Use of Rotifer in Aquaculture of BangladeshIntroduction

Rotifers are popularly called as wheel animalcules. They are an important group of live food organisms for use in aqua hatcheries. Brachionus, which is the most known form of all rotifers, serve as an ideal starter diet for early larval stages of many fish and prawn species in marine as well as freshwater. Species of the genus Brachionus (Brachionidae: Rotifera) are well represented in different water bodies worldwide (Pejler, 1977). Depending on the mouth size of the cultured organisms, small (50 to 110 micron length) or large (100 to 200 micron length) rotifers are used. There are about 2,500 species of rotifers have been known from global freshwater, brackish water, and seawater. B. plicatilis is the species used most commonly to feed fish larvae in hatcheries around the world. It is a euryhaline species, small and slow swimming, with good nutritional value. It is well suited to mass culture because it is prolific and tolerates a wide variety of environmental conditions. The rotifer, B. plicatilis and B. rotundiformis, have been indispensable as a live food for mass larval rearing of many aquatic organisms (Maruyama et al., 1997). By way of significant developments in larval rearing technology of fishes, demand for the rotifer is further increasing.

Aquaculture

Aquaculture, probably the fastest growing food-producing sector, now accounts for nearly 50 percent of the world's food fish. The need to exchange reliable information on all related subjects is becoming a key issue for the responsible management of aquaculture. FAO (1988) introduced a definition of aquaculture, which reduces its confusion with capture fisheries: “Aquaculture is the farming of aquatic organisms, including fish, molluscs, crustaceans and aquatic plants. Farming implies some form of intervention in the rearing process to enhance production, such as regular stocking, feeding, protection from predators, etc. Farming also implies individual or corporate ownership of the stock being cultivated. For statistical purposes, aquatic organisms which are harvested by an individual or corporate body which has owned them throughout their rearing period contribute to aquaculture, while aquatic organisms which are exploitable by the public as a common property resources, with or without appropriate licenses, are the harvest of fisheries”.

Aquaculture in Bangladesh

Bangladesh is a densely populated country of 147 570 square km with a population of 130 million people. It is fortunate in having an extensive water resource in the form of ponds, natural depressions (haors and beels ), lakes, canals, rivers and estuaries covering an area of 4.56 million ha(DoF,2005). Bangladesh is one of the world's leading inland fisheries producer with a production of 1 646 819 tons during 2003–4, with marine catch total of 455 601 tons and a total production from aquaculture of 914 752 tons during 2003–4. Bangladesh's total fish production for the year totaled above 2.1 million tons (DoF, 2005). FAO (2005) ranked Bangladesh as sixth largest aquaculture producing country with its estimated production of 856 956 tons in 2003 (FAO,

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2005). Aquaculture accounted for about 43.5 percent of the total fish production during 2003–4, with inland open water fisheries contributed 34.8 percent (DoF, 2005).

The present per capita annual fish consumption in Bangladesh stands at about 14 kg/year against a recommended minimum requirement of 18 kg/year; hence, there is still need to improve fish consumption in the country. 

Fisheries in Bangladesh are diverse; there are about 795 native species of fish and shrimp in the fresh and marine waters of Bangladesh and 12 exotic species that have been introduced. In addition, there are 10 species of pearl bearing bivalves, 12 species of edible tortoise and turtle, 15 species of crab and 3 species of lobster. 

The Ministry of Fisheries and Livestock (MoFL), Department of Fisheries (DoF), Bangladesh Fisheries Development Corporation (BFDC) and the Bangladesh Fisheries Research Institute (BFRI) are the main organizations responsible for aquaculture and its development. Universities, organizations within other ministries and local and international NGOs are also involved in this area.

Present Status of Aquaculture in Bangladesh

The country’s fish production has nearly doubled over the last decade—thanks to popularity of fish farming. According to data of Bangladesh Department of Fisheries, farmed fish production was 2 million tons in 2013-14. This was only 0.8 million tons back in 2001-02.Consequently, the overall fish production increased spectacularly by around 88 percent during this period. This is great news for the nation as fish accounts for about 56 percent of Bangladesh’s intake of animal protein. According to fisheries department, the country produced about 3.5 million tons of fish in the 2013-14. Of this, 2.9 million tons came from farms and catches from various Inland water sources, and 0.6 million tons were from the sea. A decade back, the total fish production stood at 1.9 million tons—majority of it came from various inland water sources.

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This rise in fish culture is due to increase in high quality fish feed and seed production, and widespread induced breeding technology, the department says. Another reason that boosted fish production is the Jatka Conservation project that imposed a ban on Hilsa fishing.

The most dominant source of fresh water fish in the country is the several thousand ponds in the villages, comprising of about 0.4 million hectare area that produce half of the Inland water fish. One-quarter fish comes from flood plains.

Water area for Inland fisheries comprises of approximately 4.7 million hectares across the country. Dhaka division tops fish production with claiming 28 percent of the stake; followed by Chittagong, Khulna and Rajshahi ranging between 13 percent and 20 percent. Rangpur contributes the least with just 5 percent production.

