Jin Guan Li 13-3-603, Wangdingdi, Tianjin 300191, China

Abstract

This paper describes the ecological principles and procedures in the production of the rotifer Brachionus plicatilis in outdoor earthern ponds in north China.

In high eutrophication waters, copepods, small shrimp and fish, the predators of rotifers, can not survive longer and are ultimately eliminated from the waters. But rotifers have higher ability to tolerate eutrophication than the predators. The rotifers can survive, grow and reproduce very well in high eutrophication waters.

So we artificially create high eutrophication waters to mass produce the rotifer Brachionus plicatilis. Commercial availability of rotifer resting eggs could be the solution by eliminating the need to maintain stock cultures and reduce the chances for contaminations with ciliates and pathogenic bacteria. We have produced dry rotifer resting eggs ( cysts ). One gram contains 2,000,000 resting eggs with about 80% of hatching rate at 28 C and 20 ppt in 36 hours. 

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Introduction

Rotifers are valuable live food for larval fish and crustacean culture. Several characteristics of rotifers, including their nutritional quality, body size and relatively slow motility have contributed to their usefulness as good prey for active larvae ( Snell et al., 1984 ).

The rotifer Brachionus plicatilis has been most widely used as essential food source in raising marine fish, shrimp and crab larvae due to its tolerance to the marine environment ( Lubzens, 1987; Dhert et al., 1994 ).

There are a large array of culture methods to produce the rotifer B. plicatilis. These can be sorted into three basic methods: (1) batch culture, (2) semicontinuous cultures and (3) feedback culture systems.

The widely applied batch culture procedure appears to be simple, using microalgae and/or bakers' yeast or other formulated products as food source. The development of the larval rearing industry is primarily due to the advancement of mass culture technology of the marine rotifer B. plicatilis ( Fulks et al., 1991 ). But rotifer mass cultures are still sometimes unstable due to the change of bacterial flora or contamination of other species ( Hino, 1993 ).

The rotifer B. plicatilis is an euryhaline species. In north China along the Bohai Bay, great populations of rotifers appear in natural brackish waters near village and/or processing plants in April, when Chinese shrimp and crab larvicultures have begun. The wild rotifers are harvested by fishermen and sold to local hatcheries at price about 1 US dollar per kilogram, which includes about 60,000,000 live rotifers.

With the rapid development of Chinese white shrimp Penaeus chinensis and the freshwater crab Eriocheir sinensis larvicultures, the supply of wild rotifers can not reach the demand. In 1988, Chinese experts invented rotifer mass production technology. Since 1988, we have been producing the rotifer B. plicatilis in outdoor earthern ponds planedly.

Principles

We observed that: in nature, great populations of the rotifer B. plicatilis only appeared in waters with high eutrophication near village and/or processing plants of aquatic and/or poultry products. In these waters there is no copepods, the main predator of rotifers. If waters includes copepods, the rotifer population is very small or there is no rotifers in the waters.

Our observations can be easily explained by ecological principles. Domestic sewage and waste water is about 99.9 percent water and 0.02-0.04 percent solids of which proteins and carbohydrates each compose 40-50 percent and fats 5-10 percent.

In other words, sewage includes mostly biodegradeable pollutants such as human fecal matter, animal wastes and certain dissolved organic compounds ( carbohydrates, urea, etc. ) and inorganic salts such as nitrates and phosphates of detergents and sodium, potassium, calcium and chloride ions. Under natural processes most of the biodegradeable pollutants of sewage are rapidly decomposed and when they accumulate in large quantities, they create eutrophication.

The great amount of basic nutrients such as ammonia, nitrogen, nitrate, nitrites and phosphates stimulate algae growth and lead to green algae blooms. Under these conditions, copepods and other predators ( small shrimp and fish ) can not survive longer and are ultimately eliminated from the waters. But rotifers have higher ability to tolerate eutrophication than copepods, small shrimp and fish. The rotifers can survive, grow and reproduce very well in high eutrophication waters. So great populations of rotifer B. plicatilis apear in the brackish eutrophication waters.

In nature waters with low N and P content, copepods are predators of rotifers. Once appearance of copepods, the density of rotifers can not be large. In opening outdoor earthern ponds, it is impossible to avoid entering of copepods.

We used ecological principles into rotifer production procedures. Artificially create high eutrophication waters by applying fermented bean cake and chicken manure, the grow of copepods is inhabited and then the copepods are eliminated from the system. But this circumstance is very suitable for mass production of rotifers.

Rotifer production

1. Construction of earthern ponds

Rectangular ponds are recommended with suitable size of about 1,000 sq. m. of surface area and about 1 m of water depth. 5-10 ponds are needed according to demands for rotifers. The rotifer B. plicatilis is an euryhaline species, but the optimal salinity for rotifer growth is 10-20 ppt. So the ponds should be constructed near freshwater source.

2. Applying manure and algae culture

In north China during early spring as soon as the ice melts, sea water and freshwater are pumped directly into the earthern ponds. Salinity remains about 15 ppt. Disinfecting the ponds is not needed. About 500 kg of fermented bean cake and chicken manure ( 1:4 ratio ) are applied into each ponds. The dominant marine algal species are good food sources for rotifer growth. The algae can provide rotifers with sufficient protein and especially essential fatty acids. Practical applications have proved that the produced rotifers are suitable for larvicultures of marine shrimp, crab and fishes. The bacterial populations in these ponds can provide rotifers with proteins and especially essential vitamins such as VB12 for rotifers growth ( Yu et al., 1988 ).

3. Inoculation of live rotifers or hatching of resting eggs

In end of March at about 15 C, wild rotifer populations have appeared, which are harvested to inoculate the rotifer ponds at initial density of 1-5 rotifers per l. In later years, wild rotifers are not needed, because there are a great number of rotifer cysts on the bottom of the rotifer ponds. They can easily hatch at suitable conditions. In regions where there is no rotifers, commercial rotifer cysts are recommended to start rotifer mass cultures.

Rotifer culture practice requires large volumes of phytoplankton. The volume ratio between rotifer and algal ponds is 1:3 ( Hagiwara, 1989 ). So 3/4 of ponds are used only as algal culture ponds. The algal ponds must not be used to culture rotifers, because if all the ponds are inoculated with rotifers, the rotifers can rapidly consume all the algae and the algal culture will not forever recover.

2-3 Rotifer ponds are selected to hatch the resting eggs. DO and light significantly affect the hatching of rotifer resting eggs ( Hagiwara et al., 1989). The rotifer resting eggs are located on the bottom of the rotifer ponds, where DO is very low or zero and light is not sufficient. So the algae water in the rotifer ponds should be pumped into other ponds, and then the empty ponds are exposed to sun and air in order to oxidize the reductive pond bottom. 2-3 Days later brackish water ( 10-20 ppt; sea water + freshwater ) is pumped into the empty rotifer ponds upto a level of 20-30 cm, so that the resting eggs on the bottom of the ponds receive sufficient sunlight.

Meanwhile a number of resting eggs float from the bottom to the water surface ( Hagiwara et al., 1985 ). At 15 C after about 72 hours, most of the eggs have hatched. The rotifer resting eggs on the bottom of algae ponds with about 1 m of water depth do not hatch due to the deficiency in DO and light.

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