Culture of fish food organisms

Unit 4 - Copepods

4.5.Culture techniques
A continuous production system for the calanoid copepod Acartia tonsa consists of three culture units: basis tanks, growth tanks and harvest tanks. The Acartia tonsa are isolated from natural plankton samples or reared from resting eggs onwards. The basis tanks (200 l grey PVC tanks: 1500 x 50cm) are run continuously and the eggs produced are used to adjust population stocks. These tanks are very well controlled and kept under optimal hygienic conditions: using filtered (1 μm) seawater (salinity 35 g/l) and fed with Rhodomonas algae. Temperatures are kept at 16-18°C and a gentle aeration from the bottom is provided. Adult concentrations with a ratio of 1:1 males to females are maintained at less than 100 per l by adjusting once a week with stage IV-V copepodites. Approximately 10 l of the culture water is siphoned daily from the bottom of the tanks (containing the eggs), and replaced by new, clean seawater. Eggs are collected from the effluent waters by the use of a 40 μm sieve; production averaging 95,000 eggs/day, and corresponding to a fecundity rate of 25 eggs/ female per day. The basis cultures are emptied and cleaned two to three times per year, by collecting the adults on a 180 μm sieve and transferring them to cleaned and disinfected tanks. Collected eggs are transferred to the growth tanks where maximal densities reach 6000/l The nauplii start to hatch after 24 h with hatching percentages averaging 50% after 48 h incubation. Initially Isochrysis is given at a concentration of 1000 cells.ml/1 and after 10 days a mixture of Isochrysis and Rhodomonas administered at a concentration of 570 and 900 cells.ml/1, respectively. The generation time (period needed to reach 50% fertilized females) is about 20 days with a constant mortality rate of about 5%/day. After 21 days, the adults are collected using a 180 μm sieve and added either to the basis or harvest tanks. Harvesting tanks are only in use once the fish hatchery starts to operate. Cultures are maintained in 450 l black tanks under the same conditions as described above. Each tank receives a daily amount of 16X108 Rhodomonas cells, harvested from bloom cultures. These tanks are emptied and cleaned more regulary than stock tanks. To facilitate the harvesting of solely nauplii or copepodites of a specific stage (depending on the requirements), eggs are harvested daily and transferred to the hatching tanks; the aeration levels within these tanks being increased to maintain 80% oxygen saturation. Nauplii of appropriate size (and fed on Isochrysis) are harvested on a 45 μm screen and by so doing cannibalism by the copepod adults is also minimized.