25May2019

Articles

Fertilization

Female shrimp secretes eggs from the base of its third set of legs. Simultaneously it releases semen from the sperm sack stored in its body via the root of the fourth pair of legs. The eggs and sperm are then fertilized together in water.

 

Division and Hatching

The first division occurs after the eggs are released in water at a temperature of 27-29 for 30 to 40 minutes. Afterwards, divisions occur at intervals of 12 -15 minutes. After 13 -14 hours it develops into an unsegmented larva, or Nauplius.

 

Nauplius

At this stage the larva looks like a small spider and does not need to seek food from outside sources because it survives on its egg yolk. The nauplius is strongly attracted to light. After molting the larva becomes bigger. The nauplius goes through 6 stages of development. The duration of each stage depends upon water temperature. At 27 -29, the process needs about 50 hours. At 21-22 degrees the process needs approximately 110 hours. After molting for the sixth time, the larva enters the Zoea stage.

 

Note:

 

  • Fertilizers are applied to accelerate the growth of diatoms.
  • Under substantial sunlight, the tank water begins to acquire a brownish color due to the growth of diatoms.
  • Generally, the application of the chemicals (KNO3, K2HPO4, 2ppm) is made every morning until the larvae grow to the stage of P10, for the proper population growth of diatoms.
  • Nauplii grow by their own yolk, and do not take food from the outside.

 

Zoea Stage

This stage is divided into 3 periods. Although the length of time needed depends upon the water temperature and the type and amount of feed used, the process usually takes 4 days.

The first stage is characterized by the body becoming bigger and more elongated than in the nauplius stage; the head and thorax, however, remain unchanged. During this stage direct heat and light from the sun is detrimental to the development of the larvae. Under direct sunlight they become deformed and are unable to absorb nutrients. At this time, it is advisable to let algae grow on the water or to use a special water conditioner to cause the water to become a light brownish color.

The zoea will begin to catch plankton such as Spirulina and Skeletonema Costatum, oyster eggs or oyster larvae. At this stage the mortality rate is relatively high, the survival rate being only 30-50%. After the nauplii have reached their fifth molting (that is, on the second day after hatching), they should be provided with the above mentioned nutrients. In order to judge whether the larvae have reached the sixth stage, one should observe whether or not spines have developed at the endof the tail section. If not, the nauplii are still in their fifth stage.

Whether or not the larvae are ingesting food can be seen from their excretion. Sometimes the excretion of larvae is 3 or 4 times the length of their bodies.

This is known as the "excretion-dragging phase", because the excrement is drawn along behind the protozoeal shrimp in long strands. When the larvae have eaten, they fill their intestines exhibiting a yellowish green color (Spirulina and Skeletonema Costatum), or white color (oyster eggs). If the excrement is abnormal or lacking, the breeder should examine the following thing; (1) whether the amount of feed is enough,(2) if the digestion of the larvae is normal,(3) if the excrement is watery or(4) whether the water quality has worsened. Only after the cause of the problem is determined, can actions be taken to solve the problems.

 

Note:

  • By the time the zoea larvae appear the tank water becomes rich in fertility from the repeated applications of chemicals and contains swarms of diatoms.
  • Feed at a rate 1.5 gm in weight for the feeding of 10,000 zoea in a day.
  • Fine particle : <100 mesh.
  • The zoea swims continuously forward by vigorous moving of the swimming legs dragging a thread-like feces.

The determining factor that decides whether artificial breeding is successful or not depends on whether the correct amounts of feed is provided at proper times to maintain the normal functioning and metabolism of the digestion system. Poor artificial feed or overfeeding will cause the water quality to worsen. This is one of the major causes of failure in shrimp breeding.

 

Genchem`s Shrimp Larva Feed GAP is prepared according to the nutritional requirement of shrimp fry. It contains quality nourishments and is rapidly encapsulated at low temperature by the most advanced scientific and technical methods. All particles are homogenous which has the advantages of suspending (no problems of nutrient leaching, particle break-down, and water fouling) and of being easy to digest and absorb. GAP also contains microorganisms which have water-cleansing properties, being able to break up and dissolve remaining suspended feed particles as well as providing the necessary nourishment for the shrimp larvae. This can make up for difficulties resulting from lack of unknown growth factors in artificial feed.Live feed such as the slenderSpirulina are floating organisms which can maintain good water quality and are also abundant in nutrients, proteins, natural vitamins, minerals and carotenoid. They are suitable food for the shrimp larvae from the Zoea through to the Mysis stage of development. It is possible that carotenoid is transformed inside the body into astaxanthin, essential to shrimps, and which strengthens the immune system.

