Biology of reproduction

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7. BIOLOGY OF REPRODUCTION

Fish culture practices are based on the biology of the species under cultivation. Reproduction control is particularly important. Here are some of the basic principles for the common carp.

8. In adult carp (1) the reproduction cycle initiates in the gonads the ovaries of the females and the testes of the males with the development of the sexual products, eggs and spermatozoa. (2)

When spawning takes place, the eggs are fertilized (3) and attach themselves to aquatic vegetation (4). They develop and hatch into carp larvae (5). Within a few days, these larvae start feeding and become early fry (6).

Further development and growth in new environments successively produces advanced fry (7), fingerlings (8) and adults (9).

9. As you may observe in Figures 1-5, the development of the eggs in the ovary goes through several stages during which single cells grow to 800-1000 microns in size at the end of yolk accumulation.

The egg has now reached the resting stage and it may remain in this state for several months: this is a dormant egg, with a micropyle and a central nucleus.
(6) In favourable conditions the dormant egg will start developing again until final ripening or (7) ovulation and (8) spawning. In the absence of favourable conditions, dormant eggs will degenerate and be reabsorbed by the ovary.

10. The presence of favourable spawning conditions stimulates the further development of the dormant eggs in the ovaries.

First, the central nucleus migrates toward the micropyle, and hydration slightly increases the egg size. Then, ovulation occurs during spawning: the membrane of the nucleus disappears and the chromosomes become visible, the first cell division takes place, the egg follicle dissolves liberating the egg from the ovary wall, and the ripe egg is spawned.

Eggs develop independently; eggs at different stages of development are found in the ovary simultaneously.

11. The development of the sexual products in the gonads is mostly determined by water (1) temperature and (4) availability of food. Other factors such as (2)/(3) dissolved oxygen and the diurnal rhythm of light also influence this development.

To produce dormant eggs efficiently female carp need warm water above 17C and food which is rich in protein.

The production of sperm is far less complicated than that of eggs and, except for water temperature, the environmental requirements are not so rigid.

12. Final ripening of the dormant eggs and spawning are regulated by gonadotropin hormones, formed and stored in the hypophysis or pituitary gland.

Through its sense organs, the female carp gathers information about its environment, such as morning light, water temperature, atmospheric pressure, presence of male carp and vegetation. This information accumulates in the hypothalamus of the brain.

When environmental conditions become suitable, this part of the brain gives the order to the hypophysis to release gonadotropin hormones into the blood stream. When these hormones reach the ovaries they trigger the final ripening of the dormant eggs and spawning. Males also become ready for spawning on gonadotropin command.

13. According to the climate, carp may spawn once or several times a year.

In temperate regions (Figure A), spawning occurs once a year in late spring (1). New eggs start to develop in early summer (2) and by autumn dormant eggs are present in the ovaries (3). They remain in this dormant stage through winter (4) and early spring (1), until suitable spawning conditions occur.

In tropical regions (Figure B), carp may spawn several times a year. High water temperature accelerates the development of new eggs after each spawning and the dormant stage may be reached when suitable spawning conditions still exist. A similar process may be repeated three to four times a year (1-4).

14. In tropical and subtropical regions, carp usually become sexually mature in their first year, but in cooler climates it takes them 3-4 years.

Groups of mature carp naturally spawn in freshly flooded grassy areas along rivers and lakes. The fertilized eggs are very sticky and they adhere firmly to the submerged vegetation.

15. (1) Male and female carp spawn side by side swimming close to the submerged vegetation. (2) As the ripe eggs fall into the water, they are surrounded by a multitude of fast moving sperm. (3) One of them penetrates the micropyle, fertilizing the egg. (4) The micropyle closes as the egg swells and becomes very sticky.

In such conditions some ripe eggs are not fertilized because the high motility of the sperm lasts for only 30-60 seconds and the egg micropyle closes within one minute, after their contact with water.

16. When a ripe egg falls into the water, it becomes round (I) and within a short time begins to swell (2).

Water seeps in between the shell and the cell kernel (nucleus and yolk mass), creating the perivitelline space. If it is fertilized, the egg soon start to develop.

By the time the swelling is complete (3), the animal pole of the kernel rises as a small hillock on the yolk mass. It divides into several cells (4) and reaches successively the morula (5), the blastula (6) and the gastrula (7) stages.

The embryo finally appears with tail, head and eyes (8). It develops into a larva and hatches, breaking out of the egg shell.

17. The newly hatched larva (1) is still very different from the adult carp: it has no mouth and is nourished from the yolk sac.

After about 4 days at 20-24 C, the mouth is formed, the swim bladder is inflated with air, and exogenic feeding starts (2).

This is the beginning of the fry stage. Well-fed carp fry grow rapidly, from 10-12-day-old fry (3) into 30-day-old advanced fry (4).

18. It may be said that the natural reproduction of mature carp is essentially controlled by two kinds of environmental factors: basic and stimulating factors.

(A) The three basic factors are: (1) the water temperature which should preferably range from 18 C to 24C, (2) the dissolved oxygen content which should be between 5 to 10 mg/l, and (3) the light which should be the same as the light at sunrise.

(B) Among the stimulating factors are (1) specific weather conditions such as a stable atmospheric pressure, (2) the presence of males, and (3) the presence of grassy vegetation.

19. Both semi-artificial and artificial reproduction techniques are commonly practiced in carp culture.

The factors controlling the spawning of females with dormant eggs differ accordingly. Semi-artificial reproduction (orange arrows) is controlled by the same basic (A) and stimulating (B) factors as shown above, except that (4) the injection of gonadotropin hormone becomes a major stimulating factor.

On the other hand, in artificial reproduction (red arrows) where (4) the artificial fertilization of the eggs is carried out after two injections of gonadotropin hormone, two basic factors control its success: (1) water temperature and (2) dissolved oxygen content.

20. Why should artificial fertilization be carried out under dry conditions?

The reason why eggs and sperm should be collected avoiding any contact with water is because (a) as soon as eggs come into contact with water they start to swell and their micropyle closes within a minute; (b) as soon as spermatozoids come into contact with water, they become highly motile, but this activity lasts for only 30-60 seconds.


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