DESCRIPTION OF THE SPECIES AND USE IN AQUACULTURE

Species presentation

Mullets are members of the Order Mugiliformes, Family Mugilidae. Mullets are ray finned fish found worldwide in coastal temperate and tropical waters and, for some species, also in freshwater. Taxonomically the family is usually treated as the sole member of the order Mugiliformes, but as Nelson (1994) reports, “.... there has been much disagreement concerning the relationships .....” of this family. Most species commonly reach about 20 cm in total length, but some (e.g. Mugil cephalus) may attain 80–120 cm.

The head is broad and flattened dorsally in most species. The snout is short and the mouth is small. The gill arches of many species are specialized, forming a characteristic pharyngobranchial organ that has an expanded, denticulate pad used for filtration of ingested material. In many species of mullet, the tiny teeth are positioned on the lips.
The eyes may be partially covered by adipose tissue. There are two short, well separated dorsal fins, the first with four spines and the second with eight to ten segmented rays. The anal fin is short; with two or three spines, and seven to twelve segmented rays in adults. The pectoral fins are high on the body, and the caudal fin is weakly forked. The lateral line is absent. The scales are moderate to large in size, with one or more longitudinal grooves. There are two or more pyloric caeca associated with the stomach, which also has a thick-walled, muscular gizzard in most species. Mullet are usually grayish green or blue dorsally, and their flanks are silvery, often with dark longitudinal stripes. They are pale or yellowish ventrally (Harrison, 1999; Nelson, 1994).

Most mullets are found in coastal marine and brackish waters. They are nektonic, usually in shallow inshore environments, such as coastal bays, reef flats, tide pools, and around harbor pilings and in brackish water estuaries, lagoons and mangroves. They usually swim over sandy-muddy bottoms and seagrass meadows, in relatively still waters. They commonly occur at water depths of 20 m but may be found offshore or in deeper waters. Many species are euryhaline and move between marine and freshwater environments of rivers and flooded rice fields. Some species occasionally swim far up river, while a few species spend their entire adult lives in rivers (Smith and Smith, 1986; Cardona, 2006).
Mullets migrate in large aggregations from their feeding grounds in rivers, estuaries, lakes or lagoons to the sea for spawning in a single spawning cycle each season. Spawning seasons differ according to species and regions. Fecundity is high in all species and is estimated at 0.5–2.0 million eggs per female depending on the size of the adult. Eggs are scattered on the bottom substratum in open waters and left unguarded. The eggs develop at sea and hatching occurs about 48 hours after fertilization, releasing larvae of approximately 2.4 mm in length. When the larvae reach 16–20 mm they migrate to inshore waters and estuaries (Saleh, 2006; Maitland and Campbell, 1992).
Out of the 17 genera and 80 species belonging to the family Mugilidae (Nelson, 1984) only three species are of aquaculture importance. Due to its higher growth rates and market acceptance, the flathead gray mullet Mugil cephalus, thinlip mullet Liza ramada and the bluespot mullet Valamugil seheli are the most commonly cultured species of mullet in Egypt.
The flathead grey mullet, Mugil cephalus, is a very important aquaculture species in the Mediterranean, Southeast Asia, Taiwan Province of China, Japan and Hawaii (Saleh, 2006). The species can reach a length of up to 120 cm making it the largest mullet species (Figure 1). Externally, males are difficult to distinguish from females, except for the more slender shape of males when sexually ripening (Virgona, 1995). Their color is olive-green dorsally, with sides that are silvery shading to white ventrally. They have thin lips and the pectoral fins are short, not reaching the first dorsal fin.
The grey mullets, found in coastal waters of the tropical and subtropical zones of all seas, are catadromous, frequently found in estuaries and freshwater environments (Figure 2). Adult mullet have been found in waters ranging from zero salinity to 75 ppt, while juveniles can only tolerate such wide salinity ranges after they reach lengths of 4–7 cm. Adults form large schools

Figure 1
Flathead grey mullet, Mugil cephalus

near the surface over sandy or muddy bottoms and dense vegetation and migrate offshore to spawn in large aggregations (Eschmeyer, Herald and Hammann, 1983). The larvae move inshore to extremely shallow water, which provides protection from predators as well as a rich feeding ground. After reaching 5 cm in length, the young mullets move into slightly deeper waters (Saleh, 2006).

