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Species Name: 
Mugil cephalus Linnaeus, 1758

Common Name:
Striped mullet, black mullet, sea mullet, flathead mullet, gray mullet.

Striped mullet is generally the most widely used of the common names for this species; however, black mullet and sea mullet have been extensively used, depending upon geographic location.

Species Description:
The striped mullet, Mugil cephalus, can attain 18" in length and reach approximately 3 pounds. Body shape is cylindrical anteriorally, becoming somewhat laterally compressed toward the posterior. Adult coloration is bluish-gray or greenish above, becoming silver along the sides of the body, and white on the ventral surface. There are 6-7 black horizontal bars along the sides of the body, and no obvious lateral line. The pectoral fins are placed high on the shoulders, and the pelvic fins are abdominal. M. cephalus has a blunt snout, and a small, somewhat upturned mouth.

Nine species of mullet occur in the west central Atlantic ocean (Ditty and Shaw 1996). In Florida, M. cephalus is the most common of the mullet species, but also occurs with M. curema, the white mullet, and M. gyrans, the fantail mullet. Differences in fin rays and fin morphology help separate species. There is an apparent seasonality of larvae, which also assists in separating larvae of M. cephalus from other species, especially M. curema. Larvae of M. cephalus are most abundant in the northern Gulf of Mexico from November through December, while those of M. curema are most abundant from April through May.

Other Taxonomic Groupings:
Division Teleostei in the Class Actinopterygii replaces the former class Osteichthyes as the newer taxonomic grouping for the ray-finned fishes (Compagno 1991, Nelson 1994, Bond 1996). The teleost fishes are further divided into subdivisions, with the mullets ordered as follows: Subdivison Euteleostei, Superorder Acanthopterygii, Series Mugilomorpha, Order Mugiliformes.

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Mugil cephalus occurs worldwide from approximately 42° N to 42° S Latitude (Bok 1979, Render et al. 1995), where it inhabits estuarine intertidal, freshwater and coastal marine habitats. In the western Atlantic Ocean, M. cephalus ranges from Cape Cod to Brazil, including the Gulf of Mexico, Caribbean, and West Indies (Amos and Amos 1997).

IRL Distribution:
Mugil cephalus occurs lagoon-wide, with juvenile fishes most common in impounded areas, around mangroves, in seagrass beds, and offshore throughout the late fall and winter.

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Mugil cephalus attains an adult size of 46 cm (18 inches). In the first year, it grows to 17.8 - 22.2 cm (7 - 8.5 inches), and weighs 64 - 119 g (2.3 - 4.2 oz.).

Prejuvenile mullet from 0.17 - 0.35 cm (0.07 - 0.14 inches) standard length (SL) are a distinct silvery color, with evident countershading (i.e., they are generally darker on the dorsal surface than on the ventral surface). Prejuveniles and small juveniles form loose schools of tens to hundreds of individuals that occupy shallow, warm, near-shore water where they feed intensely and continuously. Prejuveniles undergo metamorphosis to the juvenile stage before they reach 0.50 cm (0.2 inches). The most striking change seen after metamorphosis is the loss of silver body color , especially along the dorsal side. Countershading is still evident, however. Other metamorphic changes in juvenile mullet include elongation and convolution of the intestine, development of adipose eyelids, transformation of some soft anal fin rays into spines, and changes in the morphology of the teeth and lips (Major 1978). The ontogenetic shift in the diet of M. cephalus from feeding primarily on copepods and other small zooplankton, to feeding on detritus and algae is coincident with metamorphic changes in the intestine, teeth and lips of the fish as it becomes a juvenile (Major 1978).

Abundance:
M. cephalus is the most abundant of the mullet species throughout much of its range, especially in fresh water and estuarine areas.

Locomotion:
Rulifson (1977), in tests to assess maximum burst swimming speed of mullet, found that most juveniles between 2.5-6.5 cm (1 - 2.5 inches) SL could sustain maximum swimming speeds of at least 12.7 body lengths (L) per second, for 30 seconds. These findings led Rulifson to suggest that M. cephalus juveniles could reach a maximum burst speed of over 20 L/s for approximately 2 seconds.