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As per the Food and Agriculture Organisation (FAO), global fish production was 158 million tons in 2012 and per capita fish consumption was 19.2 kg. Annual consumption of fish and fish products in Bangladesh was 12 kg/per person in 2010, with Chittagong having highest annual consumption of about 17 kg/per person. In Bangladesh, about three fourth of consumed fish are fresh water fish, among which Pangas, Rui, and Tilapi tops our chart. Pangas farming has become an established agro-processing industry in the country over the last decade. Pangas is most easy to produce due to its short culture-cycle, high growth rate and unselective feeding habit. It also has good resistance to diseases. These characteristics are somewhat also present in Tilapia.

Export of fish and fish products increased by 86 percent from 2001-02 and reached 77,000 tons in 2013-14.

Rotifers in Bangladesh

Many species of rotifers have been identified from different types of freshwater bodies in Bangladesh. Brachionid rotifers seem to be predominant and the common species recorded are Brachionus diversicornis, B. falcatus, B. quadridentatus, B. calyciflorus, B. havanensis and B. forficula. The other common genera in Bangladesh are Keratella, Monostyla, Lecane, Lepadella, Filinia, Trichocerca, Polyarthra, etc.

Importance of Rotifers

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As a live food, rotifers are the perfect size (105-160 micrometers in width) for many species of fish larvae that are cultured commonly. For rotifers to be nutritionally suitable for fish larvae, they must be "enriched", fed with various combinations of microalgae or commercially prepared products. Once enriched, rotifers are an excellent hatchery food for many species of larval fish. The nutritional value of rotifers for larval fish depends on the rotifers' food source. Researchers have determined that highly unsaturated fatty acids (HUFAs) are essential for the survival and growth of finfish larvae. Rotifer feeds containing DHA, 22:6n-3, docosahexaenoic acid, and EPA, 20:5n-3, eicosapentaenoic acid, can be valuable, with DHA the more essential for marine fish larvae. Depending upon their food source, rotifers are about 52 to 59 percent protein, up to 13 percent fat, and 3.1 percent n-3 HUFA.

Biology and Life History

The life span of rotifers has been estimated to be between 3.4 to 4.4 days at 25°C. Generally, the larvae become adult after 0.5 to 1.5 days and females thereafter start to lay eggs approximately every four hours. It is believed that females can produce ten generations of offspring before they eventually die.

The life cycle of Brachionus plicatilis can be closed by two modes of reproduction. During female parthenogenesis the amictic females produce amictic (diploid, 2n chromosomes) eggs which develop and hatch into amictic females. Under specific environmental conditions the females switch to a more complicated sexual reproduction resulting in mictic and amictic females. Although both are not distinguishable morphologically, the mictic females produce haploid (n chromosomes) eggs. Larvae hatching out of these unfertilized mictic eggs develop into haploid males. These males are about one quarter of the size of the female; they have no digestive tract and no bladder but have an over-proportionated single testis which is filled with sperm. Mictic eggs which will hatch into males are significantly smaller in size, while the mictic fertilized eggs are larger and have a thick, faintly granulated outer layer.

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Figure 1. Parthenogenetical and sexual reproduction in Brachionus plicatilis (modified from Hoff and Snell, 1987).

These are the resting eggs that will only develop and hatch into amictic females after exposure to specific environmental conditions. These can be the result of changes in environmental conditions eventually creating alternations in temperature or salinity or changing food conditions. It should be emphasized that the rotifer density of the population also plays an important role in the determination of the mode of reproduction. Although the mechanism is not completely understood, it is generally believed that the production of resting eggs is a survival strategy of the population through unfavorable environmental conditions such as drought or cold.

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Strain Differences

Only a few rotifer species belonging to the genus Brachionus are used in aquaculture. Brachionus plicatilis is a cosmopolitan inhabitant of inland saline and coastal brackish waters. It has a lorica length of 100 to 340 mm, with the lorica ending with 6 occipital spines.

However, for use in aquaculture, however, a simple classification is used which is based on two different morphotypes, namely Brachionus rotundiformis or small (S-type) rotifers and Brachionus plicatilis or large (L-type) rotifers. The differences among the two types can be clearly distinguished by their morphological characteristics: the lorica length of the L-type ranging from 130 to 340 mm (average 239 mm), and of the S-type ranging from 100 to 210 mm (average 160 mm). Moreover, the lorica of the S-type shows pointed spines, while of the L-type has obtuse angled spines.

Figure 2 : Brachionus rotundiformis (S-type) and Brachionus plicatilis (L-type) (modified from Fu et al., 1991).

Who uses rotifers?Rotifer is used in-

• Finfish hatcheries• Ornamental fish breeders• Shrimp hatcheries

Uses of Rotifer in Larviculture

The successful use of rotifer in commercial hatchery operations of red sea beam (Pargus major) encouraged investigations in the development of mass culture techniques of rotifer. Twenty years ago, rotifer was first used in larviculture feeding. Fingerling production is one of the many challenges faced by those interested in promoting industrial production of emerging marine and fresh water species. In spite of huge efforts to use artificial feeds, the of fish larvae during the primary nursing phase still depends heavily on natural food. Live feeds include Rotifers, Artemia and other tiny organisms are often the enhancing larval production. These live foods are used in larviculture of carps, catfish and shrimp. Though there is not enough study on rotifer culture in Bangladesh. The benefits they are getting is discussed below.