The species is mainly diurnal and feeds on zooplankton, benthic organisms and detritus. Adult fish tend to feed mainly on algae while inhabiting freshwater. Reproduction takes place in the sea from July to October. Females spawn 5 to 7 million eggs provided with a notable vitellus.
The flathead grey mullet was the first species of mugilidae used for aquaculture. In Egypt, this species has been used for traditional aquaculture and culture-based fisheries since the late 1920s and is still of major importance today also in other Mediterranean countries and Taiwan Province of China (Faouzi, 1936; Saleh and Salem, 2005; Basurco and Lovatelli, 2003).
The thinlip grey mullet, Liza ramada, although the second choice in the aquaculture of mullet constitutes the majority of the aquaculture harvest of mullet in Egypt. This species has a lower growth rate than Mugil cephalus, but exceeds that of all other Mediterranean mullet species. The availability and abundance of the wild fry of this species as compared to those of Mugil cephalus makes it the dominant aquaculture species (Sadek and Mires, 2000) (Figure 3).
Thinlip grey mullet can reach a body length of up to 70 cm. The species is characterized by an elongate fusiform body; slightly compressed from side to side, with a massive short head, flattened above the eyes and with a broad terminal mouth with very small, barely visible, teeth. The upper lips are thin and smooth and the snout is short

Figure 2
Global distribution of the flathead grey mullet, Mugil cephalus
Figure 3
Thinlip grey mullet, Liza ramada
Figure 4
Global distribution of the thinlip grey mullet, Liza ramada

and blunt. The thinlip grey mullet has two well-separated dorsal fins, the first with 4 to 5 spines. The pectoral fins are placed high on the flanks and the caudal fin is deeply forked. There is no external lateral line and the scales are large and adherent. Scales on the top of the head extend forward to the anterior nostrils and the eye is not covered by a thick adipose lid. At the base of pectoral fin there is a scaly appendix. The colour on the back is grey-dark brown, while the belly whitish-grey often with 6–7 lengthwise stripes (FAO, 1973; Rochard and Elie, 1994).
The thinlip grey mullet is a fast swimmer, leaping out of the water when disturbed. It enters estuaries and rivers for feeding, but spawns in the sea. Juveniles often concentrate

Figure 5
Bluespot mullet, Valamugil seheli
Figure 6
Global distribution of the bluespot mullet, Valamugil seheli

in the vicinity of freshwater outflows. It feeds on minute bottom living or planktonic organisms (e.g. diatoms and amphipods) and also on suspended organic matter. Liza ramada is native in the Eastern Atlantic from the coasts of southern Norway to Morocco, including the Mediterranean and the Black Sea (Figure 4) (Wonham et al., 2000).
The bluespot mullet (Valamugil seheli) although it has a lower growth rate, fetches a higher market price compared to the other cultured mullet in Egypt. The fish is highly appreciated for its taste and usually consumed at an individual body weight of 120–180 grams. The bluespot mullet can reach a body length of 60 cm. The body is compact, pressed from both sides and the head is small (Figure 5). Adults are bluish brown or green dorsally, flanks and abdomen silvery with a dusky spots on the upper row of scales, giving indistinct longitudinal stripes. The dorsal and upper lobe of the caudal fin has a dark-blue tip. Anal, pelvic,

and pectoral fins are yellow. Pectoral fins are also with dark blue spot dorsally at origin (Harrison and Senou, 1997).
The species usually swims in schools and inhabits coastal waters, but enters estuaries and rivers where it feeds on microalgae, filamentous algae, forams, diatoms, and detritus associated with sand and mud (Harrison and Senou, 1997). The bluespot mullet is found in the Indo-Pacific and the Red Sea all the way south to South Africa; to the east its distribution reaches the Hawaiian and Marquesan islands; north to southern Japan, and to the south to New Caledonia and Norfolk Island (Figure 6). In Egypt, the species is caught mostly from the Red Sea, Gulf of Suez, Suez Canal and the Bitter Lakes. On the Mediterranean coast of Egypt, the bluespot mullet is only caught in the coastal waters of the area extending from Damyitta to the northwest of the Sinai Peninsula.