Reproduction:
Female mullet reach sexual maturity in their fourth year, when they are between 40 - 42 cm (15.8 - 16.5 inches). Males mature in their third year, once they reach a size of 33 - 38 cm (13 - 15 inches). The minimum spawning size of females is between 31 - 34 cm (12.2 - 13.4 inches) (Apekin and Vilenskaya 1979). The general reproductive pattern of Mugil cephalus involves migration from either fresh or estuarine waters to offshore waters where they spawn in large schools. Larvae and prejuveniles then migrate to inshore estuaries where they inhabit shallow, warm water in the intertidal zone.

Beginning in the early fall, large schools of mullet aggregate in the lower reaches of estuaries and at river mouths in preparation for offshore migration to spawning grounds.  Environmental cues such as falling water temperatures, passage of cold fronts and falling barometric pressure are thought to trigger aggregation and subsequent migration (Mahmoudi 2000).  Spawning occurs in deep, offshore waters from mid-October through late January, with peak spawning occurring in November and December (Ditty and Shaw 1996).   Mugil cephalus are isochronal spawners, with all oocytes reaching maturity at the same time. However, based on the size of the female body cavity, it is unlikely that a female's entire store of eggs is hydrated at the same time in preparation for spawning. Rather, females are likely to hydrate eggs in batches (Thompson 1958, Render et al. 1995) and spawn on successive evenings until their supply of yolked eggs is depleted.  Female fecundity ranges from 270,000 - 1.6 million eggs per individual per season (Render et al. 1995); absolute fecundity is between 2.9 - 16 million eggs (Apekin and Vilenskaya 1979).

M. cephalus has a generally well defined and short recruitment period throughout its range. In South Africa, Bok (1979) found that recruitment of 0.15 - 0.40 cm (0.06 - 0.16 inches) fork length (FL) fry takes place from July to October, with fewer numbers in May, June and November. Chubb et al. (1981), studying mullet in Australia, found that, based on the first appearance of small juveniles between 0.20 - 0.30 cm (0.08 - 0.12 inches), spawning in Australia occurs between March and September. In Hawaii, the reproductive season of the striped mullet is between September and March (Kelly 1990). In Florida, M. cephalus spawns offshore from October through mid-January, with spawning completed by late February (Render et al. 1995, Ditty and Shaw 1996).

Larvae become abundant in the waters of the northern Gulf of Mexico between November and December (Ditty and Shaw 1996) in water temperatures between 23-25° C. Tag returns along the U.S. Gulf of Mexico coast indicate that M. cephalus do not make extensive migrations in this region, but instead remain in a relatively small area and return to their original bay system after spawning (Funicelli 1989, Mahmoudi et al. 1989, Ditty and Shaw 1996).

Embryology:
The eggs of Mugil cephalus contain a single oil globule. Egg size varies according to location, and with water temperature. Apekin and Vilenskaya (1979) measured oocyte diameter in Black Sea M. cephalus as between 425 - 632 um. In Hawaii, Shehadeh et al. (1973) measured oocytes between 650-700 um., with mature eggs reaching up to 930 um. Kou et al (1974), in a later study, determined the egg size of Hawaiian mullet to range from 0.621 mm - 1.09 mm.

Eggs are shed and fertilized in the water column, and hatch within 48 hours (Render et al. 1995).  Newly hatched larvae of M. cephalus measure approximately 2.2 - 2.6 mm (0.87 - 1.0 inch) (Bensam 1987; Eda et al. 1990). Larval pigmentation consists of thick, stellate chromatophores covering the body, except in the posterior region. Additionally, larvae and early postlarvae of M. cephalus possess a midlateral row of stellate melanophores. This pigmentation pattern helps distinguish M. cephalus larvae from those of other mullet genera (Bensam 1987).

