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.
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).
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.
Fishes Commonly Cultivated in Warm and Temperate Freshwater Ponds
|Family||Speciesa||Natural distribution||Countries/ region where cultured||Food and feeding habits||Spawning habit||Remarks|
|1. Acipenseridae||Hybrid of Huso huso x Acipenser ruthenus||Rivers of Eastern Europe flowing into the Black Sea and Caspian Sea||U.S.S.R. and Eastern Europe||Bottom feeder||Can be artificially bred||Attains good growth in well-oxygenated ponds|
|2. Chanidae||Chanos chanos (milkfish)||Seas of the Indo-Pacific region||India and the Far East||Herbivorous, feeding on small plants, filamentous algae and detritus. At times, feeds on small animals too||Sea spawner, with pelagic eggs||Can tolerate wide ranges of temperature and salinity, grows well in freshwater, fry and fingerlings can be collected from coastal water|
|3. Plecoglossidae||Plecoglossus altivelis (Ayu)||Sea and fresh waters of Japan, Korea and China||Japan||Herbivorous, feeding on diatoms, blue-green algae, and algae attached to pebbles on river beds||Spawns in lower reaches of rivers||Has tiny adhesive eggs which hatch in 10–24 days|
|4. Coregonidae||(i) Coregonus peled (Peled, “Syrok”)||Lakes and rivers of Siberia (U.S.S.R.)||U.S.S.R.||Zooplankton feeder||Spawns in rivers during winter on gravelly and/or sandy bottom||Can tolerate extreme cold temperature|
|(ii) C. lavaretus||Northern Europe||Czechoslovakia||Zooplankton feeders||Winter spawners in deep lakes||Culture limited because of high oxygen demand and poor temperature tolerance|
|(iii) C. marena (white fish)|
|5. Anguillidae||(i) Anguilla anguilla (common eel)||European and Mediterranean coastal waters, coastal rivers and accessible lakes||Europe||Carnivorous||Sea spawner||A catadromous fish; larvae can be collected in estuaries; is being increasingly used in intensive and super-intensive culture; its artificial propagation is yet to be achieved|
|(ii) A. japonica (Japanese eel)||Japan, China and Vietnam||Japan and China||Carnivorous||Sea spawner||Is similar to A. anguilla with regard to migration, breeding, culture, etc.|
|6. Characidae||(i) Colossoma spp.||Amazon and Orinoco river systems in South America||Venezuela, Colombia, Peru, Ecuador and Brazil||Omnivorous, column feeders||River spawners||Fast growing; seed readily available in inundated areas adjacent to the rivers; induced propagation is likely to be achieved soon|
|(ii) Milossoma spp.|
|(iii) Brycon spp.|
|7. Anostomidae||Leporinus copelandi (Piava) and other species||Rivers of South America||South America||Omnivorous||River spawners||Have been bred through hypophysation in Brazil|
|8. Prochilodontidae||(i) Prochilodus argenteus (Curimata pacu)||Amazon river system||Brazil||Omnivorous, bottom feeder||River spawner, has non-sticky floating eggs||Can be bred through hypophysation|
|(ii) P. mariae (Coporo)||Orinoco river system||Venezuela||Detritus feeder||River spawner||Can be bred through hypophysation; fast growing|
|9. Citharinidae||(i) Citharidium spp.||Rivers in the Congo and Nigeria||-||Detritus feeders||River spawners||Cultivated in Africa|
|(ii) Citharinus spp.|
|(iii) Distichodus spp.|
|10. Catastomidae||(i) Catastomus commersonii (white sucker)||North America||North America||Bottom feeder||River spawner; spawns during spring in shallow waters||Cultivated only sparingly, likes clear water; can be bred through stripping around gravelly riffles; eggs are buried in loose gravel|
|(ii) Ictiobus bubalus (small mouth buffalo)||North America||U.S.A.||Bottom feeders||Spawn on submerged vegetation in inundated terrains (like common carp); can also be made to breed in spawning ponds||Cultivated only sparingly, as supplemental fish in catfish ponds|
|(iii) I. cyprinellus (big mouth buffalo)|
|(iv) I. niger (black buffalo)|
|11. Cyprinidae||(i) Cyprinus carpio (common carp)||Europe to China||Worldwide||Omnivorous, mostly bottom feeding||Spawns on submerged vegetation in freshly inundated terrain or in ponds||Fast growing; artificial and semi-artificial propogation techniques are commonly employed on a wide scale|