 

Breaking Out of the Zoea Bottleneck

It is our observation that the highest mortality during hatcheries and breeding in shrimp occurs at the transition from molting to stages 2 and 3 of Zoea, and the last mysis stage of molting to Post-larvae. I concluded from many years of field studies that one of the main reason for this growth-period dependent mortality is the feeding habit migration. Zoea and Post-larvae experience tremendous stress in the process of feeding habit change. In these transition periods, the digestion system is not well developed yet and is very sensitive to any change in their digestion systems.

This stress-related indigestion can eventually lead to death. The stress is easily induced in the above mentioned two stages because (1)Zoea begins to take external food since the yolk in its body has been consumed during the nauplius stage.(2)Post-larvae also is the stage when larvae changes their food pattern-from suction to biting.(3) Certain kinds of stress occurring during metamorphorsis and stress due to certain larvae not having fully functioning enzyme glands.

 

The Principal Benefits of Genchem POLYTASE

1.It is very specific for the young larvae to cure digestive system disorders at the stages of zoea.

2.The bottom of the culture pond will become clear and no more black sludge, after using this drug in the culture pond.

 

Mysis Stage

After molting three times, the Zoea enter the Mysis stage during which they begin to look more like adult shrimp and swim in a characteristic fashion with head and tail pointed downwards at right angles and occasionally performing a sudden retrograde jumping action.

This is known as the inversion state, the fry being suspended upside-down in the upper middle region of the pond water. This is, therefore, also called the " inverted suspension phase". During this stage the fry adapt more easily than the first two stages to external changes in environment, such as water temperature and salinity. The main feed at this time consists of Brine Shrimp larvae, oyster eggs and diatoms (Spiruline, Skeletonema costatum). However other zooplankton such as Copepoda and Rotifera can also be used.

During the Mysis stage the bodies of the fry become larger and so are able to ingest the nauplii of the Brine Shrimp along with Spirulina and GAP. If this method of feeding is employed, the survival rate of the shrimp fry is surprisingly high. The reason for this is that the Brine shrimp larvae also feed on Spirulina and GAP. Consequently the survival rate of nauplii of Brine shrimp is increased and the water quality is maintained. For this reason, the water does not need to be changed so frequently.

Usually in water temperatures of 27-29,with a salinity level of 32 0/00 the three phases of the Mysis Stage take 3 days to complete. The shrimp fry then enter into the Postlarva Stage.

 

Note:

  • During the mysis substages there is a gradual transition from phytoplankton to zooplankton.Feed on: Diatoms, animalcule such as A. salina, oyster eggs and larvae as well as on rotifers and nematodes.
  • The successful breeding of the larvae is usually accompanied by the proper growth of diatoms, which gradually changes the color of the water from brownish to dark coffee brown.
  • Sometimes the water becomes greenish in color due to the growth of protozoans such as the flagellates, replacing the diatoms which have died out. Such water is known as "green water". If "green water" unfortunately appears, it is generally irremediable; it is better to start all over again.

 

The factors or causes responsible for the "green water" are listed below:

1).Sea water put into the tank was polluted

2) Cleaning of the tank was insufficient

3) Dead shrimp were left in the tank.

4) The setting of the aeration system was improper

5) The supply of sunlight was insufficient.

6) A contaminated dip-net was carelessly immersed in the tank water.

