3. NATURAL PROPAGATION OF FINFISH

3.1 Significance of Propagation

Fishes, as other living organisms, must adapt to the environment if they are to survive and flourish. The most vital point in this struggle for existence is their ability to successfully propagate repeatedly during their life span and add a large number of offspring to the population. The survival of a sufficient number in the face of innumerable hostile environmental factors holds the key to their success, since at this stage they are weakest and most vulnerable.

3.2 Biology of Propagation

The propagation habit is the most vital adaptation of the fish to its environment with respect to survival. For successful propagation, the place where eggs are released should have optimal conditions with respect to oxygen, temperature, food, etc. and should be almost free of enemies. Those fishes unable to find such conditions have gradually been eliminated.

The propagation habit of any fish runs according to a more or less predetermined pattern and is connected with some sort of parental care. The parental care in some fishes is hardly traceable, while in others it is highly developed. In the latter case, either one of the parents or both take care of the eggs, larvae and fry. The nature of the propagation habit of any fish is determined by the age or time of sexual maturity, the season of propagation, the place of propagation, and the extent of parental care (Figure 1).

3.2.1 Age of sexual maturity

Some fishes, such as Tilapia spp., become sexually mature within a few months, while others may take as long as a few years. Sexual maturity depends on several factors. Sexual maturity is delayed in cold climates, while it is accelerated in warmer environments. This aspect is very well exemplified by the common carp and the Chinese carps. The common carp usually becomes sexually mature in its first year in tropical and subtropical regions. On the other hand, the same carp takes three years to mature in Central Europe and four years in northern Europe. Chinese carps become sexually mature in their second or third year (sometimes even in their first year) in tropical and subtropical regions. In Europe, however, they may take five to seven years and may have attained a weight of 5–10 kg. It is known that fish which reproduce two or more times a year mature earlier than the season-bound spawners; i.e., those which spawn only once a year during a particular season.

Table 1

Fishes Commonly Cultivated in Warm and Temperate Freshwater Ponds

^a Parenthetically stated names refer to common names
^b T. mossambica, T. andersonii, T. galilea, T. nigra, T. sparmanii, T. variabilis, T. hornorum, T. macrochir, T. macrocephala, T. zillii, T. nilotica, T. rendalli, T. leucostiota, etc.

3.2.2 Season of propagation

Some species of fishes reproduce two or more times a year. Such spawners usually exhibit well-developed parental care, which ensures the survival of offspring in spite of numerous adverse environmental conditions.

Among the cultured fishes, Tilapias are year-round spawners; this creates problems for the culturist. The gonadal development of these fish takes its own course and is hardly influenced by food and temperature. Spawning starts when the eggs mature and as soon as the female finds its male partner.

The season-bound spawners propagate only during a particular season of the year. However, they may spawn more than once during that season, as in the case of wild common carp. The development of gonads of the season-bound spawners proceeds only to a certain stage following which the gonad lies dormant till the advent of suitable environmental conditions. This dormant phase may last several months. The advent of the appropriate season triggers further gonadal development, which finally results in propagation. This final part of egg development, once triggered, cannot be stopped or turned back. If the changes in the environment are not strong enough to trigger further development of the eggs, the dormant phase continues until one of the environmental factors (i.e., oxygen or temperature) changes for the worse, whereupon the resorption of the eggs begins. During that particular year the fish has no further chance to spawn.

It often happens in nature that the spawning of ripe fish fails to come about. If a river spawner is kept enclosed in confined water, its gonad develops only up to a certain stage and remains dormant until resorption sets in. This process can be repeated year after year, without ever leading to spawning. However, the propagation of such fishes can be achieved by the artificial induction of ovulation at the proper time. After spawning, the development of new eggs starts immediately and proceeds until it reaches the resting phase. By resorting to the artificial inducement of ovulation, the same fish, which is by nature a season-bound, one-time spawner, can be propagated two or three times in a year.

