Culture of fish food organisms (1+1)

Unit 2 - Rotifers

2.3.3.1.Stock culture of rotifers
Culturing large volumes of rotifers on algae, baker’s yeast or artificial diets always involves some risks for sudden mortality of the population. Technical or human failures but also contamination with pathogens or competitive filter feeders is the main cause for lower reproduction which can eventually result in a complete crash of the population. Relying only on mass cultures of rotifers for re inoculating new tanks is too risky an approach. In order to minimize this risk, small stock cultures are generally kept in closed vials in an isolated room to prevent contamination with bacteria and/or ciliates. These stock cultures which need to generate large populations of rotifers as fast as possible are generally maintained on algae.

The rotifers for stock cultures can be obtained from the wild, or from research institutes or commercial hatcheries. However, before being used in the production cycle the inoculum should first be disinfected. The most drastic disinfection consists of killing the free-swimming rotifers but not the eggs with a cocktail of antibiotics (e.g. erythromycin 10 mg.l-1, chloramphenicol 10 mg.l-1, sodium oxolinate 10 mg.l-1, penicillin 100 mg.l-1, streptomycin 20 mg.l-1) or a disinfectant. The eggs are then separated from the dead bodies on a 50 µm sieve and incubated for hatching and the offspring used for starting the stock cultures. However, if the rotifers do not contain many eggs (as can be the case after a long shipment) the risk of loosing the complete initial stock is too big and in these instances the rotifer should be disinfected at sublethal doses; the water of the rotifers being completely renewed and the rotifers treated with either antibiotics or disinfectants. The treatment is repeated after 24 h in order to be sure that any pathogens which might have survived the passage of the intestinal tract of the rotifers are killed as well. The concentration of the disinfection products differs according to their toxicity and the initial condition of the rotifers. Orientating concentrations for this type of disinfection are 7.5 mg.l-1 furazolidone, 10 mg.l-1 oxytetracycline, 30 mg.l-1 sarafloxacin, or 30 mg.l-1 linco-spectin.