Role of Plankton in Aquaculture

Farming organisms such as fish receive nutrients other than minerals through taking food. In nature, most organisms survive by consuming live food such as plankton from the environment. However, some living organism takes dead organic particles as food. Phytoplankton is the primary food of many aquatic larvae. The larvae of most species eat completely zooplankton or both plant and animal plankton after a certain period of time. The plant components of all types of plankton are known as phytoplankton and they are the primary producer for the entire reservoir or any water bodies. The animal components of plankton are known as zooplankton. These are the primary consumer. Generally, plankton enhance the productivity of the any water bodies. 

Role of Phytoplankton

Fish farmers pay more attention to the color of the water. In other words, they give more importance to the production of phytoplankton in ponds and other fish farms.  Phytoplankton is produced in nursery ponds and reservoir ponds as a result of the initiation of a biological cycle by inputting the mineral nutrients in the ponds.  Phytoplankton play the following roles:

  • It increases the dissolved oxygen in the pond water and reduces toxic gases like ammonia, nitrate, hydrogen sulfide, methane, carbon-dioxide, etc.
  • It maintains the quality of pond water and reduces the amount of toxic substances.
  • Plankton is used as a natural food for fish.
  • They provide fish shade to protect fish from sunlight and reduces cannibalism.
  • They increase water temperature and bring its stability.
  • They help to reduce the population of pathogens and microorganisms by competing for the nutrients available in the water.

Therefore, phytoplankton plays an important role in stabilizing the ecosystem of the pond or other water bodies and keeping the water quality at the right level.

A suitable phytoplankton population enriches the aquatic ecosystem through photosynthesis during the day and maintains low levels of gases like carbon dioxide, ammonia, nitrite, hydrogen sulfide, methane etc. Phytoplankton can take ammonia, nitrogen and heavy metals as food. Therefore, a healthy phytoplankton bloom can reduce toxicity. A healthy algal bloom maintains proper turbidity resulting in stability in shrimp farming and reduction in cannibalism. It stabilizes water temperature by preventing winter heat dissipation. As food density increases, phytoplankton competes for food with other microbes and lower class pathogenic bacteria for nutrients.

Phytoplankton play a major role in water quality management by donating a suitable color to the water. The abundance of plankton species in the water can be inferred from the color of the water. Sometimes the color of water changes due to extensive outbreak of phytoplankton. When the phytoplankton reaches the end of its reproductive cycle, such conditions occur when the physical chemical environment of the reservoir suddenly becomes unfavorable for the phytoplankton. Such as when there is a drastic change in salinity or temperature, or when nutrient deficiencies are observed, or when the presence of zooplankton is greatly increased.

The number of plankton increases rapidly during extreme heat when there is adequate nutrient content in intensively cultivated ponds.
Excessive phytoplankton mortality on hot days causes problems for the survival of prawns, shrimps and fish.

High temperatures cause dead plankton cells to decompose rapidly. O2 is used in this work which also cause the lack of oxygen. This results in the production of ammonia and sulfide in the anaerobic sediment, which has a detrimental effect on benthic shrimp and prawn. In this situation, the water of the fish farm has to be changed quickly.

Role of zooplankton

In nurseries and stocking ponds, tiny larvae, juveniles and even older fish, shrimps and prawn etc. need protein-rich food. In this case, natural food play an crucial role for proper growth. Different types of zooplankton such as Brine Shrimp (Artemia), Rotifer (Brachionus sp., Lecane sp., Filinia sp., Keratella sp., Asplanchna sp., Monostylla sp., Polyarthra sp., Etc.), Cladocerans (Moina sp., Daphnia) sp., Ceriodaphnia sp., Diaphanosoma sp., Alona sp., Bosmina sp., etc.), Copypods (Cyclops, Diaptomas sp., Braocamptus sp., Naupleus larva, etc.), the larval stage of various aquatic organisms are treated as food for prawn, shrimps and fish.

Economically important fish feed on live foods such as phytoplankton and zooplankton. Usually zooplankton eats phytoplankton. Thus phytoplankton forms the basis of the food chain. Live food can swim in the water. These foods are specific and readily available for fish and shellfish larvae. The presence of these foods stimulates the larvae to feed (David, 2003). In aquatic ecosystems, these live foods create more valuable resources for fish farming.

Most fish and shellfish larvae survive by eating small phytoplanktonic and zooplanktonic foods in nature. Clear pond water does not have an abundance of natural food organisms. However, they are abundant in the water of the green pond. The green color indicates the presence of phytoplankton and other natural foods in the water.

The natural fish food such as zooplankton provides the important part of the food for the larvae of fish. It is generally thought that Copepods can meet nutritional needs for fish larvae (Evejemo et al, 2003).

