Production Methods for the Flathead Grey Mullet

Habitat and biology

In the western Atlantic Ocean, it is found from Nova Scotia, Canada south to Brazil, including the Gulf of Mexico. It is absent in the Bahamas and the Caribbean Sea. In the eastern Atlantic Ocean, the striped mullet occurs from the Bay of Biscay (France) to South Africa, including the Mediterranean Sea and the Black Sea. The eastern Pacific Ocean range includes southern California south to Chile.

The flathead grey mullet is catadromous, frequently found coastally in estuaries and freshwater environments. Adult mullet have been found in waters ranging from zero salinity to 75‰, while juveniles can only tolerate such wide salinity ranges after they reach lengths of 4–7 cm. Adults form huge schools near the surface over sandy or muddy bottoms and dense vegetation and migrate offshore to spawn in large aggregations. The larvae move inshore to extremely shallow water, which provides cover from predators as well as a rich feeding ground. After reaching 5 cm in length, these young mullet move into slightly deeper waters.

Flathead grey mullet is a diurnal feeder, consuming mainly zooplankton, dead plant matter, and detritus. Mullet have thick-walled gizzard-like segments in their stomach along with a long gastrointestinal tract that enables them to feed on detritus. They are an ecologically important link in the energy flow within estuarine communities. Feeding by sucking up the top layer of sediments, flathead grey mullet remove detritus and microalgae. They also pick up some sediment which functions to grind food in the gizzard-like portion of the stomach. Mullet also graze on epiphytes and epifauna from seagrasses as well as ingest surface scum containing microalgae at the air-water interface. Larval flathead grey mullet feed primarily on microcrustaceans. Copepods, mosquito larvae, and plant debris have been found in the stomach contents of larvae under 35 mm in length. The amount of sand and detritus in the stomach contents increases with length, indicating that more food is ingested from the bottom substrate as the fish matures.

Trials on the artificial propagation of flathead grey mullet have been carried out, but most of the commercial aquaculture production of flathead grey mullet still depends on fry collected from the wild, which is cheaper.

Production

Production systems

Seed supply

Most of the flathead grey mullet fry used in commercial aquaculture are collected from the wild, especially in the Eastern and Southern Mediterranean, Saudi Arabia and Gulf States and South East Asia. Seed produced through artificial propagation is used on a limited scale in Italy and Hawaii.

During the autumn and winter months adults migrate to the sea in large aggregations to spawn. Fecundity is estimated as 0.5–2.0 million eggs per female, depending upon the adult size. Hatching occurs about 48 hours after fertilization, releasing larvae approximately 2.4 mm long. When the larvae are 16–20 mm, they migrate to inshore waters and estuaries, where they can be collected for aquacultural purposes during late August to early December.

Shoals of fry are collected by fine seine nets, transported in seawater to hapas or shore aggregation tanks for a few hours. They are then transported by trucks to separate nursery units, or nursery facilities in grow-out farms. On arrival, they need to be acclimatized, especially in terms of salinity; this takes place over several hours, during which water from the nursery pond is gradually added and mixed with the transport water. Mortality rates of up to 100 percent can occur during the following two weeks if this process is neglected or not properly carried out before stocking fry into the nursery.

Hatchery production

Full-scale commercial production of Mugil cephalus is not yet common. Induced spawning and production of fry has been achieved on an experimental and semi-commercial basis in the United States of America and Taiwan Province of China, and the production of mullet fry on a limited scale for aquaculture has been reported in Italy, Israel and Egypt.

In these systems large numbers of sexually mature individuals (over two years of age, 32–50 cm long and 1.0–2.1 kg each) are kept under optimum environmental conditions with limited physical disturbance. Prior to spawning, fish are kept at 32–35‰ and 12–15 °C. Ripe fish are selected and placed in plastic or fibreglass tanks filled with seawater saturated with oxygen at a 2–3:1 male:female ratio shortly before spawning. Females are injected with regulated and successive doses (2–3 injections) of pituitary gonadotropin. Females spawn 12 hours after the last injection. Spawning is heralded by a violent quivering of the male, which liberates sperm as a response of the release of eggs. Eggs are produced at a rate of 650–850/g female body weight. Mullet eggs are spherical (880–980 µm) and transparent, with a smooth surface and a single large oil globule making the egg extremely buoyant. Eggs are carried with the overflow of water, sieved and transferred to incubation jars. Eggs are incubated at a temperature of 22–24 °C in seawater (30–32‰) saturated with oxygen. Hatching takes place after 50–64 hours. After hatching, the larvae are transferred to fibreglass indoor tanks and fed with live food (rotifers, and later with Artemia nauplii). Larvae are kept in indoor tanks for 14 days, and then transferred to larger tanks until they reach 10–2 mm before transport to outdoor nursery ponds.

Nursery

After acclimatization, fry are stocked in earthen nurseries at high densities (up to 125/m²), where they depend mainly on natural food. From 2.5 to 5.0 tonnes/ha of animal manure are added to the soil before filling with water; then chicken manure and chemical fertilizers (usually phosphate and nitrates) are added in suitable amounts on a weekly basis to keep secchi disc readings of 20–30 cm. Rice or wheat bran is sometimes used as an additional source of food.

