Mass culture of the rotifer Brachionus plicatilis and its evaluation as a food for larval anchovies

Growth rates of anchovy larvae, Engraulis mordax, reared for 19 days under constant environmental conditions on a diet of laboratory-cultured organisms, exceeded the growth rates of anchovies fed on a diet of wild plankton. The rotifer Brachionus plicatilis was found to be a nutritious food source when fed to the larvae in concentrations of 10 to 20/ml and in combination with the dinoflagellate Gymnodium spendens (100/ml). Optimum conditions were determined for mass culture of the rotifer. A high food concentration was the most important parameter needed to assure a high yield of rotifers. Large volumes (464 l) of the unicellular flagellate Dunaliella sp. were cultured for feeding the rotifers. The rotifer culture technique described produces approximately 2.5 x 10 organisms/day, providing a reliable food source for rearing studies. The lengths of B. plicatilis (without eggs) ranged between 99 and 281 μ, most rotifers being larger than 164 μ and less than 231 μ. Individuals weighed 0.16 μg and contained 8 x 10cal. Introduction As part of a larger program on the biology of the northern anchovy Engraulis mordax, a study was initiated at the Fishery-Oceanography Center, La Jolla, California, USA, to rear larval anchovies using a variety of cultured food organisms. In an earlier report (LASKER et al., 1970) we showed that first-feeding anchovy larvae grew well initially on the cultured dinoflagellate Gymnodinium splendens (53 μ diameter) but soon required a larger food organism. Veligers of the bubble snail Bulla gouldiana were of the proper size (140 μ) and could be cultured in the laboratory, but growth rates of larval anchovies fed on them never reached those of larvae fed on a diet of wild plankton. Because Japanese workers have used the mixohaline rotifer Brachionus plicatilis in fish culture, particularly for feeding larvae of the yellowtail Seriola dorsalis (HARADA, 1970) we investigated the possibility of using this rotifer as food for larval anchovies. Although ITO (1960) published a study on factors affecting B. plicatilis reproduction in small cultures, he did not study reproduction in mass cultures which are needed to produce large numbers of rotifers to feed fish larvae. VASIL’EVA and OKUNEVA (1961) reared the rotifer Brachionus rubens in outdoor concrete tanks, but the environmental conditions were not controlled. In contrast to B. plicatilis, fecund female B. rubens attach to a substrate, therefore, only a portion of the latter’s population is available as a forage organism. In any rotifer culturing procedure, where the maximum number of individuals is desired, parthenogenetic (amictic) reproduction must be optimized and conditions avoided which cause sexual (mictic) reproduction, which produces smaller males and resting eggs. A number of factors are known to alter the ratio of mictic to amictic females; these are species composition and crowding (GILBERT, 1963), temperature, and the quantity and quality of algal food (EDMONDSON, 1965; KING, 1965) and salinity (ITO, 1960). Quantitative studies are not yet available on the mass culture of rotifers, nor of a rotifer’s nutritional adequacy with respect to larval fish growth, therefore, in this study we define laboratory conditions which can be used for mass culturing Brachionus plicatilis with the maximum production of amictic females. We compare reproduction of B. plicatilis on different algal diets, and assess the rotifer as a food for the larval anchovy Engraulis mordax.