Production of Grey Mullet

The grey mullet, Mugil cephalus is one of the important brackish water cultivable species. The fish is notable with greyish green colour on the dorsal surface and silver-white on the ventral side, an elongated body, broad and flattened head with a small, inferior mouth. Eyes are often covered by adipose tissue.

Grey mullet is euryhaline and eurythermal. It is cultured in seawater and brackish water. They feed at low trophic levels, consuming microorganisms, decaying organic matter, algae, insect larvae and small molluscs from the bottom. Due to its benthic feeding behaviour, the species is considered as an efficient bio-remediator in aquaculture, and forms part of polyculture systems to manage the quality of sediments.

Natural breeding of grey mullet occurs in the sea, and the fry drifts back towards estuaries. These are areas from where wild seeds of grey mullet are collected. The seed of grey mullet collected from the wild may often be mixed with other unwanted species.

Seed Production

Broodstock management

Similar to broodstock management of the milkfish, the grey mullet broodstock are also maintained adhering to the principles of high-quality feed, water quality and health management and it is almost similar.

Water quality management

It is also practiced in a similar way to that explained for the milkfish.

Spawning

The male and female fish attains sexual maturity within 2-3 years and 3-4 years of age, respectively. Based on the gamete quality, male and female fish are selected and released into breeding tank of 50 m³ having 1.5 m depth. Female is induced to spawn using a total dosage of LHRHa, 400 microgram/kg as priming and resolving injection in two split doses. Two running males are released with the female after second injection and spawning occurs by 12 hours. Fertilisation is external, and the fertilised eggs are buoyant (pelagic), spherical and transparent with 0.8-0.9 mm diameter size. Eggs are collected as explained for the milkfish. Fecundity is 5-10 lakh/kg body weight.

Incubation

The fertilised eggs are measured and incubated as explained for the milkfish. The eggs hatch out by about 30 hours at 32°C. The newly hatched larva has 2.0-2.5 mm size with a large yolk sac, and it remains suspended in water in an inclined position with the ventral side directed upwards and avoids light.

Rearing of larvae

The larvae of 2 dph are transferred to indoor FRP tank (2-30 t capacity) containing filtered seawater at a density of 10-25 no./l. No feed is given for the first two to three days and rotifer is used as live feed from 3-5 dph onwards. Artemia nauplii are given as feed from 20 dph to 35 dph. Fry attains a length of 2 cm within one month of rearing.

Live feed production

It is practiced in a similar way to that explained for the milkfish.

Rearing of fry

Earthen nursery pond of 0.2 ha area with 1 m depth is ideal for the rearing of fry. The nursery pond is prepared by de-siltation, bund strengthening, installation of sluices, draining, drying, tilling, eradication of aquatic weeds and liming as explained for the major carps. Use of bleaching powder is ideal for the removal of pathogen, predator and weed fish. Before filling water to the pond, cow dung is added to the pond at 2500 kg/ha. Subsequently, poultry droppings at 500 kg/ha or chemical fertilizers (Urea at 100 kg/ha, Superphosphate at 200 kg/ha) is added as per the requirement to ensure sufficient plankton in the pond. After acclimatisation, fry of 2 cm size is stocked in the nursery pond at a density of 100-300 no./m². Ideal transparency of the pond water is 20-30 cm. Rice bran or wheat bran is used as a supplementary source of feed depending upon plankton production. In the nursery pond, the fry is grown to fingerling stage (5-8 cm) in two months.

Non-availability of hatchery-produced seed is the main bottleneck for the expansion of grey mullet farming. Presently, wild seeds of the mullet concentrating in shallow estuarine regions during the breeding season are collected for aquaculture; however, their survival is low due to handling associated stress and injuries.

Packing and transportation

The fingerlings are caught by netting or by draining the nursery pond. Packing and transportation are being carried out similar to that of major carps.

Pond Farming

Site selection & pond construction:

All the aspects of site selection and pond construction are same as that of the milkfish.

Pond Preparation

Before stocking, the pond is prepared by de-siltation, bund strengthening, installation of sluices, draining, drying, ploughing, eradication of aquatic weeds, liming and manuring as similar to that explained for the major carps. Then the pond is fertilized with poultry droppings at 500 kg/ha or cow dung at 2000 kg/ha. Water is added to a level of 25-30 cm and maintained for 7-10 days to develop natural feed. Then the water level is increased to above 1 m before stocking of fingerlings. The productivity is maintained at the required level by adding subsequent doses of poultry droppings and chemical fertilizer.

Water quality requirements

• Salinity           : 5-30 ppt      
• pH : 7.5-8.5
• Temperature  : 26-30°C
• DO : >4ppm
• Transparency  : 30-40cm

Stocking

Stocking in the grow-out pond is done after the development of natural food and increasing the water depth to l m. As mullets can be cultured in salinities ranging from freshwater to seawater, the seed should be properly acclimatised. The fingerlings of 4-6 cm are preferably stocked in hapa or pen installed in the same pond till it reaches 10 cm size and thereafter released at a stocking density of 10,000 no./ha for monoculture. Being a peaceful herbivore, grey-mullet is also stocked (5000/ha) along with pearlspot (15,000/ha) in the polyculture system.

Feeding

Generally, grey-mullet thrives on organic material available at the pond bottom. Rice bran or wheat bran is given as supplementary feed daily at 1-5% of the body weight. Its protein requirement is 20-32%. All the aspects of feeding are the same as that of the milkfish.

Care and maintenance

Periodic liming at 250 kg/ha is done according to the pH of water. Whenever the density of plankton in the pond decreases, subsequent doses of cow dung are added at 500-1000 kg/ha in the pond to enhance production. Monthly sampling is done to assess the growth and health. All the aspects of care and maintenance are similar to that of milkfish.

Harvesting

Within a period of 8-12 months, the fish attains a size of 750-1000 g. The harvesting aspects are almost same as that given for the milkfish.

Source : Department of Fisheries, Government of Kerala