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Brachionus plicatilis

B. plicatilis is the species used most commonly to feed larval fish in hatcheries in Bangladesh. It is a euryhaline species, small and slow swimming, with good nutritional value. It is well suited to mass culture because it is prolific and tolerates a wide variety of environmental conditions.

Brachionus calyciflorus

The freshwater rotifer B. calyciflorus is widely used in aquaculture as a live food to raise larvae of many species during the first two weeks of exogenous feeding. No other live food has been developed for larval feeding that can replace rotifers. The two types of protease enzymes in rotifers assist fish larvae to digest them. Weight, rearing environment and feeding protocol are among the key factors affecting the migration and fishery returns of fish fingerlings produced by artificial breeding programs for the purpose of restocking and rehabilitation of stocks.

Brachionus angularis

Brachionus angularis is a cosmopolitan freshwater rotifer. Its potential use as live food for small-mouthed freshwater fish larvae. In the past, there was less demand for small live food in freshwater aquaculture, but the demand in recent years has been increasing for several reasons. Ornamental freshwater fishes that produce small larvae which require small planktonic rotifers for fry production. This rotifer is used in Bangladesh for Larvae culture.

Now, In Bangladesh; Different hatchery owners prefer different rotifer spp. For feeding their freshwater larvae. Sometimes they use rotifer as live capsule by enriching rotifer with different nutrient. As rotifer is a highly nutritive for freshwater fish larvae, hatchery owners of Bangladesh are used this for better survival of fish larvae.

Uses in Catfish

Larval nutrition and live feed culture like Artemia and rotifers is one of the most important and obligatory matter for successful fish culture. Feeding with live prey for fish larvae are most essential because during first few days of their life they have no complete develop digestive tract, especially their digestive enzymes. Moreover, live increases feeding by predatory larvae, resulting reduce cannibalism of some species like Clarius gariepinus the enhancing larval production. Clarius gariepinus is generally considered as predator or omnivorous and also commercially important fish for aquaculture because they are able to tolerate to extreme environment conditions and worldwide demand. Awaiss and Kestemont studied the suitability of the freshwater rotifer Brachionus calyciflorus as starting food for the larviculture of African catfish. Rotifer also acts as an important food for stinging catfish Heteropnuestes fossilis in their early larval stage of life.

Uses in Shrimp Culture

Now-a-days rotifers are used as replacement of Artemia in shrimp culture. In some coastal shrimp hatcheries of Bangladesh rotifers are used as successful larvae feed. Many marine fish

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hatcheries incorporate rotifers in their larval diets because rotifers are smaller than Artemia. With shrimp, Artemia is usually introduced in the Mysis stage, but most species can survive on rotifers from earlier stages. The success of extensive penaeid larvae rearing systems using rotifers in areas where Artemia and enrichment diets are prohibitively expensive or unavailable indicates that they are an alternative worth considering.

Producers of Rotifers

Hatcheries equipped with modern facilities produce rotifers for their culture operations to prevent mass mortality of species that fetch high market value. The larval stages of stinging catfish popularly known as shing and African catfish are very sensitive. Therefore, culture operation of these two species is not easy because of appropriate feed. Live feed like Artemia and rotifer can overcome the problem. Bangladesh Fisheries Research Institute (BFRI) and live feed laboratories from different public universities are aiming to disseminate the effective and efficient technique of culturing rotifers and other live feed organisms.

Problems associated with Rotifer Culture

The culture of rotifer needs intensive care and the environment should be friendly. Inexperienced personnel can ruin up the whole operation. Therefore, they need to be trained up to run the culture successfully. There is another problem concerning rotifer culture. Food of rotifer is important. They feed on phytoplankton like Chlorella sp. Before setting up rotifer culture, farm owners should consider set up phytoplankton culture. And establishment of advanced live food laboratory is essential.

Prospects of Rotifer Culture in Bangladesh

Artemia is widely used in different hatcheries across Bangladesh. If there is any 2nd choice that is rotifer. Bangladesh is a culture based fish producing country. The environment exists here is suitable for rotifer culture. To produce huge number of fish fry, live food like rotifer is essential to provide balanced nutrition in their larval stage and prevent mass mortality.

Conclusion

There are about 2000 rotifer species all requiring moisture and most of them inhabit freshwater (around 1950 species). Most rotifers are free swimming and form an important part of the freshwater zooplankton. This types of Rotifers are highly nutritive food for larvae culture. So, we need to put great emphasize for rotifer culture in Bangladesh. Then we can make more profit in aquaculture.

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References

Pejler, B., 1977. On the global distribution of the family Brachionidae (Rotifera). Arch. Hydrobiol. Suppl., 53: 255–307.

Maruyama, I., Nakao, T., Shigeno, I., Ando, Y., Hirayama, K., 1997. Application of unicellular algae Chlorella vulgaris for the mass culture of marine rotifer Brachionus. Hydrobiologia, 358: 133-138.

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