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Chapter: Aquaculture Principles and Practices: Grey Mullets and Milkfish

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Fry collection and artificial propagation of Grey mullets

Fry collection methods, Artificial propagation - Fry collection and artificial propagation of Grey mullets

Fry collection and artificial propagation

 

As mentioned earlier, most of the fry or fingerlings used in grey mullet culture the world over are obtained from natural sources. In the extensive type of farming in impoundments like the bheris of India and the Mediterranean vallis, most of the stocking is done by taking advantage of the tidal flow and the habit of mullets of swimming against currents. Very small fry may be brought in with the incoming high tide, but larger ones enter at low tide, when there is a slow flow of water from the impoundments. Suitable screens and traps are maintained at the sluice gates to prevent the escape of fish from the impoundments.

 

Fry collection methods

 

At present, most farmers supplement the stock obtained through tidal flow with fry caught from the estuaries. The more important species, including M. cephalus, breed in the sea and the fry and fingerlings migrate towards the shore and estuaries, where they can be found to congregate in schools. Only fry and small fingerlings below 25cm in length school in large numbers. In any estuarine area there are certain locations where they congregate in large numbers, but variations in occurrence according to species and location have been observed. In Israel, for example, M. saliens usually concentrate in the lower reaches of rivers, whereas M. cephalus and M. capito ascend to the higherreaches. On the east coast of India, fry of one or more species can be found in schools in the estuarine waters up to the tidal limits, through-out the year. The most suitable areas for the collection of fry are the marginal areas of rivers, tidal streams, creeks, swamps and inundated fields (Sarojini, 1958). Where a fresh-water stream flows into a river or a brackish-water lagoon, the mullet fry can be observed to congregate and swim against the slow current. Such areas are ideally suited for the collection of large numbers of mullet fry, taking advantage of the tidal flows. According to Hepher and Pruginin (1981), catches of fry are made in Israel after sea storms and rainy periods.

Grey mullets are difficult species to identify even when they reach the adult stage, and this is much more difficult in the fry and fingerling stages. Since some of the species are very slow-growing, it is important to sort them before stocking in rearing facilities. The seasonality of occurrence of different species in the coastal areas can be of initial help in obtaining the desired species. Perlmutter et al. (1957) observed that the fry of each species of mullet appear regularly at the mouth of rivers on the Israeli coast in particular seasons. Similar seasonality in the occurrence of fry has been observed by Sarojini (1958) and Luther (1973) on the east coast of India in West Bengal.

 

Different types of equipment are used for fry collection, the most common being seines and dip nets. Short-bagged drag nets and beach seines are the most usual equipment in Taiwan. Fry collectors normally acclimatize the fry through gradual decreases in salinity, when the fry are meant for rearing in fresh water.

 

However, it has been shown experimentally that the fry can be directly transferred to fresh water without any special acclimatization. Fish farmers in Taiwan, Israel and certain parts of India stock rearing ponds directly without any major mortality. It is believed that sudden changes in temperature and low pH affect the survival of fry more than salinity. If the fry are to be transported long distances, it is considered advisable to condition them for a day.

 

Generally the fry are stocked in production ponds, directly from the collection grounds or after a brief period of conditioning. In Israel the fry are first reared in small ponds for two to three months before stocking in larger production ponds. Mullet fry are stocked at the rate of about 30000/ha, often with 200–300 young carp per ha to reduce the growth of filamentous algae. The mullets are stocked in larger ponds when they have reached a weight of around 3g.