Artificial feeding of fish larvae cannot be compared with natural food in terms of acceptability. Different species of fish have different diets in natural water bodies. However, all fish need protein-rich live food for suitable growth, efficient breeding and survival (Mandal et al, 2009).

The strength and importance of food can be increased through live food development strategies for the growth of aquatic species larvae. Easy availability of fish fry in hatcheries for fish farming in stocking ponds mainly depends on the availability of live food suitable for fish larvae, fry and fingerling (Lim et al., 2003).

Large quantities of live food such as marine rotifer (Brachionus plicatilis) and (Brachionus rotundiformis) and nauplius of Artemia have been successfully used for at least 60 marine fin fish and 16 species of crustaceans at different stages of fish fry production (Dherti, 1996).

Adding small micro-algae such as green water with zooplankton has become a very popular practice nowadays in intensive farming of fish larvae and prawns.

Live food organisms contain all kinds of nutrients such as essential proteins, lipids, carbohydrates, vitamins, minerals and fatty acids (New, 1998). This is why this food is commonly called living capsules of the production of nutrients.

Proper supply of live food in a timely manner can play an important role in achieving maximum growth and survival of larvae and fry of finfish and shellfish. In order to maximize production and financial gain, natural food ingredients must be identified and quantified. The nutritional value of live food organisms can be improved by applying various strategies for nutrient enhancement and bioen capsulation.

The success of fish farming depends on stocked healthy fish fry. A disease-free healthy stockpile can be built by supplying artificial foods along with live food. The artificial food supplied does not provide all the nutrients for fish growth. Therefore, fish and shellfish must be fed live food. To get good results in fish larvae and shellfish rearing, they must be fed nutritious food.

Larvae rearing are a risky business of fish farming, but it is one of the more profitable businesses. Through special techniques and planning, the risk of higher mortality in this stage of fish farming can be reduced. Many farmed fish require zooplankton as their primary food. Zooplankton play a role in the rapid growth and high survival rate of fish.

In many cases, fish and shellfish larvae can eat supplement food. In this case, they need small sized live food for nutrition. Live food is an easily digestible protein food which is suitable for fish and shellfish. These live foods can be purchased from the market which is very costly and not available when needed. Moreover, the cost of food production increases the cost of fish farming. However, these live foods can be easily and cheaply cultivated. Live food refers to both phytoplankton and zooplankton.

Chlorophyll-containing organisms such as Microcystis, Volvox, Eudorina, Oscillatoria, etc., plants or saproplanktons such as bacteria and fungi that do not participate in photosynthesis but they are treated as phytoplankton. On the other hand, animal plankton,  are called zooplankton. The main zooplankton in the tropic regions are protozoans (eg Arcella sp., Difflugia sp., Actinophrys sp., Vorticella sp.,), Rotifers (e.g. Brachionus spp., Keratella spp., Asplanchna sp.). Polyarthra vulgaris, Filinia, And planktonic crustacea (Artemis spp.), Cladosaran (Moina spp., Daphnia spp., Ceriodaphnia spp., etc), Ostracodes (Cypris spp., Stenocypris sp., Eucypris sp., etc.) and Copipodes(Cyclops spp., Mesocyclops leukarti , M. hyalinus, Microcyclos varicans, Heliodiaptomus viduus, etc., ) and their larvae.

Microalgae are used as live food for all stages of growth of bivalve mollusks, crustaceans, larvae or juvenile stages of certain fish species. Hundreds of micro-algae species have been tested as food over the last forty years. However, perhaps less than 20 species are being used as food in aquaculture.

Microalgae play an important role in the production of zooplankton for fish and other larval food. In addition to protein (essential amino acids) and energy, other major nutrients such as vitamins, essential poly-unsaturated fatty acids, dyes, sterols are provided which are transferred through the food chain. In nature, zooplankton is one of the primary food of fish larvae. The two main zooplankton groups are Rotifera and Copepoda. Shrimp are taken as two foods and are being used extensively in fish farming as their live food. Intensive farming of most fish larvae is heavily dependent on zeoplankton suppliers

The different types of water in fish ponds and the relationship of plankton with them are mentioned below:

Golden brown and reddish brown: If there is more diatom in the water, this color is more visible. This condition is caused by low temperatures and high levels of organic matter in salt water. In this case, the transparency of the water is 25-35 cm. It happens as a result of blooming of diatoms.

This type of color is seen if algae species such as Chaetoceros, Navicula, Nitzschia, Skeletonema, Cyclotella, Synedra, Amphora, Euglena, etc. In this case, especially in the first three species are found in ponds.  Diatoms are rich in nutrients and are easily digestible. The golden brown color is usually associated with fish or healthy shrimp. Their bodies are brightly colored which indicates the desired production.