Postlarva Stage

From the P1 to P5 phases of this stage, the Brine Shrimp's larvae and zooplankton such as Copepoda should be used as feed. The amount of feed used should be increased as the shrimps grow, and the feed should be administered several times a day. In the P5 to P7 phases, most shrimp settle into life on the bottom or sides of the pond, they need only Annelida and Nematoda. At this time, Genchem's shrimp flakes (in ground form), as well as zooplankton should be administered as feed. This can not only supplement a possible lack of plankton and lower costs, but also produce healthy and active shrimps. The ingredients of Genchem`s shrimp flakes are in accordance with the national regulations governing shrimp breeding and nutrients. Its contents are based on Akio Kanazawa`s research report. (Akio Kanazawa is the Professor of Marine Resources, Nutrient Chemistry at Kagoshima University, Japan)

 

Note:

  • The post-larvae, growing a chela on the walking leg, begin to devour other small actively swimming animalcules, and the slow moving Bs-N (brine-shrimp nauplii) becomes easy prey.
  • The post-larvae come to resemble the Bs-N in brownish red color.
  • The larvae change their food habit from omnivorous to carnivorous.
  • When the larvae reach the PL5~6stage, the feeding is shifted to either clam or formula feed.
  • As stated before, the post-larvae metamorphosed from mysis start to feed on animal food for which brine-shrimp are used, but the feeding of brine-shrimp lasts only until the P5 stage, when it is mixed with minced clam meat.
  • After the P6 stage only clam meat is given or formula feed.

 

Items to observe when feeding

1. The amount of feed to be used can only be decided after observation of the condition of the artificial feed passing through the shrimp intestines.

2. At night the intervals between feeding can easily become longer, so it is necessary to increase the feed used at this time. However, care must be taken not to use an excess of feed as this would cause the quality of the water to worsen and the breeding program to fail.

3. If too much feed is used it will often congeal on the surface of the water. This scum must be removed.

● Use of appropriate amounts of feed is the key to successful breeding. Using too much causes the water quality to worsen, but too little causes cannibalism among the shrimp.

● Causes of shrimp mortality:

a) poor water qualityb) unbalanced nutritionc) lack of feedd) shrimp being preyed upon while molting.

 

Factors affecting feed digestion and the cultivation environment

Water Temperature: At below 25 the amount of feed ingested tends to drop. Salinity: Between 9-19 0/00 there is no difference in feed ingestion, but under this level there is a marked drop.25 - 30 is most suited to the breeding of shrimp. Dissolved oxygen (oxygen saturation) level should be 85 - 120 %

 

Controlling the spread of Epistylis and other protozoa

1. Methyl blue solution (0.5-2ppm) added into the water for 3-5 days.

2. 0.3ppm of Copper Control to be used continuously.

3. 0.6ppm of Quinacrine HCl can control the spread of Epistylis Sp.

(ZC-100 is more specific)

 

Causes of Stress

● Water quality and collection of sludge on the bottom of the pond.

● The transference of fry from the hatchery to the pond.

● Inability to adapt to changes in pH levels and salinity and temperature changeswhen introduced into the pond.

● Problems with levels of ammonia, nitrates, nitrites, hydrogen sulfate, acidity and alkalinity, lack of dissolved oxygen in the water, hard water, and abrupt changes in temperature.

 

Methods for reducing stress

The problem often faced by shrimp farmers is isolating factors which cause stress to the shrimp. These factors include water quality and pollution of the pond bottom. In fact, perhaps 80-90% of diseases in shrimp cultures can be prevented by eliminating stress. Stress weakens the shrimp's immune systems, and makes them susceptible to various diseases.

Without proper acclimation and stocking, the shrimp larvae will die. Hatcheries should endeavor to maintain the salinity and pH levels of their water as near as possible to that of the pond where stocking will take place. The shrimp fry are often unable to adjust to the changes in these levels when they are transferred into ponds.

Late evening or early morning is the best times to stock the ponds. Stocking in the heat of mid-afternoon is not advisable. High temperatures and higher PH levels are harmful to the larvae.

The shrimp should be transported in cool temperature. Some cold water can be used to cool the outside of the bag which contains the shrimp fry. The water temperature of the pond should not differ vastly from the temperature at which the fries were transported. The temperature difference should not be more than 2 degrees, and the difference in pH levels should not exceed 0.5.The salinity difference should not be more than 2ppt. These parameters must be measured continually and carefully before the shrimp can be introduced into the pond. The shrimp fry should be gradually distributed over a large area of the pond.

 

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