 

Artificial propagation

 

Considerable effort has been devoted to the artificial propagation of the grey mullets which do not breed in confined waters, especially M. cephalus. A number of species have beeninduced to breed by the administration of pituitary extracts or gonadotropins. Wild M. cephalus has been induced to spawn by theadministration of mullet pituitary homogenate, often combined with Synahorin (Tang, 1964; Ling 1970). Mature four-to-six-year-old females are injected intramuscularly with homogenates of two to five pituitaries from the same species, with 10–60 rabbit units of Synahorin (a mixture of chorionic gonadotropin and mammalian hypophyseal extract). Mature males do not require any injections, except towards the end

of the breeding season. The best results are obtained by giving two injections at an interval of 24 hours. The treated fish can be stripped easily and a female M. cephalus of about 1.5kg weight is reported to yield 1–1.5 million eggs (Chen, 1976). The eggs measure 0.9–1mm in diameter, and are fertilized by the dry or the wet method. At temperatures of 20–24°C the eggs hatch out in 16–30 hours. The larvae are very small, ranging from 2.5–3.5mm in size, and tend to avoid strong light.

Captive M. cephalus have also been spawned by the administration of partially purified salmon gonadotropin, with the potency of 1mg, equivalent to 2150IU human chorionic gonadotropin (HCG) (Kuo et al., 1975). The easily available and less expensive HCG can also be used effectively for induced breeding of M. cephalus. Female spawners with oocytesof at least 600 mm diameter require a dose of approximately 60IU HCG per g body weight, administered in two injections. The first injection of about 20IU per g body weight is followed in 24 hours by an injection of 40IU per g body weight (Kuo et al., 1973).

 

Both M. cephalus and M. capito raised in fresh-water ponds have been induced to breed by injection of carp pituitary homogenates. The brood fish are acclimatized to full sea water for at least two weeks, after which the females are given a series of three injections of homogenized carp pituitaries. The first injection contains one half of a pituitary for every kg of the recipient; the second, given after about seven hours, contains one pituitary per kg; and the third injection, after 14 hours, contains two pituitaries per kg. Males are also given one half of a pituitary per kg, at the time of the last injection of the females. The females can be stripped 16–24 hours after the third injection and the eggs fertilized with milt stripped from the males. After rinsing in sea water, the fertilized eggs are transferred to incubators. The hatchlings are fed with brine shrimp and zooplank-ton. The larvae grow to fry stage in about a month and can be acclimatized back to fresh water.

 

The Chinese mullet M. so-iuy is another species which has been successfully bred by the administration of hormones (Zheng, 1987). Three- to four-year-old brood fish are selected for spawning. Mature females with oocytes

measuring 600–700 mm are injected with homogenates of carp pituitary, mullet pituitary, HCG or LHRH-A. Generally two injections are given at an interval of 24 hours. When the water temperature is between 15 and 16°C, spawning takes place in one to three days. Incubation can be carried out in sea water or brackish water with salinities above 7ppt. Hatchlings are reared in indoor concrete tanks, fed with eggs and larvae of oysters and mussels, rotifers and brine shrimp nauplii. Larvae can also be reared in manured nursery ponds with an adequate growth of zooplankton. Soybean milk is given as a supplementary feed initially. Later, soybean or peanutcake is given in the form of a paste. In 30 to 40 days the fry reach the stocking size.

 

Mugil macrolepis has been successfullyinduced to spawn in India by the administration of pituitary of the same species (Sebastian and Nair, 1975). The experiments were conducted with mature wild fish caught in Chinese dip nets. The effective dose was reported to be three to four glands per female of 40–130g weight, injected intramuscularly at six-hour intervals. The males did not require any injection. When the injected fish were kept in hapas together with males in salt water of 29–31ppt salinity, spontaneous spawning occurred, but the rate of fertilization was poor. However, by stripping and artificial fertilization better results were obtained. At temperatures ranging from 26 to 29°C most of the developing eggs hatched in about a day. Three- to four-day-old larvae could be reared in cement tanks in brackish water containing rich growths of chlorella, diatoms, copepods, etc.

 

Mugil parsia has also been induced to breedwith a single low dose of mullet pituitary at the rate of 5mg per kg body weight of the female (Radhakrishnan et al., 1976). Spontaneous spawning of untreated fish was also observed. Despite experimental success in induced spawning, farmers continue to depend on wild stocks of fry and fingerlings for culture operations.

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