|(ii) Ctenopharyngodon idella (grass carp)||Siberia and China (Amur River)||In many countries throughout the world||Herbivorous, feeding on submerged and floating plants; can also feed on green loams, terrestrial grass, and artificial feed||River spawner; can be bred through hypophysation||Fast growing; can be cultured both in temperate and tropical regions; useful in polyculture and biological control of weeds|
|(iii) Hypophthalmichthys molitrix (silver carp)||Siberia and China (Amur River)||In many countries throughout the world||Phytoplankton feeder; column feeder||River spawner; can be bred through hypophysation||Fast growing; useful in polyculture; can tolerate a wide range of temperature|
|(iv) Aristichthys nobilis (bighead or bullhead carp)||Siberia and China (Amur River)||In many countries throughout the world||Feeds on bigger algae and zooplankton; column feeder||River spawner; can be bred through hypophysation||Fast growing; useful in polyculture; can tolerate a wide range of temperature|
|(v) Mylopharyngodon piceus (black carp)||South and Central China||Malaysia, Vietnam, Thailand and Japan||Feeds on water snails||River spawner; can be bred through hypophysation||Fast growing; useful in polyculture; can tolerate a wide range of temperature|
|(vi) Cirrhinus molitorella (mud carp)||South and Central China||China, Taiwan, Thailand and Malysia||Scavenger||River spawner; can be bred through hypophysation||Fast growing; useful in polyculture; can tolerate a wide range of temperature|
|(vii) Megalobrama amblicephala (wuchan fish)||Rivers of China||China||Feeds on planktons when young; adults herbivorous||River spawner||Cultivated as associate fish in China, along with major Chinese carps|
|(viii) Parabramis pekinensis (white amur bream)||Rivers of China||China||Feeds on zooplankton when young and later on vegetation and animal matter||River spawner|
|(ix) Catla catla (catla)||Rivers of India||Bangladesh, India, Pakistan, Burma, Nepal, Malaysia and other Far Eastern countries||Surface plankton feeder, feeding mainly on zooplankton||Spawns in inundated terrain; can be bred by hypophysation||Fast growing; large number of spawn and fry of this and other Indian major carps are regularly collected from rivers and they form the mainstay of Indian fish culture|
|(x) Labeo rohita (rohu)||Rivers of India||As above||Column feeder; feeds also on vegetable matter||As above||As above|
|(xi) Cirrhinus mrigala (mrigal)||Rivers of India||As above||Bottom feeder; omnivorous||As above||Of medium growth rate|
|(xii) Labeo calbasu (kalbasu)||Rivers of India||As above||Omnivorous; also feeds on detritus||As above||Of medium growth rate|
|(xiii) Puntius spp. (P. javanicus, P. gonionotus, P. carnaticus, P. belinka, P. orphoides and P. schwanefeldi)||Rivers and other freshwater bodies in the Far East||Indonesia, Malaysia, Thailand, India, etc.||Omnivorous||Some are river spawners, others also spawn in ponds; can be artificially bred||Of medium growth rate|
|(xiv) Tinca tinca (tench)||Europe and western Siberia||Europe, India, Indonesia, Japan, and Australia||Omnivorous, bottom feeder||Pond spawner; can be bred artificially||Cultured as an associate fish in carp ponds in Europe|
|(xv) Tor tor (mahseer)||India and Nepal||India||Omnivorous||River spawner; can be bred artificially||At present used mainly for restocking rivers|
|(xvi) Osteochilus hasselti (nilem)||Far East||Indonesia, Malaysia and Thailand||Feeds on phytoplankton and soft or decayed leaves of higher plants||Spawns normally in rivers and lakes; also breeds in special spawning ponds with strong flow of water||Cultured in Far East|
|12. Siluridae||Silurus glanis (sheat fish or European catfish)||Danube River system, Europe||Europe||Carnivorous||Confined water; can be artificially bred||Used as carnivorous fish in European carp ponds; rearing of fry and juveniles is not easy|
