Pisciculture Study Notes

Pisciculture

Aquaculture is the production of animals or plants in an aquatic environment (freshwater, brackish water, or marine).

It includes:

  • Pisciculture (fish farming): Salmon farming is the most well known (trouts, salmons)
  • Conchyliculture (shellfish farming): Includes oyster farming and mussel farming
  • Crustacean farming: especially shrimp and crayfish farming
  • Algae culture (algoculture)

Characteristics of Fish for Fish Farming

The chosen species should:

  • Have flesh appreciated by consumers.
  • Be hardy and easy to handle. Hardy means able to withstand artificial living conditions and robust to withstand a high concentration without being subject to epidemic diseases. The fish should also be easy to handle, without dangerous spines.
  • Be able to reproduce easily in captivity, either naturally in ponds or artificially in hatcheries.
  • Have rapid growth, which depends on the species, the feed, and the rearing conditions. Each fish can be improved by selection; however, poorly fed fish or fish in too great a number for the volume of water will remain small all their lives, consuming food unnecessarily. Predator fish are used in some ponds.
  • Have an economical diet. This generally implies fish with a short food chain, capable of exploiting plankton and plant foods. Carnivorous fish can only be used when by-products of animal origin are available in sufficient quantity, which is rarely the case in the tropics. Predator fish are used to limit the proliferation of species with high natural reproduction; they will consume fish that are too small to be marketable.

Reproduction of Fish

There are several methods of reproduction for farmed fish. The choice depends on the reproductive biology of the species, local environmental conditions, and available facilities. These methods can be classified into three categories:

  1. Natural Reproduction: Male and female fish are placed together in a spawning area, such as a small pond or enclosure, where they spawn naturally. This method is commonly used for inexpensive tilapia production.
    • Effective reproduction of some species may require modifications to the environment, such as a new supply of water and a significant rise in the water level of the pond for African catfish (Clarias), the presence of grassy vegetation used as an egg collector for common carp, or the presence of artificial nests for American and European catfish (Ictalurus and Silurus).
  2. Semi-Natural Reproduction: Fish (usually only females) initially receive an injection of chemical products, such as pituitary gland extract, which triggers the reproductive process. Males and females are then brought together in a specially prepared spawning area. The fertilized eggs are generally collected and then raised under privileged conditions, either natural or artificial.
  3. Artificial Reproduction: Females receive one or more injections of chemical substances to control the final maturation of the eggs at rest in the ovaries. Once these eggs have reached maturity, they are extracted from the females' bodies. Males also usually receive an injection. The eggs are fertilized artificially with the sperm of the males and raised under controlled conditions.

Each of these reproduction methods is primarily conditioned by a set of environmental factors (temperature, oxygenated water, food, light).

Preparation of Breeders for Spawning

Obtaining Breeders

The success of fish propagation requires healthy individuals of both sexes that have reached sexual maturity. This will be the stock of breeders.

Methods for Obtaining Breeders:
  • Capture fish in the wild using fishing gear and transport the live fish to the fish farm, where they will be stocked either in breeder ponds until they have reached sexual maturity or in holding ponds if they were captured during the spawning season and have already reached maturity.
  • Farm breeding at the farm itself, which facilitates the progressive improvement of the stock through careful management.
Management of Breeder Ponds

The farm ponds dedicated to breeders must be well adapted to the species, having easy access. Maintaining carefully controlled temperatures and well-oxygenated water will contribute to the success of the rearing. In addition, the fish must have an abundant supply of natural food, well adapted to the particular diet of the breeders. During the maturation period, it is necessary to provide a supplementary food consisting of protein-rich ingredients. The stocking density of the breeders must be adapted to the food availability but should always remain relatively low.

It is preferable to keep a stock of relatively young breeders, consisting of small to medium-sized fish depending on the species. Fish of this type produce more eggs of better quality, with better food utilization efficiency.

Choosing Appropriate Breeders

When the breeding season arrives, the breeders must be carefully chosen. Only fish ready to spawn should be used. It is advisable to choose fish with the following characteristics:

  • The abdomen of the males must release a few drops of milt when slightly compressed.
  • The genital opening of the females must be swollen and protuberant, of a pink/reddish color; their abdomen must be rounded and soft, indicating the presence of gonads that have reached the dormant stage.

When there is a risk of aggressiveness in males (for example, in the case of catfish) or uncontrolled spawning (tilapia and common carp), fish of both sexes should be placed in separate ponds after being selected.