The mouths of larval mullet are open by the second day of post hatch, with the yolksac fully absorbed by the fifth day. Active feeding begins prior to full absorption of the yolksac, as early as 70 hours post hatching, with young larvae beginning to take rotifers and microalgae as food. Adverse effects from withholding food become evident as early as 3.5 days after hatching. Larval mullet die within 192 hours (8 days) if not fed (Eda et al. 1990).

IV.  PHYSICAL TOLERANCES

Temperature:
Embryos of M. cephalus develop optimally at temperatures of approximately 21°C. Greatest hatching success was achieved at 22 - 25°C (Sylvester et al. 1974, 1975).  Growth of embryos was retarded at temperatures above 26°C (Kou et al.1974). Critical thermal maxima (CTM) for juvenile mullet ranges from 30 ° C to 42.5 ° C depending on acclimation temperature.

Acclimation to higher temperatures appears to be a selective advantage to prejuvenile and juvenile mullet, which routinely choose shallow, warm waters in the intertidal zone. In studies of M. cephalus in the Pacific Ocean, Major (1978) and Chubb et al. (1981) found that prejuvenile mullet approximately 20 mm SL leave the open ocean to enter estuaries where they select shallow waters with extensive diel fluctuations in both temperature and salinity. Prejuvenile fish < 50 mm (1.97 in.) SL were found in shallow pools with high, often near lethal temperatures of between 34 - 42.5 ° C, and salinities ranging from 2 - 30 ppt. (Major 1978). This choice of marginal habitats, in conjunction with the schooling behaviors observed in prejuveniles and juveniles, is thought to be beneficial as a predation refuge for small fish. As mullet grow larger and their predation risk is lessened, they are then able to move away from the high intertidal zone into open waters where temperature and salinity are more stable. When mullet complete their metamorphosis to the juvenile stage (at approximately 50 mm SL), they begin moving into somewhat deeper areas of the intertidal zone. In a laboratory experiment, Major (1978) showed that as body size increased, the temperature range within the chosen habitat tends to decrease. Older juveniles (> 50 mm SL) maintained themselves seaward of the tide line in waters of lower temperature and more uniform salinity (Major 1978). These observations have lead some authors to speculate that the biochemical and hormonal changes that result from metamorphosis result in the preference of older juveniles and adults for reduced temperatures.

Kulikova, Shekk and Rudenko (1986) studied M. cephalus juveniles under conditions of cold stress. Findings from this study showed that juvenile mullet are active at temperatures above 10 - 12 ° C. With decreasing temperature, they begin to sink toward the bottom where they form dense schools. At temperatures below 5 ° C, juveniles respond sluggishly to food, and will eventually cease feeding. At 2 - 3 ° C, they remain nearly motionless at the bottom. During the first week under these conditions, red hemorrhages appear on the snout and fins. In the next week, hemorrhages appear under the skin, and on the internal organs, followed by tissue necrosis, and eventually, death. 

The Kulikova, Shekk and Rudenko (1986) study also found that short, cold periods have little long-term effect on mullet. Older mullet were found to overwinter in deep bays where temperatures sometimes reach as low as 7 ° C. In the laboratory, young mullet were able to endure brief cooling to temperatures of 1 - 1.5 ° C for up to 1 day. There were no adverse effects on mullet when they were gradually warmed to their original acclimation temperature. Cold stress did occur, however, if fish were repeatedly subjected to alternate cold/warm cycles.

Changes in red blood cells also occur in thermally stressed mullet. Low temperatures decrease the functionality of blood hemoglobin and lead to asphyxia. Additionally, lipids in the body are not catabolized (Kulikova, Shekk and Rudenko 1986) to maintain bodily function. Rather, mullet that are cold stressed increase the catabolism of bodily proteins. This results in the alteration of blood chemistry with respect to increased ammonia nitrogen excretion.   

Salinity:
Adult M. cephalus are highly euryhaline, and survive in a range of salinities from 0 ppt. in fresh water, to hypersaline waters with salinity as high as 90 ppt. (Lee and Menu 1981). However, Hu and Liao (1981) showed that female M. cephalus are generally unable to ovulate in fresh water.