|13. Ictaluridae||Ictalurus punctatus (channel catfish)||North America||U.S.A. and Central American countries||Carnivorous, as well as omnivorous; readily accepts pellettes feed||Spawns on inundated terrain; can be induced to breed in confined waters by providing egg receptacle, or through hypophysation||Suitable for intensive and super-intensive cultures|
|14. Claridae||(i) Clarias batrachus (magur)||S.E. Asia and Indian subcontinent||Thailand, India, Pakistan, Malaya, and Vietnam||Zooplankton feeders in early stages; omnivorous scavengers later on||Spawn in confined waters; natural spawning can be promoted by digging holes of 20–30 cm diameter near the banks; can be readily spawned through hypophysation||Possess accessory respiratory organs and are hence capable of surviving in oxygen-deficient water and for several hours out of water|
|(ii) C. macrocephalus|
|(iii) C. lazera||Africa||Africa||Omnivorous||Can be bred through hypophysation||As above|
|(iv) Heteropneustes fossilis (Singhi)||India, Pakistan, Sri Lanka, Burma, Thailand, Kampuchea and Bangladesh||India and Pakistan||Carnivorous||Pond spawner; can be bred through hypophysation||As above|
|15. Pangasidae||(i) Pangasius sutchi (Asian catfish; Pla swai)||S.E. Asia||Thailand and Vietnam||Carnivorous in open waters and omnivorous in ponds||River spawner; can be bred through hypophysation|
|(ii) P. pangasius (Pangas)||India and Bangladesh||India||Omnivorous; eats almost anything, with a special liking for molluscs||River spawner||Can be utilized in composite culture|
|16. Esocidae||Esox lucius (Pike)||Europe, Asia and North America||Europe and North America||Carnivorous||River spawner; can be propagated under controlled conditions from eggs collected from wild spawners; induced breeding is also practised with moderate success||Excellent game fish|
|17. Anabantidae||(i) Helostoma temmincki (kissing gourami)||Tropical areas of the Far East||Far East||Plankton feeder (mostly phytoplankton)||Breeds in confined waters; reaches sexual maturity in 12–18 months and spawns every six months; spawning ponds are specially prepared - dried, a portion covered with damp straw, filled with water and then the brood fish released; the floating straw gives protection to eggs and larvae from the sun||This and other gouramis are commercially cultured in ponds and rice fields. They all have accessory respiratory organs, which enable them to survive in oxygen-deficient water|
|(ii) Osphronemus goramy (giant gourami)||Tropical areas of the Far East||India and the Far East||Phytophagous; occasionally feeds on insects, frogs and worms; readily accepts artificial feeds||Spawns in ponds, which are deep enough (1–1.5 m) and have submerged vegetation; can also be made to spawn by providing nest-making material such as leaves of indjuk palm; builds sub-merged nest, in which the buoyant eggs are deposited||The largest and most important of the gouramis; is useful in the biological control of aquatic weeds|
|(iii) Trichogaster pectoralis (snakeskin gourami or Sepat Siam)||Far East||Far East||Plankton feeders; feed also on decaying matter||Spawn in confined waters, rich in oxygen and submerged vegetation; they make foam (bubble) nests|
|(iv) T. trichopterus (three spot gourami)|
|18. Mugilidae||(i) Mugil cephalus (grey mullet)||Atlantic and Pacific Oceans||In several worldwide regions (China, Taiwan, Hawaii, Hong Kong, India, Japan, Israel, etc.)||Feeds on algae, detritus, and plankton||Sea spawner; can be bred through hypophysation||Fry can be collected in inshore waters and estuaries and stocked in freshwater ponds after gradual conditioning; grows well in freshwater|
|(ii) M. tade (grey mullet)||Red Sea and coastal waters of Indian subcontinent, the Far East and Australia||India, Pakistan and Indonesia||Herbivore, feeding on unicellular and filamentous green algae, blue-green algae and diatoms matted on the pond bottom, along with the associated small animals, detritus and soft vegetable matter||Sea spawner; induced spawning||Marine fish which easily adapt to estuarine and freshwater conditions|