Use of Pituitary Gland Extracts

The use of semi-artificial or artificial reproduction methods involves the use of chemical substances (or hormones) that play a decisive role in ovulation, that is, the final maturity of dormant eggs. These chemical substances, gonadotropins, are produced, accumulated, and stored in the pituitary gland of the fish, called the hypophysis, during their passage to the stage of sexual maturity.

This small pituitary gland is located in the upper part of the fish's head, on the ventral side of the brain. The collection of these glands taken from fish that have reached maturity poses no difficulty, nor does their storage for later use, if necessary, and the extraction of the gonadotropic hormones they contain.

It is particularly important to collect pituitary glands from properly selected fish, to be sure that these glands contain enough gonadotropins to be effective.

It is necessary to choose fish with the following characteristics:

  • Sexual maturity.
  • Fish preferably alive or recently killed.
  • Appropriate size.

The pituitary gland can be taken from a recently killed fish in two ways: by cutting and opening the head, or by removing the pituitary gland with a drill. The use of a wooden frame facilitates the operations performed on the fish's head to cut it or drill a hole in it.

Fish Sexing & Pituitary Gland

Freshly taken hypophyses will be stored, collected in the same bottle (all hypophyses taken the same day) and stored in acetone, to be renewed every 8 hours for 24 hours.

The chosen spawners must be prepared for hormonal injection.

  • Prepare the holding ponds for the male and female spawners that will receive an injection on the same day. Check that the chosen individuals are ready to spawn and not far from the spawning area.
  • Store them in a net or in a tank with a good water supply, keeping males and females separate.
  • Prepare all the necessary material and the fish

If possible, anesthetize the fish to be injected by placing them in a chemical solution suitably chosen according to their size. The fish under sedation should be carefully monitored at all times and, if necessary, quickly replaced in properly aerated water.

  • Inject the hormone solution slowly at a 45° angle, depending on whether the fish have scales or not.
  • Immediately after the injection, replace the fish in properly aerated water. Spawning usually takes place a few hours later.
Egg Maturation

The time required for complete maturation of the initially formed eggs (also called the ovulation period) is expressed in the number of hours required at the ambient water temperature. This indication is measured in degree-hours (dh).

During the ovulation period, the temperature of the water (in °C) should be measured at the end of each hour and the values obtained successively added. When you are about to reach the prescribed number of degree-hours, the fish are ready to spawn.

Induced Spawning and Egg Collection

Induced spawning can take place in different types of spawning enclosures (for example: small earthen spawning ponds, fine-meshed net cages, hapas, fiberglass, brick, cinder block or concrete ponds).

When stocking the spawning ground:

It is always preferable to introduce a greater number of males (M) than females (F) into the spawning ground, for example (1 F/2 or 3 M) or (2 F/3 M) or (3 F/4 M), in order to ensure effective reproduction

The total number of spawners to be placed in the spawning ground depends on the dimensions of the spawning ground and the size of the spawners.

As soon as spawning has taken place, the spawners are usually removed from the spawning ground and stocked in a pond for later use.

Collection of Fertilized Eggs

The choice of method to use to collect fertilized eggs after spawning depends on the type of eggs obtained.

Non-adhesive eggs, such as floating Chinese carp eggs and semi-floating Indian carp eggs, are easy to collect either inside the spawning ground if it is not too large, using a fine-meshed dip net, or outside by filtering the evacuated water through a fine-meshed net.

Adhesive eggs, such as those of common carp and catfish, must be collected using egg collection devices. After spawning, these collectors fitted with adhesive eggs are generally transferred to another enclosure where incubation and hatching take place.

Incubation and Hatching of Eggs

The development of the eggs begins just after their fertilization and in contact with water until the hatching of the fish larvae. This is the incubation period.

Incubation comprises three main phases of egg development.

  • The swelling phase: the dry fertilized eggs absorb water to hydrate and the perivitelline space develops.
  • The phase of cell division and germ development: the unicellular animal pole subdivides into 2, 4, 8, 16 and 32 cells successively, which produce a blastoderm then the morula phase.
  • The phase of embryo development: the fish embryo forms around the yolk. The head and tail are formed, and the eyes can be distinguished. The movements intensify until the eggshell is broken and hatching occurs.

Eggs are said to be "embryonated" as soon as the eyes of the larva can be distinguished, in the form of two black dots.

The proper development of the egg requires a favorable environment, depending on the species:

  • Adequate water temperature, close to optimal values
  • Good water quality, rich in oxygen and without toxic chemicals
  • Adequate water Renewal to ensure a good oxygen supply and waste removal;
  • Disturbances reduced to a minimum: Noise, shocks, violent water currents
  • Reduced light intensity and protection from sunlight

If possible, sort and remove dead eggs as they may become a source of fungal and bacterial infection of live eggs.