Most freshwater fishes spawn during spring, while the others spawn when rivers and lakes are flooded. Tropical and subtropical fishes spawn during the rainy season when the offspring have a better chance for survival in the fast-flowing turbid waters.

It is interesting to note that many of the carnivorous (predatory) fishes spawn earlier in the season than the bulk of non-carnivorous fishes. This ensures an adequate supply of prey for the young predators.

3.2.3 Place of propagation

Freshwater fishes are known to spawn in three different types of sites: (1) confined waters, (2) flowing waters, and (3) inundated terrains. Within these main sites there are many distinguishable spawning sites chosen by different fishes according to their spawning habits (Figure 2).

3.2.3.1 Spawning in confined waters. In confined waters, the chosen places for spawning are often different for different species. Those with adhesive eggs, such as the European cyprinids and pike, scatter their eggs on submerged weeds, stones, or gravel. The sciaenid Plagioscion squamosissimus scatters its floating eggs in the water column, where they develop further. Some fishes, such as pike-perch, European catfish, channel catfish, etc., lay their eggs in one place adhered together in a clump or on some sort of a nest. There are fishes which spawn in holes and gaps found in the clay and rocks (e.g., Plecostomus).

The nest spawners search and find objects (bushy roots of water plants or trees, rocks, etc.) which they clean free of silt and debris and then make their nest. Some build their nests by collecting nest material piece by piece (e.g., some species of Tilapia, the giant gourami, etc.), while some others, such as Trichogaster, Hoplosternum spp. of the Far East and of South America, make nests of foam (bubble nests). Mouth-breeding is a very effective method of parental care, as found in Tilapia.

In general, there are very few tropical or subtropical pond spawners which abandon their eggs and leave them to the mercy of nature. Most of them practise active parental care by guarding, defending, and aerating the developing eggs and larvae. Some guard their fry as well.

3.2.3.2 Spawning in flowing waters. Spawning in flowing waters has a definite advantage in that the turbid, continuously moving water under flooded conditions very effectively protects the eggs and larvae. The non-adhesive, floating, semi-floating, or rolling eggs get enough oxygen for their development, and the hatchlings are well concealed from predators. The water current causes the eggs and larvae to drift downstream and toward the shore. Many of them are taken to inundated areas, which are rich in food organisms required for the developing fry and fingerlings.

Many cultured fish are by nature river spawners, even though they grow well in confined waters. They migrate in large shoals or in pairs upstream when the river starts to swell and spawn there when the preconditions are suitable. Some lay their eggs on certain objects on the river bed, such as roots, branches or leaves of trees, stones, gravel, etc., where the turbid water protects the egg mass from being discovered by their enemies.

Among the cultivated and the potentially cultivable river spawners with non-adhesive, floating, or semi-floating eggs, mention may be made of the Chinese major carps, Barbus spp., Pangasius spp., and members of Characidae, Anastomidae, and Prochilodontidae. This type of spawning habit is very common among the fish of tropical and subtropical rivers.

3.2.3.3 Spawning in freshly inundated areas. The freshly inundated field is an ideal place for spawning and for growth of the juveniles. It is nearly free of their enemies because the inundating water kills terrestrial fauna and there is insufficient time for aquatic fauna (predators) to develop. The water is usually warm and rich in oxygen, factors that favour the rapid development of eggs and larvae. A rich micro- fauna and flora develops on the decaying remnants of terrestrial plants and this provides the fry and fingerlings with abundant food. Fish spawning in this environment generally have adhesive eggs and their larvae are mostly of the hanging type.

Prominent among fish breeding in inundated areas are the common carp and other European cyprinids, the buffalo fish of North America, and the Indian major carps.

3.2.4 Parental care

Parental care is a very important adaptation among fish for ensuring the survival of their offspring. The parent fish look after their offspring during their most critical stage of life when they are defenceless and very sensitive.

In a broader sense, almost every fish practises some sort of parental care, either passive or active.