Fry are kept in the nursery ponds for 4–6 months (from August or November till April) until they are about 10 g BW. Optimum temperatures are 20–26 °C, both in the nursery and grow-out stages. The fingerlings are then caught, either by draining the nursery ponds into catch ponds or by netting. Over-wintered mullet fingerlings are sold for ongrowing in various culture systems but especially for semi-intensive aquaculture. In the rare cases that fry supply exceeds demand, they are retained and grown-on to market size in the nurseries.

Ongrowing techniques

In many countries mullet fry and fingerlings are stocked in inland lakes and reservoirs as a form of fisheries enhancement (culture-based fisheries). They have been transferred into inland water lakes of the El Fayyum area of Egypt since the 1920s, and into the Black Sea and Caspian Sea regions of Russia since 1930.

Cultured flathead grey mullet are usually grown in polyculture in semi-intensive ponds and netted enclosures in shallow coastal waters. Mullet can be polycultured successfully with many other fish, including common carp, grass carp, silver carp, Nile tilapia and milkfish, and can be reared in freshwater, brackishwater and marinewater.

Prior to stocking, aquaculture ponds are prepared by drying, ploughing and manuring with 2.5–5.0 tonnes/ha of cow dung. Ponds are then filled to a depth of 25–30 cm and kept at that level for 7–10 days to build up a suitable level of natural feed. The water level is then increased to 1.5–1.75 m and fingerlings are stocked. Productivity (measured by secchi disc – see nursery section) is kept at the required level by adding chicken manure and/or chemical fertilizers. Optimal dissolved oxygen is maintained by the use of various types of aerators, especially after sunset. Extruded feed is supplied to semi-intensive ponds to cover the feeding requirements of both carps and tilapia grown in the same ponds.

The growing season is normally about 7–8 months. If mullet are monocultured, manuring may be sufficient to reach the required feed level. In many cases, mullet has been found to feed directly on chicken manure and good levels of production have been recorded. Growth is checked by sampling, and if growth rates are not as expected, rice and/or wheat bran is added daily in amounts of 0.5–1 percent of biomass to supplement the natural feed in ponds. When mullet are reared in polyculture, they are usually stocked with tilapia, common carp and silver carp. In this case, feeding and fertilization programmes are usually targeting the other cultured species and the mullet feed on the natural feed, detritus and feed leftovers.

Acclimatized to the appropriate salinity, and stocked as 10–15 g individuals at 6 175–7 410/ha, a harvest of 4.3–5.6/tonnes/ha/crop can be obtained. In semi-intensive polyculture with tilapia and carp, mullet fingerlings are stocked at 2 470–3 705/ha together with 1 850–2 470/ha of 100 g common carp juveniles and 61 750–74 100/ha 10–15 g Nile tilapia fingerlings. Total harvests are typically 20–30 tonnes/ha/crop, of which 2–3 tonnes are mullet.

After an ongrowing season of 7–8 months in either culture systems in the subtropical region, flathead grey mullet reach 0.75–1 kg; if kept for two ongrowing seasons, they reach 1.5–1.75 kg each. Rearing for a second year depends on the market requirements; in some countries mullets are marketed at a size of 1.5 kg and larger. The two seasons are continuous, as fish are kept in ponds for over wintering, fish then grow in the same ponds through the next spring and summer until they reach that size. As usual, the choice of rearing technique depends on market demand and economics.

Feed supply

In monoculture, mullet feeds on natural food and on the by-products of grain mills and rice polishing plants. In polyculture, manufactured extruded pellets are produced either in feed mills specialized in the production of fish feed or, in many cases, in chicken feed mills that have a line for fish feed production. Feed is formulated according to the dietary requirements of the major cultured species (i.e. tilapia and common carp).

Harvesting techniques

Harvesting can be partial in ponds or net enclosures. Daily harvesting, according to market demand, can be carried out using gillnets of suitable mesh size. Nets are stretched in a zigzag line across ponds at sunset and collected at the early morning.

In semi-intensive culture, total drain-harvesting is used in late autumn or early winter. Fish usually move with the flow of water to a concrete catch pond at the pond outlet. A seine net can be used to collect those that do not reach the catch pond.

Handling and processing

Fish are collected from the catch ponds by scoop nets and transferred into plastic boxes, washed in running water, and then sorted according to species and sizes on a sorting table. Sorted fish are weighed and packed in plastic boxes with crushed ice or ice flakes.

In the Mediterranean region, mullet is usually marketed fresh or chilled. Fish are marketed whole but gutted mullet are accepted. Harvested mullet is marketed daily and consumed fresh and never kept on ice for more than one day. Older mullet is considered of inferior quality and does not usually gain a good price. Frozen mullet is considered of much lower value in this region. Mullet is also preserved by wet salting and consumed salted.

Production costs

Production costs vary considerably, depending on the culture system, geographical area and the level of technology applied. Costs also vary depending whether hatchery-produced fry or fingerlings are used or not, or the system is monoculture or polyculture. In Egypt, hatchery-produced 10 g fingerlings cost US$ 0.3 each, while wild fingerlings of the same size cost US$ 0.1–0.12. These figures may be different in other regions, where the collection of wild fry is forbidden or if hatchery production is well established. The total cost of producing 1 kg of flathead grey mullet in semi-intensive aquaculture in Egypt is US$ 0.75–1.00.

May 2009