Characteristics of Eggs in Good Condition and Defective Eggs

Healthy Eggs

  • Transparent
  • Bright
  • Clear content

Defective Eggs

  • Opaque
  • Whitish
  • Cloudy content

Incubation of eggs should begin as soon as possible after fertilization, and should not be transported while they are vulnerable.

Duration of the Incubation Period

The time required depends mainly on the species considered, as well as the temperature and dissolved oxygen content of the water, commonly expressed in degree-days.

It is possible to estimate the duration of the incubation period at water temperatures close to optimal values. The period is shorter the higher the water temperature is within this range of variation.

Avoid temperatures that are too high, which may cause deformations and a low egg survival rate; it is preferable to incubate the eggs more slowly to obtain better quality larvae.

Larval Rearing

Larval rearing occurs from the hatching of the eggs until the larvae become what are  generally called young fry. They fill their swim bladder with air; begin to swim  like adult fish, and begin to eat exogenous food.

This larval period is marked essentially by the development of the alimentary and respiratory organs. The yolk sac provides the substances necessary for growth and development. Its size decreases progressively until complete disappearance, a little after the larvae have begun to absorb outside food. The duration of this period therefore generally depends largely on the initial size of the yolk sac, which varies from one species to another, and on the rate of development of the larvae, which depends above all on the temperature of the water. For each species, there is an optimal temperature range, similar to that defined for egg incubation. Similarly, the duration of the larval rearing period is defined in degree-days (dj), like the incubation period. This duration is generally three to four days for most warm water fish, but it is longer for colder water fish.

Conditions for Successful Larval Rearing
  • Appropriate water temperature, with as few fluctuations as possible
  • Oxygen-rich environment (preferably saturated), especially for passive larvae (the water flow should not be too high, however).
  • Clean environment (egg shells and defective eggs must be removed).
  • Availability of adequate food, carefully dispersed among the larvae.

The behavior of the larvae may  during their development. It will therefore be necessary to modify the conditions of breeding accordingly.

Transfer of Larvae and Young Fry

In the case of certain fish, for example Chinese carp, it is possible to automatically transfer the newly hatched larvae directly into a rearing device, by the technique known as "swimming out".

Principles of Feeding for Marine Fish in Intensive Farming

Introduction

In contrast to extensive or semi-intensive farming systems where the fish finds all or part of its food from the natural environment, fish in intensive farming are totally dependent on an external supply (complete diet) and this throughout the entire rearing cycle.

Traditionally, complete diets are in the form of granules, or semi-moist foods prepared from various ingredients that come as close as possible to the nutritional needs of the fish, resulting in maximum growth. Alternatively, a complete food may consist of a single food with high nutritional power (e.g. fish waste, living organisms, Artemia, or a combination of the two).

For the majority of farmed marine fish, information on basic nutritional requirements is insufficient, as opposed to intensive terrestrial farming where dietary needs with regard to proteins, essential amino acids, essential fatty acids, vitamins and minerals are well identified.

Until now, to remedy this lack of information, a high-nutrition diet based on fresh or frozen fish (compound dry food) has been used. The use of such food is economically justified by good management and by the high market price of the species being farmed (e.g. sea bass, sea bream, turbot, sole, eel).

The Essential Principles of Feeding

From a nutritional point of view, the principles governing the feeding of fish in intensive farming are as follows:

  1. The food must be appetizing and consumed with a minimum of effort and waste.
  2. The composition of the food comes as close as possible to the known diet necessary for that fish.
  3. The food is digestible, keeps the fish healthy and allows it to grow normally.
  4. Ideally, the food should be efficiently converted into new tissue and therefore in the form of fish growth (e.g. optimal conversion rate).

In addition, in practice, from a commercial (breeder) point of view, the choice of a particular diet is based on the evaluation of 3 criteria (intensive farming)

  1. Availability of food and its transport
  2. Qualities of the food
  3. Cost per unit of production/unit of time of the food and feeding and the rate of return.

For example, 4 basic hatchery feeding strategies are commonly available for the intensive rearing of marine fish larvae, from first feeding through metamorphosis and up to the post-larval stage. This includes:

  1. The exclusive use of a succession of living planktonic organisms (e.g. algae, diatoms, flagellates, yeasts, rotifers, nauplii of Artemia salina).
  2. The use of selected live or frozen plankton in combination with fresh or frozen fish, mollusk or crustacean waste.
  3. The use of selected live or frozen plankton in combination with a dry food or a "formulated" diet.
  4. The exclusive use of larval diets from microcapsules or microparticles.