION OF AN ADVANCED PHYTOPLANKTON CULTIVATION METHOD FOR HATCHERY FEED WITH SPECIAL EMPHASIS ON ROTIFER PRODUCTION

Microalgae are utilized in aquaculture as live feeds for the production of zooplankton and all growth stages of commercially important mollusks, larval stages of crustaceans and fishes in aquaculture. The objective of the study is to describe the design, construction, and operation of an advanced phytoplankton cultivation method for the mass production of rotifer. The present design (Raceway reactor system) is one of the economical and high yield production systems for open cultivation of phytoplankton. The raceway reactor system of 5 m length, 2 m breadth, 0.3 m depth and one-tonne capacity equipped with online DO and pH sensor, automated dosing pump, mass flow controller and a paddle wheel aerator for the daily production of 100-200 gram dry weight phytoplankton biomass was fabricated. A standard operating procedure is set up to achieve the efficacy with quality output while to reduce miscommunication with the workers. The work focussed on the mass production of Isochrysis galbana using the reactor system to demonstrate the quality of the algae for the mass production of Rotifer, Brachionus plicatilis. Inoculated microalgal culture (20% inoculum) has reached a concentration of about 3.2 million cells/ml (dry weight around 0.28g/l) within 48-72 hours in the raceway tank by consuming 24.9 g of NO3 and 7.1 g of PO4 under 224 W/m2. Paddlewheel agitation was maintained for 14 hours per day; it is a semi-continuous culture where 20% was harvested on a regular basis on the earlier days and later 50% were harvested and topped up with the same quantity of treated seawater to maintain the water volume. 200 L of the culture harvested daily comprised 0.229g/l dry weight (46 g for 200L per day). The culture was continued for 24 days and harvested live culture was used as a feed to rotifers. Water temperature ranged from 27.6-32.40C, salinity ranged from 34 to 37 ppt, pH 8.2 to 9.5 and DO fluctuate from 7 ppm to 12 ppm. The algae thus developed from the raceway system were used for testing the quality by feeding it to B.plicatilis and compared it with the outdoor cultured algae. The rotifers had grown well in the raceway culture showing an optimized number of 108 rotifers per ml, which is more or equal to the growth from other culture. The technology available in India for mass culture of phytoplankton requires large outdoor culture facilities which are labour-intensive, time-consuming, unpredictable and uneconomical. The installation of the raceway reactor in the hatchery helps to achieve high production of algae on a continuous basis from small culture volume, saving space, labour and construction cost. Based on the requirement, mixed consortium feed for nursery phase can also be developed from the reactor, which ensures higher feed quality.