Light or brown green: This type of water is due to the growth of green algae, especially herbs. Moreover, Dunaliella, Platymonas, Carteria, Chlamydomonas etc. also exist. Scenedesmus and Euglena are found in less saline water. This type of water is fairly stable. The death rate of fish or prawn in such water is lowest. In this case, the clarity of water is 20-70 cm.

This condition is observed in new ponds with sandy bottom and slightly organic sediment. The growth rate of fish/shrimp and prawn is fairly stable in this type of  environment.  The color of this type of water can be easily controlled. When the color of the water is dark green, the farmed animals become susceptible to the disease. In this case, rarely high mortality occurs.

Deep green or dark green color: When the water temperature of the pond is very high, the dead organic matter decomposes at the bottom. In this case, bloom of bluish green  algae is formed from green algae so fast. Blue-green algae such as Ocillatora, Phormidium and Mirocoleus are predominant (about 90%).

In this condition, the survival rate of fish or shrimp is high but the growth rate decreases. In this type of water, the color of shrimp is dark green or black. Sometimes shrimp look bad which is attacked by protozoa and germs. In this type of environment, the growth of fish or shrimp is less and there is a wide variation in size of fish and shrimp which ranges between large and small in size. Moreover, this results influence the  high rate of soft shell shrimps.  

This condition also weakens the shrimp, which is not the desired color in the semi-intensive system. In this case, this condition has to be improved through proper management by changing water.

Dark brown color: Weak ponds management such as over feeding or use of large quantity of rotten fish influence the rapid growth of dinoflagellated and brown algae. Such color of water is not desirable. If such color is created, the water of the pond has to be changed partially. Such type of  condition indicates super neutrification, known as eutropication.

This condition is usually caused by over-feeding, damage to the bottom of the pond, low average water depth, presence of excess organic matter, high consumption of rotten fish. The clarity of such water is 15 cm. Using an aerator creates a lot of bubbles. This is not the desired character in case of pond management. It usually weakens the fish or shrimp and sometimes causes dropsy and makes the animal susceptible to scabies. The rate of fish production in such waters is low.

This brown color of water is not formed due to algae. This condition is also caused by dyes or tannic acids. Due to acid sulphate soils in the mangrove region, high concentration of this material is produced at the bottom.

Yellowish color: As the number of Chrysophyta increases, yellow color of water is formed. Moreover, green flagellated can also grow fairly. Benthic diatom, bluish green algae and green algae also grow in this colorful pond water. This type of water is more stable. Although it is not desirable for fish, shrimp or prawn.

This condition is seen in older fish ponds where high levels of organic matter accumulate over a long period of time and bacterial activity occurs. Such ponds hamper the growth of fish, shrimp or prawn. It also increases the high mortality rate of fish, shrimp and prawn.

Foggy white water: This water contains zooplankton, clay particles, and dead organisms. It is an ideal environment for the growth of larvae and juvenile prawn as it provides natural food. On the other hand, during cultivation, it creats a problem in competing with the environment of adult prawn . In water foggy white water, the zooplankton gather together to form a whitish spot. The color of such water has to be well controlled during fish farming.

Turbid water: Turbid water is formed due to mud particles, zooplankton and dead organic particles. Such water can be beneficial or harmful depending on the quality and quantity of the degraded material. It is made from silt particles that are dissolved in freshwater rivers or rainwater. Sometimes it can be caused by strong winds. As a result, aquatic organisms, especially shrimps, move hesitantly on the bottom. In addition to improving the quality of water and the stability of the shrimp habitat, this muddy water provides some other benefits of water such as nutrient effects, mud particles, organic particles and microorganisms to form mud flocules which are used as natural food source for shrimp.  When the water is clear and not of any other color, excessive mud particles have a positive effect on the growth of fish / shrimp.

Clean water: Clean or clear water is caused by a lack of nutrients, heavy metallic contaminants such as copper, manganese, iron, or the presence of sedimentary particles in the bottom acid. In this condition no organism can grow well. Such water is not good for the growth of shrimp, prawn and fish. In this case, it is assumed that there are three defects of water or meter quality. Therefore, in the case of fish, shrimp or prawn cultivation, the water condition should be improved from the beginning.

From the above discussion it is seen that some colors of water are desirable and some colors are not desirable. To get any specific color of water, algae has to be used. It is thought that ammonia salt is beneficial for the growth of green algae. Urea, on the other hand, is harmful. Water quality cannot be changed by partial change of clean water. Food intake is greatly influenced by the color and quality of the water. So extra food cannot be provided, excessive use of rotten fish causes blooming of flagellates which is not desirable in fish farming.

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