Before being spawned, the eggs of M. cephalus are isosmotic to the blood of the parent (Lee and Menu 1981). In the laboratory, fertilization rates were optimal at 22.5 ppt. and eggs incubated at temperatures between 22 - 24 °C hatched optimally at salinities between 22 - 23 ppt. (Hu and Liao 1981). Hatching was successful at salinities from 15 - 42 ppt., but increased to over 50% at salinities above 19 ppt. At 11 ppt., the majority of fertilized eggs died, and those that did develop further, ultimately died in the gastrula stage. However, Lee and Menu (1981) showed that naturally spawned eggs developed to the embryonic stage at salinities between 5 - 60 ppt. Hatching in this study occurred at salinities between 10 - 55 ppt., but no larvae survived at either 10 or 55 ppt. In the Lee and Menu (1981) study, the optimal salinity range at an incubation temperature of 22 to 25 °C, was between 30 - 40 ppt., peaking at 35 ppt.

In aquaculture trials, young M. cephalus subjected to various salinites (Murashige et al. 1991) showed short-term differences in growth. Fish raised in tanks with a salinity of 22-23 ppt. grew faster than those held at higher salinity. However, in the long term, there was no significant difference in growth rates among the groups. This finding led the authors to suggest that there is no particular advantage or disadvantage to maintaining uniform salinity levels for tank-reared mullet.

Other Physical Tolerances:
Mullet are particularly susceptible to red tide organisms.  It has been estimated that as many as 16 different pathologies can be involved in cases of red tide-induced death in mullet, and that concentrations of 250,00 cells/L of Gymnodidium breve are sufficient to cause mortaility in mullet (Mahmoudi 2000). 

V.  COMMUNITY ECOLOGY

Trophic Mode:
Mugil cephalus is a heterotroph that, as an adult, is primarily a detritus feeder. Mullet are highly flexible in their food habits, with possible ontogenetic shifts in the diet as they age and make the transition from post-larva to adult. The mullet has a gizzard-like pyloric stomach and intestine that is 3-5 times the length of the body, indicating a principally herbivorous diet (Service et al. 1992). Many studies of feeding habits in mullet (Suzuki 1965, Odum 1968 and 1970, Zismann et al. 1975, Bishop and Miglarese 1978) found that juvenile M. cephalus (< 30 mm) are primarily carnivorous. However, De Silva and Wijeyarante (1977) found that young mullet (20-55 mm) feed primarily on diatoms (55.5%), followed by green algae (22.3%), Xanthophycea (15.5%), Cyanobacteria (6.1%) and animal matter (principally foraminiferans and copepods, 0.6 %).

In one study, detritus and sand first began to appear in mullet over 25 mm in length (0.9 in.), and increased in percent occurrence as body length increased (De Silva and Wijeyarante 1977). This finding led the authors to conclude that 25 mm may represent a transitional size in mullet where they gradually begin to alter their trophic mode from being primarily planktonic or carnivorous feeders to being primarily benthic feeders. The findings of De Silva and Wijeyarante (1977) contradicted other studies in that these authors found a diurnal pattern of feeding in mullet, with peaks of activity occurring around dawn and midday, regardless of the state of the tide. Others (Odum 1970) found that M. cephalus feeds almost continuously throughout the day, and their feeding intensity varies with tidal state.

Bishop and Miglarese (1978) found that the principal food sources of adult mullet are detritus and epiphytic algae. However, these authors also observed Mugil cephalus feeding opportunistically on swarming polychaetes of the Nereis genus. This observation lead the authors to suggest that since M. cephalus lack the mouthparts for tearing and cutting, predation must be limited to bite-sized prey, or to prey items which break apart easily. Odum (1970) made a similar observation about the opportunistic nature of feeding in mullet, stating that mullet will select food with higher caloric value whenever presented with the opportunity.