3.2.4.1 Passive parental care. This is actually the “hereditary foresight” of the females to provide more yolk for the embryo to sustain life for a long time or to place the eggs on such sites where the optimum environmental conditions are met (or are most probably met) and beyond the reach of enemies. Some fish have in their eggs a poisonous substance which keeps predators away.

3.2.4.2 Active parental care. In active parental care, either one or both of the parents take an active part in caring for and defending their eggs, larvae, and sometimes the fry as well. This includes the selection and preparation of a suitable place for depositing the eggs, selection of a good substrate to which the eggs can adhere, collection of nest making materials, and preparation of the nest (Figure 3).

The so-called “nest cleaners” have the most primitive type of “nest”, consisting of just the bushy roots of plants which are only cleaned by the fish. There is no actual collection of nest building materials involved nor actual nest building. On the other hand, the “nest builders” collect pebbles, leaves, roots, etc. and build their nest (e.g., giant gourami). Some of the gouramis prepare their nest out of objects which are cemented together with a foamy sticky substance. The “foam nest builders” make their nest out of foam, which being unpalatable, protects the eggs hidden in the foamy mass (e.g. Trichogaster spp.). Then, there are the mouth breeding cichlids (Tilapia leucosticta, T. galilea, T. macrochir, T. nilotica, T. variabilis, T. macrocephala, Haplochromis spp., Astatoreochromis alluandi, Serranochromis spp., Petenia spp., etc.). These fishes take the eggs into their mouth, and keep them there until they hatch. Among the tilapias, the female incubates the eggs in the case of T. leucosticta, T. macrochir, T. nilotica, and T. variabilis; the male in the case of T. macrocephala and both the sexes in the case of T. galilea. In the case of the pearlspot, the eggs are attached to the underside of submerged objects, where they are fertilized and remain until hatching. The newly hatched larvae are taken to nearby shallow pits prepared by the male and are shifted from one pit to another about once a day. Later, the fry are guarded by the female. The non-mouth-breeding tilapias (T. zillii, T. melanopleura, and T. sparmanii) deposit their eggs on stones or other substrata and zealously guard the eggs and larvae.

Apart from depositing the eggs in a safe and congenial place, parental care extends further to the aerating and protection of eggs, and in some cases the protection of the spawn and fry as well. Aeration is done by one of the parents by producing a continuous water current over the eggs by means of its fins, the parent fish guarding its eggs or larvae from small enemies and larger predators. It also cleans the developing eggs and removes the unfertilized ones.

In general, the following conclusions can be drawn with respect to the influence of parental care on the number of eggs produced:

1. fish with passive parental care produce more eggs than those exerting active parental care;

2. fishes with active parental care produce far fewer eggs than those which do not exhibit parental care, and

3. fishes that abandon their eggs after laying them produce more eggs compared to those not abandoning their eggs.

The number of eggs produced per kg of body weight depends on the size of eggs. Fish producing very small eggs (0.3–0.5 mm in diameter) produce 500 000–1 000 000 eggs per kg of body weight, while those with eggs of medium size (0.8–1.1 mm) produce only 100 000–300 000 eggs per kg of body weight. Those with larger sized eggs, 1.5–2.5 mm, produce only about 5 000–50 000 eggs per kg of body weight.

In culturing fishes with well developed parental care, the fish culturist need not bother about artificial propagation. However, these naturally propagating fish, such as tilapias, often pose a major problem by overpopulating the pond. Therefore, the fish culturist has to take certain measures to prevent excessive propagation. Monosex culture or culturing them with some predator species is often practised.

Hence, such fishes (tilapias and others) have been excluded from the scope of this manual.

Figure 1 Adaptation for the survival of the species - Reproduction

Figure 2 Spawning places of freshwater fishes

(A) Oxygenating eggs	(E) Nest building
(B) Cleaning eggs	(F) Foam nest building
(C) Keeping off predators	(G) Incubating in mouth
(D) Attacking predators	(H) Laying eggs in a protected place

Figure 3 Principal types of parental care in fishes

3.3 Development of the Sexual Products

The development of sexual products (ova or eggs and spermatozoa or sperm) in the gonads is a long and complicated process in which several stages or phases can be differentiated (Figure 4).