|(iii) M. dussumieri (grey mullet)||Coastal waters of Indian subcontinent, Far East, New Guinea, Australia and Sri Lanka||India, Pakistan, and Indonesia||Bottom feeder subsisting on benthic algae, microfauna, decayed vegetable matter and detritus||Sea spawner; induced spawning||Marine fish which easily adapt to estuarine and freshwater conditions|
|19. Percidae||(i) Stizostedion (Lucioperca) (lucioperca) (pike-perch)||Europe||Europe||Carnivorous||Spawns on “nests” in confined water||Cultivated as an associate fish in carp ponds|
|(ii) S. vitreum vitreum (walleye)||North America||North America||Carnivorous||Spawns in confined waters|
|20. Centrarchidae||(i) Lepomis spp. (sun fish)||North America||North America||Mostly feed on invertebrates and sometimes on algae||Spawn in confined waters||Cultivated only occasionally and do not respond well to hormone treatment|
|(ii) Micropterus salmoides (black bass)||North America||In many parts of the world||Carnivorous; invertebrates and small fishes||Spawns in confined waters; parental care well developed||Excellent game fish; used to control trash fish population|
|21. Sciaenidae||Plagioscion squamosissimus||South America||South America||Carnivorous, feeding mainly on shrimp||Breeds in confined waters|
|22. Cichlidae||(i) Tilapia spp.b||Africa||Africa, Asia and America||Omnivorous, herbivorous, or planktonivorous; some of the species can be used for the biological control of aquatic weeds||Breed readily in confined waters; start breeding when only 3 months old and breed several times in a year; display a high degree of parental care; some are mouth breeders||Very widely cultured in tropical and subtropical regions; very hardy fishes, easy to handle and to transport; often pose problems by quickly overpopulating fish ponds; grow well both in freshwater and brackishwater|
|(ii) Haplochromis mellandi||Africa||Zaire and the Zambia||Carnivorous, feeding on molluscs||Breed readily in confined waters; mouth breeders||They are also used for Bilbarzia control in Africa and excess of reproduction by Tilapia in ponds|
|(iii) H. carlottae|
|(iv) Astatoreochromis alluandi|
|(v) Cichla spp.||Latin America||Latin America||Carnivorous||Spawn readily||As above|
|(vi) Astronotus spp.||Latin America||Latin America||Mostly carnivorous||Spawn readily||As above|
|(vii) Serranochromis spp.||Africa||Katanga (Congo)||Feed on insects||Mouth breeders||As above|
|(viii) Etroplus suratensis (pearlspot)||India, Pakistan and Sri Lanka||India||Mainly herbivorous, feeding on blue-green and green algae, and also on decaying plant remnats; also feeds on tender leaves of water plants and zooplankton||Breeds freely both in freshwater and brackishwater ponds; exhibits parental care, in which both male and female participate||A brackishwater fish, which can be easily acclimatized to freshwater|
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.
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.
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).
18.104.22.168 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.
22.214.171.124 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.
126.96.36.199 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.
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.
188.8.131.52 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.
184.108.40.206 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:
fish with passive parental care produce more eggs than those exerting active parental care;
fishes with active parental care produce far fewer eggs than those which do not exhibit parental care, and
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
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).
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.
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.
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).
Non-adhesive eggs can be further distinguished into the following types, on the basis of their specific weight:
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.
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:
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