Mullet fisheries

Mullets are an important component of Egyptian fisheries and are considered as one of the most important cash crops from artisanal fisheries in the numerous lagoons throughout the country. The fish is commonly caught with gill, trammel and veranda nets by artisanal fishers operating in the sea, lakes and coastal lagoons. Based on the statistics published by the Food and Agriculture Organization of the United Nations (FAO) the world total catch of mullet in 2004 was about 261 000 tonnes representing only 0.3 percent of the world fish catch (FAO, 2004).
In Egypt, the 2005 production of mullets was recorded by the General Authority for Fish Resources Development (GAFRD) at 186 000 tonnes, representing about 21 percent of the total national fish production (GAFRD, 2006). Mullet harvest was about 30 000 tonnes in 2005, 81.6 percent of which was from lakes and coastal lagoons.
While production of the capture fisheries slightly increased during 1985–2005, harvest of cultured mullet sharply increased during the
Although there is no accurate published statistics on the catch of the five species of mullet found in Egypt, estimates were found in the landing records in some landing sites. According to these estimates, during the last ten years, the thinlip grey mullet, Liza ramada, constituted an average of 58 percent of the catch, while the flathead grey mullet Mugil )

Figure 7
Mullet production changes from capture fisheries and aquaculture from 1980–2005

Figure 8
Increase in mullet aquaculture production in percent 35

Figure 9
traditionally practised in the “hosha” system in the Nile Delta region for centuries (Eisawy and El-Bolok, 1975). Currently, Egypt is a leading country in mullet aquaculture with a

Tunisia and Turkey (Sadek and Mires; 2000). Reliance on collection of wild seed was a result of either insufficient supply of hatchery produced seed or its higher price. Commercial hatchery production of mullet seed is carried out in some countries. 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. The production of mullet fry on a limited scale for aquaculture has been reported in
Italy, Israel and Egypt (Saleh, 2006). The development of hatchery production techniques are only practiced commercially for flathead grey mullet as the techniques for seed production of other important species (e.g. thinlip grey mullet) are not yet developed.
In Egypt, mullet fry were first produced in a hatchery near Alexandria through a project in the early 1990s funded by the United States Agency for International Development (USAID). The hatchery production capacity was limited and was capable of producing annually 1–2 million fry of flathead grey mullet. The production cost was high and the fry sold for as much as 15 times the price of wild fry. The failure of marketing the product resulted in shifting the production to species with higher market value and demand, such as the European seabass, gilthead seabream and shrimp. The availability and abundance of mullet fry in the coastal waters of Egypt and accumulated experiences in collecting wild fry developed over more than eight decades are the main reason preventing the development of hatchery production of mullet seed (Saleh, 1991).
In Italy, mullet farming is almost entirely based on extensive techniques, with coastal lagoons and semi-intensive ponds being restocked with wild juveniles. Artificial reproduction trials are currently underway, attempting to establish standard reproduction techniques for mullet (Landoli, 2000).
Mullets are usually grown in extensive, 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 fresh, brackish and marine waters.
In Egypt, where most of cultured mullet are produced, pre-farming preparation of ponds is of great importance. Prior to stocking, aquaculture ponds are prepared by drying, plowing and manuring with 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 the fingerlings are stocked. Productivity 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 have 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 as a supplement to the natural feed in ponds. When mullet are reared in polyculture feeding and fertilization programmes usually target the other cultured species while mullets feed on the natural feed, detritus and feed leftovers.
Acclimatized to the appropriate salinity, and stocked as 10–15 g individuals at about 6 200–7 400/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 about 2 500–3 700/ha together with 1 900–2 500/ha of 100 g common carp juveniles and about 62 000–74 000/ha of 10–15 g Nile tilapia fingerlings. Total harvests are typically 20–30 tonnes/ha/crop of which 2–3 tonnes are mullets. After an on-growing season of 7–8 months in the subtropical region, flathead grey mullet reach an individual weight of 0.75–1 kilogram. Mullet grown for two successive seasons, reaches 1.5–1.75 kilograms. The choice of rearing period and technique depends on market demand and economics.