Mullet also actively ingest "marine snow," a composite material which consists of detritus, mineral grains, phytoplankton, microorganisms, and small nematode worms all bound together in a mucous matrix. Particles range in size from 0.5 mm to several cm in size. Marine snow is nearly always present in coastal and estuarine environments (Larson and Shanks 1996), and is a valuable food source to detritivores such as M. cephalus.

Competitors:
Juvenile mullet > 50 mm, as well as adults, both appear to compete with prejuveniles in estuarine regions, with some evidence suggesting that limited habitat partitioning occurs. Major (1978) observed larger mullet moving into an intertidal estuarine region during high tide to feed on the same food resources used by smaller mullet during low tide. As the larger mullet moved in on the high tide, the smaller fishes moved closer to the shoreline in the high intertidal zone.

Predators:
Lizardfish, needlefish, crabs, etc. prey on juvenile M. cephalus. Larger mullet are subject to larger predators such as snook, snappers, barracuda, dolphins, etc. In the presence of large predators, prejuvenile, juvenile and adult mullet organize into tightly formed schools and tend to cease feeding activity. In the absence of predators, schools become more loosely organized, and individuals will feed constantly during low tides (Major 1978).

Habitat:
Juveniles occupy the high intertidal zone of estuaries where water temperatures and salinity fluctuate greatly. Older mullet inhabit deeper, more stable waters.

VI. SPECIAL STATUS

Special Status:
Fisheries.

Benefit in the IRL:
M. cephalus is an important commercial and recreational fishery species in the Indian River Lagoon. Adult mullet are line caught as food fish and for roe, while juveniles are commonly used as bait for larger sportfish.

Fisheries Importance:
          COMMERCIAL FISHERY:
In the Western Atlantic, M. cephalus is commercially valuable on the east coast of Florida and in the Gulf of Mexico, especially in western Florida and Louisiana, where historically a large portion of the commercial catch consists of mullet (Render et al. 1995). Florida's fishery for mullet is primarily centered around fresh and smoked mullet, as well as roe harvesting for shipment to Asian markets (Render et al. 1995, Ditty and Shaw 1996).

Mugil cephalus is one of the most important animal protein sources for people in the Pacific Basin, Southeast Asia, India, the Mediterranean, Eastern Europe, Central America and South America (Nash 1978). It has gained popularity as a widely cultured food fish throughout Europe and Asia (Lee and Menu 1981).

The striped mullet is a high value fishery species within Florida. The statewide commercial catch of Mugil cephalus between the years 1987 - 2001 was 232.9 million pounds, with a dollar value of over $115.2 million.  Over the same time period within the 5 county area encompassing the IRL (Volusia, Brevard, Indian River, St. Lucie and Martin Counties) the commercial catch of M. cephalus accounts for approximately 10% of the statewide total, with a harvest of 23.9 million pounds, and a value in excess of $11 million.   This ranks the striped mullet eleventh in commercial value within the IRL, and sixth in pounds harvested.   

Figure 1 and Table 1 below show the dollar value of the striped mullet fishery to IRL counties by year.  As shown, commercial catch ranged from a low of $456,700 in 1991 to a high of over $1.1 million in 1994.  Volusia County annually accounted for the largest percentage of the catch with approximately 39% of the total (Figure 2, Table 2),  followed by Brevard County, which accounts for 23% of the total.  Indian River, St. Lucie and Martin Counties account for approximately 14%, 15%, and 10% of the harvest respectively. 

Of interest is the change in the mullet harvest following implementation of the gill-net ban in 1995.  With the exception of 1996, landings of mullet within IRL counties decreased, but have held at a relatively stable level, with a commercial value of approximately $600,00 annually.  This trend is reflective of the mullet harvest throughout Florida.  Mahmoudi (2000) reported that fishing effort was reduced 54% from 1995 - 1999 following the ban on gill-netting.

Figure 1.   Annual dollar value of the commercial catch of striped mullet to the 5-county area
     of the Indian River Lagoon. 

Figure 2.  Total striped mullet dollar value and percentage by county for the years 1987 - 2001.

Table 1.  Total dollar value of the IRL harvest of striped mullet, Mugil cephalus, between
     1987 -2001.