Ch03

When earthen ponds are used for the large-scale production of carp juveniles, these become part of the biological production cycle.

Solar energy, through photosynthesis, contributes to the production of vegetal organic matter in the ponds: this phytoplankton forms the basis for the development of a mass of minute animals known as zooplankton. Juvenile carp feed voraciously on zooplankton, choosing the size which best corresponds to their own mouth size.

Predators consume small carp and losses can be great.

11. Looking more closely at the biological cycle, it can be seen that several other factors are involved in determining the productivity of an earthen pond.

(1) Soil erosion washes in nutrients, and (2) bacterial activity at the bottom of the pond releases others. (3) The organic matter of the bottom mud, which consists of thousands of dead organisms, is in this way partly recycled into the production system. (4) Oxygen in the water originates mostly from absorption at the surface and from plant photosynthesis. (5) Carbon dioxide (CO₂) is generated through the respiration process of animals and utilized by plants.

When no juvenile carp are present, the volume of mud in the ponds steadily increases.

12. On the other hand, when juvenile carp are present, plankton and the organisms at the bottom of the pond are consumed regularly and transformed into fish flesh, as shown by the yellow arrows.

The biological cycle (1-5) remains basically the same, except for this accumulation of energy within the fish and their release of metabolic by-products in the form of urine, faeces and carbon dioxide.

13. In the following pictures, a closer look is taken at some of the living organisms which most influence the biological production of juvenile carp in earthen ponds.

These microscopic green algae belong to the phytoplankton. Some examples are species of (1) Pediastrum, (2, 3) Crucigena, (4, 6, 8) Scenedesmus, (5) Chlorella, (7, 9) Tetraedron, (10) Richteriella, and (11) Gloeococcus.

14. High densities of both blue-green algae and of flagellates are undesirable in rearing ponds.

(4-8) Blue-green algae indicate unbalanced environmental conditions. (1-3) Flagellates develop well in the presence of water pollution.

Some examples of flagellates are species of (1) Trachelomonas, (2) Phacus and (3) Euglena. As blue-green algae: (4) Anabaena, (5) Aphanizomenon, (6 ) Dactilococcopsis, and (7, 8 ) Microcystis.

15. The slow-moving rotifers are the most important group of microscopic animals for the rearing of early fry. They belong to the zooplankton.

Some examples are species of (1, 3) Keratella, (2, 4-7) Brachionus, (8) Asplanchna, (9) Filina, and (10) Synchaeta.

16. The small cladoceres (0.2-0.5 mm) gradually gain in importance as the fry grow in size.

They also belong to the zooplankton and they may compete with the rotifers for food. Some examples are species of (1) Ceriodaphnia, (2) Bosmina, (3) Diaphanosoma, (4) Chydorus, (5) Scapholeberis, and (6) Alona.

17. Larger cladoceres become an important natural food supply during the second stage in the fry rearing process.

Before this, they are undesirable as they compete for food with smaller zooplankters. Some examples are species of (1-3, 5) Daphnia, (4) Simocephalus, and (5) Moina.

18. (A) Adult copepods are zooplankton predators which are very dangerous for early carp fry.

Some examples are species of: (1) Cyclops, (2) Diaptomus, and (3) Canthocamptus.
On the other hand, (B) juvenile copepods could be very useful natural food for carp fry, but in practice they cannot be separated from the adults.
Several juvenile stages exist such as: (1) the nauplius and (2) the copepodite stages.

19. Larvae and pupae of mosquitoes (1, 2) and chironomids (3) are an important source of food for the advanced carp fry.

The mosquito larvae and pupae live near the water surface, hidden among the aquatic vegetation such as, for example, species of (1) Anopheles and (2) Culex.

Chironomid larvae and pupae such as (3) Chironomus plumosus live in the bottom mud of the ponds.

20. In pond areas where dense aquatic vegetation is present, large insect larvae may develop in great numbers, preying voraciously on juvenile carp.

These are, for example, the water beetle larvae (1) Hydrous piceus and (2) Dytiscus marginalis, as well as the dragonfly such as (3) Anax imperator.

21. (A) Predatory aquatic insects may also cause considerable losses among carp fry in the rearing ponds, if control measures are not applied.

Some examples are species of (1) Ranatra, (2) Corixa, (3) Notonecta, and (4) Nepa
(B) Losses may also result from small crustaceans such as species of (1) Triops, (2) Lepidurus, (3) Limnadia, and (4) Branchipus.

22.Important fish losses in the rearing may also be caused by vertebrate predators such as frogs, snakes, fish and birds.

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