When conditions are not favourable for successful natural spawning in ponds, or when large quantities of fry have to be produced, it may be advantageous to resort to hypophysation techniques to induce spawning, and use hatcheries for incubation and larval rearing. This helps to improve the survival of hatchlings, by reducing predation by insects and other enemies.
Ripe brood fish kept in segregation ponds can be used directly for hypophysation. During the normal breeding season, a good common carp female produces about 150000 eggs per kg. The eggs measure 0.9–1.6mm in diameter. In practice, not all females respond to hypophy-sation and so it is considered advisable to inject double the number required. The number of males to be injected will be about two-thirds the number of females. Based on these factors, the total number of females to be hypophysed has to be determined.
It is considered advisable to disinfect the brood stock brought into a hatchery for hypophysation. Giving a 40ppm formalin bath for about two hours is a common practice in Israel. The methods of preparing hypophysis extracts have been described. The doses and sequence of injections are not standardized and practices vary considerably.
Hepher and Pruginin (1981) described a tested procedure of injecting half a pituitary per kg female spawner one day after transfer into indoor hatchery tanks, by which time the fish has acclimatized itself to hatchery conditions. A second injection is given eight hours later with 0.8 pituitary per kg female. The males are injected only once, 24 hours after transfer to the hatchery, with 0.5–0.6 pituitary per kg. Woynarovich (1975) recommended a single injection of one pituitary, at about 2.5–3.7mg dried pituitary per kg body weight of the recipient. Dried pituitary is pulverized and dissolved in a solution of 0.6–0.7 per cent common salt (NaCl) and pure glycerine in the proportion of 70 parts salt to 30 parts glycerine. This solution is injected intramuscularly between the base of the dorsal fin and the lateral line. The injected males and females are kept in separate tanks.
As indicated earlier, the time taken for the eggs to ripen for spawning or stripping depends on the water temperature. Many fish culturists anaesthetize the brood fish before stripping.
The most common anaesthetic used is a solution of ethyl-M-aminobenzoate at a concentration of 100ppm. After a bath in the solution for 3–5 minutes, the fish are completely anaesthetized. First, the female is stripped by gently squeezing the abdomen towards the tail and the eggs that flow easily are collected in bowls or basins. Then the males are similarly stripped for milt, which is collected over the eggs in the same container, and the contents are mixed immediately with a feather or a plastic spoon.
For every litre of eggs, two to three litres of milt will be needed to ensure proper fertilization. The adhesive nature of the eggs makes them clump together and hamper proper fertilization.
The stickiness can be eliminated by treating the eggs with a solution of sodium chloride and carbamide (urea (CO(NH2)2)) (40g sodium chloride and 30g carbamide dissolved in 10l clean water), equivalent in quantity to the eggs. The solution is first poured over the mixture of eggs and milt and stirred with a plastic spoon or feather for about 5–10 minutes. As the eggs begin to swell, small quantities of the solution may be added at intervals, as required. Within about one to one and a half hours, swelling of the eggs will have stopped and the first cleavage will have occurred. The sticky layer will have dissolved, but to remove it completely the eggs have to be washed with a 0.05–0.07 per cent tannic acid solution for about 20 seconds. The washing has to be repeated up to five times, the solution being diluted each time by about 0.01 per cent by the addition of water to the stock solution. Finally, the eggs are washed in fresh water for about 5 minutes. The water-hardened eggs, which measure 2–2.1mm in diameter, are ready for incubation in hatchery jars or other suitable containers.
There are different types of incubators used for hatching common carp eggs, ranging from simple double hapas (fig. 16.17) to Zoug jars with temperature-controlled water supplies.
Essentially a hatching hapa consists of a fine mesh (0.5mm) sieve-cloth tank about 2 x 1 x 1m in dimension, with an inner hapa or chamber made of the same material with a mesh of 2– 2.5mm.The whole device is placed in a protected water body where the water is well-oxygenated. The fertilized eggs are spread in the inner hapa. The hatched larvae fall through the larger meshes of the inner hapa and are retained by the outer hapa. After the hatching is over, the inner hapa is removed together with the dead eggs, egg shells and other debris to avoid deterioration of the water quality in the hapa.
The Zoug jars (see fig. 6.42), named after Lake Zoug (Switzerland) where their use originated, are large (60–70mm in height and 15–20cm in diameter) inverted bottles, with open tops and narrow bottoms. In a hatchery they are installed vertically in a series, with the narrow neck directed downwards attached to the water lines with suitable taps. Water is supplied from below and the outlet is at the top. The capacity of a jar is usually 6–8l and it can carry about 1–5l eggs. The flow of water varies between 2 and 6l/s, with an average of about 4 l/s.The MacDonald or chase jar, which is similar to the Zoug jar, is cylindrical in shape (40– 50cm in height) with a circular bottom on which it can stand. A tube is fitted inside at the bottom for the water supply and the flow of water in the jar keeps the eggs rotating slowly.
Farm-fabricated incubation funnels are also used very widely. They can be made of plexi-glass or fibreglass in any dimension required. Israeli farmers use plexiglass incubation funnels 80cm long and 60cm in diameter at the upper rim. Water is supplied from a bottom inlet, which is about 1.23cm in diameter. Water overflows from an outlet at the top and this overflow can also be used for collecting the hatchlings. Each such funnel can be used to hatch about 175000 carp eggs. Woynarovich and Horvath (1980) illustrated a number of designs of incubation funnels that can easily be made with plastic and sieve-cloth material, with simple sprinkler-type water inlets.
The eggs hatch out in the incubators within 2–7.5 days, depending on the temperature. At a temperature of about 25–26°C, the hatching takes place in 2–2.5 days, whereas at temperatures of 16–17°C, it may take up to 7.5 days. There will usually be some unfertilized eggs and these eggs are the focus of fungal (Sapro-legnia) infection. A common method ofcontrolling fungus in the incubator is by the application of malachite green at a concentration of 0.02g/l water for about 20–25 minutes, after stopping the flow of water. When the flow is resumed the chemical is washed out from the incubator.
The hatchlings are removed to indoor nursery tanks or similar containers. It is desirable to rear the hatchlings under controlled conditions, up to the fry stage. Nursery tanks are usually shallow, with a depth of about 0.5m. About a million fry can be reared in a 20m2 tank with a water exchange rate of 1l per minute per square metre. Best results are obtained when the larvae are fed on natural food organisms collected from the ponds or other water bodies or cultured in the farm. Some farmers use manufactured starter feeds and, where feasible, brine shrimp produced on the farm.