3.3.1 Development of eggs

The entire course of development of the eggs is distinguishable into the stages listed below (Figure 5). The sizes of egg cells at their various stages of development, as indicated below, refer to those of the common carp:

Stage I: The primitive egg cells (ovogonium or archovogonium) are very small, their size being hardly bigger than that of other cells (8–12 microns). They multiply by normal mitosis.

Stage II: The egg cells grow to a size of 12–20 microns, and a follicle begins forming around each egg cell. The follicle, whose function is to nurture and protect the developing egg, eventually becomes a double layer of cells.

Stage III: During this stage, the egg cell grows significantly larger to attain a size of 40–200 microns and becomes enclosed by the follicle.

These first three stages mark the period prior to the accumulation of nutrients in the developing eggs.

Stage IV: During this stage the production and accumulation of the yolk begins; this is a process known as vitellogenesis. The egg continues to grow to a size of 200–350 microns with the accumulation of drops of lipoid materials in its cytoplasm.

Stage V: This marks the second phase of vitellogenesis. The cytoplasm is now full of lipoid drops and yolk production begins. The egg size reaches 350–500 microns.

Stage VI: This is the third phase of vitellogenesis, during which the yolk plates push the lipoid drops toward the edge of the cell where two rings begin forming. The nucleoli, which take part in protein synthesis and the accumulation of nutrients are seen adhering to the membrane of the nucleus. The size of the egg is now 600–900 microns.

Stage VII: The process of vitellogenesis is completed during this stage and the egg attains a size of 900–1 000 microns. When the yolk accumulation ends, the nucleoli withdraw into the centre of the nucleus. The micropyle (a small opening on the egg shell) develops during this stage.

Stages IV, V, VI, and VII are the stages of vitellogenesis, when yolk is synthesized and accumulated in the egg cell. The egg is now materially ready. To reach this stage of development, the female fish needs a lot of protein in its food and a favourable temperature range.

On the completion of stage VII, the egg may remain as such for several months without any change, and this forms the “dormant” or “resting” phase.

This resting or dormant phase will either end in ovulation if favourable conditions occur, or in follicular putrefaction and resorption in the absence of such conditions (Figure 6).

It is interesting that the ovary has some branching blood vessels, but is not rich in capillaries. This means that the lymphatic sinuses play the main role in the transportation of materials such as hormones, lipoids, amino acids, oxygen, carbon dioxide, etc. to and from the ovarian wall and the egg cells.

Further development of the eggs toward ovulation (final ripening) is regulated by gonadotropin hormones, which are formed and stored in the pituitary gland or hypophysis. Some hormones of the hypophysis, such as FSH (follicle stimulating hormone) and LH (luteinizing hormone), are continuously produced and secreted into the blood stream. On the other hand, the steroid type of hormones (oestrogens) secreted by the oapsule (theca) of the follicle “inform” the brain about the stage of egg development.

When the environment changes for the better, the fish starts gathering, through its sense organs, all the necessary information about environmental conditions; e.g., temperature, suitable place for spawning, water current, floods, presence of fish of the opposite sex, etc. This sensory information accumulates in the hypothalamus of the brain and when a certain threshold level is reached the hypothalamus gives an order through a hormone (gonadotropin-releasing-horwone, GRH) to the hypophysis to release gonadotropins into the blood system. The gonadotropins, thus released, reach the gonad and trigger the preovulation process and final ovulation (Figure 7).

The first effect of gonadotropins on the egg is the movement of its nucleus toward the micropyle. This is followed by what is termed as “hydration”, wherein the eggs absorb water. All these are completed during the preovulation stage.

