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Dominant species of phytoplankton
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Marine species
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Skeletonema costatum, Thalassiosira coramendelina. Coscinodiscus spp., Rhizosolenia spp., Chaetoceros spp., Biddulphia spp., Fragillaria oceania. Thalassisothrix longissine, Pleurosigma spp., Navicula spp., Nizschia spp., Prorocentrum spp., Dinophysis caudata, Peridinium spp., Ceratum spp and Isochrysis galbana.
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Freshwater species
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Scenedesmus sp., Pedistrum sp., Selanastrum sp., Chorella sp., Coeleosphaerium sp., Closterium sp., Microcystis aeruginosa. Spirulina sp., Oscillatoria sp., Lyngbya sp., Asterionella sp. and Synedra formosa.
Effects of phytoplankton in fish farming systems
Phytoplankton development benefits ponds for the following reasons:
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A dense phytoplankton bloom can produce ten times more oxygen than it consumes. This oxygen production is important in compensating for respiration and supplied to the pond without added energy expenditure.
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Phytoplankton represent an efficient means of removing excess nurtrients or toxic micro nutrients especially ammonium from the water.
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Phytoplankton acts as a screen to light penetration in the water and prevents development of benthic algae. The development of scums of blue-green algae promotes production of hydrogen sulfide within the sediment which is highly toxic. These algal scums remove nutrients and postpone the return to normal growth for algae after algal crash.
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Phytoplankton serve as good food for planktivorous fish. Exploiting primary productivity is a cheap way to produce fish.
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The development of phytoplankton prevents the development of macrophtyes, which is unfavourable for fish culture.
Zooplankton
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The success and sustainable development of aquaculture practices depend upon many factors, of which , zooplankton assumes greater significance. The production of zooplankton is termed as secondary production, which directly or indirectly determines the fish yield in aquaculture systems. Zooplankton, being the intermediate link in the pelagic food web, serve as the index of the productivity potential of aquaculture systems. Furthermore, due to their high nutritive value, some planktonic animals such as copepods, cladocerans and rotifers are used as the live food for larvae and post larvae of penaeid shrimp, bivalves and fishes. Thus, studies on qualitative and quantitative aspects of zooplankton form an integral part of environmental assessment efforts in aquaculture farms as well as in areas conductive to aquaculture.
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For the rearing of fry and post-larvae of cultivable finfish species, several live food organisms are collected from natural waters or cultured under controlled conditions. The mass production of zooplankton, particularly Daphnia, Moina, instars of Artemia and rotifers is of utmost importance to run the fish seed hatchery in a successful manner. In the pond ecosystem, the production of zooplankton must be assessed qualitatively as well as quantitatively in order to assess the type of secondary producer available as natural feed.
Different groups of zooplankton
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Freshwater zooplankton include protozoans, rotifers, cladocerans, copepods and other crustaceans and insect larvae. In general, these forms are calssified on size basis as microzooplankton, composed of flagellates, ciliates and rotifers and macrozooplankton, consisting of planktonic crustaceans, cladocerans, copepods, and less frequent groups, worms and insect larvae (mainly Chaoborus).
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In the zooplankton collected from marine environments, i.e., from the brackishwater and coastal waters, a variety of organisms are present. Most of the zooplankton forms are composed of copepods (90% of the total population). The different groups of zooplankton include protozoans, coelenterates, chaetognaths, larval forms of mollusks and arthropoda and eggs and larval forms of fishes. Important freshwater and marine zooplankton commonly occurring in aquaculture systems
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Marine zooplankton
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Protozoa
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Annelida
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Chaetognatha
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Arthopoda
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Cladocera Penilia sp., zoea and mysis of shrimp, zoea and megalopa of crabs Evande sp. and Podon sp.
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Decapoda: Lucifer hanseni, zoea and mysis of shrmp, zoea and megalopa of crabs.
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Copepoda
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Calanoid copepods
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Centropages furcatus, Acartia spp., Acrocalamus sp, Temora spp., and Labidocera spp.
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Cyclopoid copepods
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Harpacticoid copepods
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Cirripedia
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Mollusca
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Chordata
Collection of plankton from pond water
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For collection of net zooplankton, from the finfish or shrimp ponds, a known quantity of the water is filtered through a conical bag made of no. 30 bolting silk. A graduated bucket is used for this purpose. The plankton so filtered by the net is concentrated and used for quantitative analysis.
Preservation of zooplankton
Quantitative estimation of zooplankton biomass
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The biomass of the zooplankton can be determined using any of the following methods
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Species dry weight
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Gravimetric analysis and
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Volumetric analysis
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The standing crop or total mass biomass of zooplankton can also be estimated by direct enumeration method. This is the commonly employed method for estimating the plankton density. The enumeration of zooplankton is done by various counting chambers such as Kolkwitz’s chamber (0.5 ml), Utermohl’s tubular chambers (2-50 ml capacities), Haemocytometer, and Sedgewick Rafter cells. Of all the counting devices, Sedgewick Rafter cell is the commonly used device for estimating the total density of zooplankton.
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Sedimentation chamber
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Enumeration of zooplankton
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An aliquot of sub sample is taken from the whole sample by using a Stempel pipette. One ml of zooplankton sample obtained from the stock through the Stempel pipette is transferred to the counting chamber, i.e., Sedgewick Rafter counting cell. The counting cell filled with the zooplankton sample is then placed under a microscope provided with a mechanical stage. It is left undisturbed for about half an hour for proper sedimentation. The organism are then counted from one corner of the counting cell. The rafter is moved horizontally along the first row of squares and the organisms in each square of the row are counted. When one row is finished, the next consecutive row is positioned using the mechanical device of the stage. By this way, all the organism in all squares are counted.
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If one lacks sufficient time to count all the square in the counting cell, we may count just a few transects considering the homogenous settlement of the zooplankton. The total number of organisms is then computed by multiplying the number individuals counted in these transects with the ratio of the whole chamber area to the area of inspected transects. A preliminary and cursory qualitative examination of the plankton sample would also be useful , especially if a list of various species whose individuals are to be counted is to be complied.
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