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4. Daily management and harvest of rotifers
After hatching of the resting eggs, the algae water should be pumped into the rotifer ponds. Fresh sea water or brackish water and fermented bean cake should be added to the algae ponds to promote growth of the algae population. After 10 days later in early April, the rotifers are ready for harvest at a density of about 100,000 ind./l at 15 C.
The production yields are about 10 billion rotifers per day per pond. By pumping continuously algae water into the rotifer ponds and changing part of the water in these ponds, the production period can last for a long time ( a month, or longer ). The harvested rotifers can be transferred into other algae ponds to start another rotifer production cycle. Introduction of boiled soya-bean milk as a food source for the rotifers can enable a reduction of the culture ponds required for green algae.
Snell et al ( 1987 ) described 2 important indicators for assessing the physiological condition of rotifer mass cultures. The first is swimming activity which is measured by observing rotifer swimming over a grid with 1 mm square. Lowered swimming activity indicates the physiological stress in the rotifer mass culture.
In this case, the water should be partly changed. Unionized ammonia has been recognized as an important factor restricting rotifer reproduction and causing mass culture instability ( Yu et al.,1986 ), but the rotifer B. plicatilis can tolerate higher ammonia concentration than copepods. In practical production of rotifers in case of appearance of copepods in the rotifer ponds, we can effectively kill the copepods by adding 500-1,000 grams of the commonly used fertilizer urea.
The second indicator for assessing the status of rotifer mass cultures is egg ratio, which is the number of eggs carried by females divided by the number of females. Egg ratios are most useful for predicating future reproductive output of rotifer populations. Egg ratios typical of exponentially growing B. plicatilis populations at 25 C ranged from 0.5-1.2 ( Snell et al., 1987 ). Rotifer populations reproducing at a replacement level ( stationary phase ) had egg ratios between 0.13-0.5. Once egg ratio fell below 0.13, populations declined.
We find that appearance of male rotifers is the 3rd indicator for assessing the status of rotifer mass cultures. The monogonont rotifer B. plicatilis reproduce both asexually and sexually. Under optimal culture conditions, the rotifer reproduces by parthenogenesis.
Rotifer bisexual reproduction is affected by both internal and external factors ( Hagiwara et al., 1991 ). The ability of rotifers to undergo mictic multiplication and to generate the resting eggs is thought to develop in an evolutionary process to cope with severe environmental changes ( Fukusho, 1989 ). Under severe conditions, the rotifers undergo bisexual reproduction. So the appearance of male rotifers indicate the culture medium need to change.
Application of rotifers in aquacultureThe harvested rotifers are rinsed with fresh sea water three times before use. Only live rotifers are feeded to larval fish and crustacean. In north China along Bohai Bay, hatcheries produce larvae of red sea bream, black sea bream, Chinese white shrimp and fresh water crab.
Daily rotifer consumption by red sea bream and black sea bream have been extensively studied ( Fukusho, 1989 ). The relationship between the total length of larva ( L mm ) and the daily rotifer consumption ( F ) can be expressed by the following equation: 3.934 F = 0.303 L , where 3.92 mm < L < 10.05 mm ( Kitajima et al., 1976).
The density of rotifers in the culture water will be very important when larvae and juveniles are reared. 3-10 Rotifers per ml is necessary for red sea bream, while 1-3 rotifers per ml is sufficient for black sea bream ( Fushimi, 1983 ). Estimates suggest that one red sea bream larva requires 12,000 to 15,000 rotifers over 25 days untill it reaches 10 mm in length ( Kafuku et al., 1983).
Practical application of rotifers in larviculture of Chinese white shrimp P. chinensis have demonstrated that rotifers have more advantages than Artemia nauplii. Higher survival rate of shrimp larvae can be obtained by using rotifers as main food from Z2 to M2 stages of shrimp larvae.
In recent years, larviculture of Chinese fresh water crab E. sinensis have developed rapidly. Successful mass production of the rotifer B. plicatilis has inhanced development in larviculture of the crab.
Production of rotifer cystsThe need for continuous maintenance of live stock cultures of Brachionus either for laboratory investigations or aquacultural purposes requires considerable routine effort and involves the risk of bacteriae and ciliates contamination.
Commercial availability of rotifer eggs could be the solution by eliminating the need to maintain stock cultures and reduce the chances for contamination with ciliates and pathogenic bacteria. Furthermore, the rotifer cysts could also be disinfected prior to hatching out in a new culture inoculum.
Under optimal culture conditions, B. plicatilis reproduce by parthenogenesis, each female produce several eggs at a time which upon hatching reach the reproductive stage in a few days only. But in undesired conditions, they reproduce by bisexual and produce resting eggs ( cysts ) which deposit on the bottom of ponds. Rotifer cysts remain dormant and hatch after stimulation by specific external conditions.
The extent of resting egg production is determined by both internal and external factors. The most important internal factors are the age of the parental female and her genotype. The external factors include temperature, photoperiod, population density and grouping, and both qualitative and quantitative aspects of diet ( Pourriot et al., 1983 ). In nature, the most important factor affecting cysts production of rotifers is the supply of diets. In spring, rotifer cysts hatch and rotifer populations rapidly grow. When the diets are consumed out, the rotifers begin to produce cysts.
On a hatchery we produce rotifer cysts by adjusting only the diet supply. First we culture rotifers with algae and yeast at 25 C and 20 ppt under natural sunlight. When rotifers reach a density of 10,000 ind./l, the diets supply are gradually reduced to zero. 10 Days later, we harvest the cysts by draining off the culture water and the resting eggs are collected by sieving through a net. Then the cysts are purified and processed to a dry form. 1 gram of processed rotifer cysts include 2,000,000 eggs with about 80% of hatching rate at 28 C and 20 ppt in 36 hours.
ReferencesDhert P., P. Sorgeloos, 1994. Live feeds in aquaculture. Proceedings of Aquatech'94, in press.
Fukusho, K., 1989. Biology and mass production of the rotifer, Brachionus plicatilis. Int. J. Aq. Fish. Technol., 1:232-240.
Fulks, W., K. L. Main, 1991. Rotifer and Microalgae Culture Systems. The Oceanic Inst., Honolulu, USA. 364pp.
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Kitajima, C., K. Fukusho, H. Iwamoto, H. Yamamoto, 1976. Amount of rotifers Brachionus plicatilis, consumed by red sea bream larvae, Pagrus major. Bull. Nagasaki Pref. Inst. Fish., 1:105-112.
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Pourriot, R., T. W. Snell, 1983. Resting eggs in rotifers. Hydrobiologia 104:213-224.
Snell, T. W., K. Carrillo, 1984. Body size variation among strains of the rotifer Brachionus plicatilis. Aquaculture 37:359-367.
Snell, T. W., M. J. Childress, E. M. Boyer, 1987. Assessing the status of rotifer mass culture. J. World Aquacult. Soc. 18:270-277.
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Yu, J., K. Hirayama, 1986. The effect of un-ionized ammonia on the population growth of the rotifer in mass culture. Bull. Jap. Soc. Sci. Fish. 52: 1509-151
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This article was originally published in Aquaculture Magazine 1996 22(3):16-22.
Used with permission.
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