After preovulation, the membrane of the nucleus disappears, the chromosomes become visible, and the first cell division of meiosis occurs (during which the total number of chromosomes is reduced to half). At the same time, the follicle keeping the egg fixed on to the wall of the ovary gets dissolved by enzymes and the “ripe for fertilization” egg falls into the cavity of the ovary. The second cell division of meiosis normally takes place in the presence of the sperm, which intrudes into the nucleus of the egg through the micropyle. Therefore, the presence of the male pronucleus is necessary for the development of the female pronucleus.

3.3.2 Development of sperm

The process of development of sperm is far less complicated than that of eggs.

The primitive spermatogonia propagate actively by mitosis in the wall of the tubules of the testis. From the spermatogonia the primary spermatocytes develop, each of which later gives rise to two secondary spermatocytes. Each secondary spermatocyte gives rise to two spermatozoa or sperm. The sperm collect in the cavities of the tubules of the testis and remain there in a dormant stage until the onset of suitable environmental conditions, when, on gonadotropin command, the male becomes ready for spawning. The spermatozoa in the testis are capable of fertilizing eggs even when they are in the dormant stage. The sperm are motionless in the testis but become motile when they come in contact with water. The motile period of the sperm is very short and depends on the temperature of the water. The sperm of warmwater fishes move actively by their filamentous tail for only about half or one minute. The fish sperm are very small, their estimated number in one cubic centimetre of milt being about 10 000–20 000 million depending on the denseness of the milt.

3.4 Fertilized Egg

The eggs that have fallen into the cavity of the ovary do not have definite shape. The egg shell is soft and tightly surrounds the cell “kernel”, which includes the nucleus and the mass of yolk. When the ripe egg falls into the water it assumes a round shape and within a short time begins to swell. The water penetrates between the shell and kernel of the egg and thus the perivitelline space develops. The micropyle closes within a minute, after which no sperm can enter the egg. The swelling usually lasts for about one-two hours. The egg then takes its final form and becomes water-hardened. The swelling, however, does not alter the size of the kernel.

The fertilized fish eggs can be of different types. However, for practical purposes, two principal categories can be distinguished; viz., non-adhesive and adhesive (Figure 8).

3.4.1 Non-adhesive eggs

Non-adhesive eggs can be further distinguished into the following types, on the basis of their specific weight:

1. buoyant eggs (whose specific weight is slightly less than that of water),
2. floating eggs (whose specific weight is slightly higher than that of water),
3. semi-floating eggs (whose specific weight is higher than that of water), and
4. rolling eggs (whose specific weight is higher than that of water).

The specific weight of eggs is determined by the size of the perivitelline space and the specific weight of the kernel. The kernel can be heavy when there are no oil droplets, or light in the presence of one or more oil droplets.

3.4.2 Adhesive eggs

These eggs have an adhesive layer on their shell, which becomes activated when the egg comes into contact with water. The adhesive layer sticks the eggs to some objects or to each other. Two types of attachment can be distinguished:

1. eggs sticking to objects, and
2. eggs sticking only to each other, forming a clump or egg mass.

The adhesiveness is sometimes very strong and the eggs suffer damage if torn off from their substratum. Sometimes the adhesiveness is quite weak and the eggs can be easily removed. There are several grades or variations between the two extremes.

The adhesiveness gradually weakens during the course of egg development. Higher salinity effects a negative influence on adhesiveness.

The fish eggs also vary in size. The factors which determine the size of the egg are the size of the egg “kernel”, the thickness of the egg shell and the size of the perivitelline space. The first two determine the size of the “dry” eggs (i.e., before they come in contact with water), while the third determines the size of the water-hardened eggs.

Further details regarding the processes of fertilization, swelling, germ and embryo development, hatching, larval development, fry development, etc. are provided in the section dealing with practical hatchery work.

Figure 4 General pattern of development of sexual products in fishes

Figure 5 Environment and maturation of fish eggs

Figure 6 The fate of developing eggs

Figure 7 The course of natural spawning

(A) Non-adhesive eggs

(B) Adhesive eggs

Figure 8 Types of finfish eggs