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  Southern flounder, Paralichthys
  lethostigma.  Image courtesy of South
  Carolina Department of Natural Resources.

Common Name:
Southern flounder, mud flounder, doormat and halibut

Body color is light to dark brown with diffuse non-ocellated dark spots and blotches. The blindside is white or dusky. P. lethostigma are characterized by the following meristic (number of structures per body part) counts:

Synonymy:
None

Other Taxonomic Groupings:
Subphylum: Vertebrata
Superclass: Osteichthyes
Subclass: Neopterygii
Infraclass: Teleostei
Superorder: Acanthopterygii
Suborder: Pleuronectoidei

Potentially Misidentified Species:
Paralichthys lethostigma is sometimes confused with P. albigutta, the gulf flounder. The two are easily distinguished based on the much smaller size of the gulf flounder, which grows only to 15 inches (38 cm). Additionally, the gulf flounder has 3 ocellated spots: 2 vertically placed posterior to the pectoral fins, and 1 placed inside the base of the tail.

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
The southern flounder, Paralichthys lethostigma, occurs from North Carolina south through Florida and the Gulf of Mexico to Texas.

III.  LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Southern flounder attain a size of up to 3 feet (91 cm) in length, and can weigh as much as 9 kg (24.1 lbs.) (Smith et al. 1999). The von Bertalanffy growth model predicts a maximum age for summer flounder of approximately of 20 years (Reagan and Wingo 1985).

Abundance:
P. lethostigma is one of the largest and most commercially valuable flounders in the western North Atlantic (Burke et al. 1991).

Reproduction:
Adults migrate to offshore spawning grounds during late fall and winter, though some remain in estuaries year-round. Spawning migrations are usually preceded by a drop in water temperature of 4 - 5 ° C. Males move seaward earlier than females, with few remaining in estuaries after November (Reagan and Wingo 1985). In North Carolina, southern flounders begin migration in the fall; in Texas, they migrate from October through December (Reagan and Wingo 1985).

Laboratory experiments from Texas indicate that approximately 3 weeks before spawning takes place, male southern flounder begin following gravid females. In tank experiments, the first spawning was in December and occurred at midday. Females swam to the surface and released eggs that were immediately fertilized by attending males. Fertilization was 30% to 50% successful, and 6% to 35% of the eggs hatched within 61 - 76 hr (Reagan and Wingo 1985).

Females become sexually mature at 2 years of age in Texas, while the youngest mature female southern flounder in northern Florida was 4 years old (Reagan and Wingo 1985).

Thirteen southern flounders examined in the laboratory, produced a total of 120,000 eggs (approximately 9,230 eggs per female) (Reagan and Wingo 1985).

Embryology:
Eggs of P. lethostigma range from 0.85 - 0.95 mm in diameter (Powell and Henley 1995), with a single oil globule. Laboratory rearing of southern flounder shows that eggs hatch after 3 days at 18° C and 30‰ salinity (Denson and Smith 1997). Larvae begin to feed when 4 -6 days old, and show signs of becoming premetamorphic by day 14. By day 16, larvae begin to settle out of the water column and congregate on the bottom. By day 21, larvae show signs of adult pigmentation, and begin to rest on their left sides, though their eyes remain in position. By day 23, metamorphosis is initiated and the left eye begins to migrate to the right side. Most animals had completed metamorphosis by day 36 (Denson and Smith 1997). In culture experiments, yolk-sac larvae began metamorphosing to postlarvae at 40 - 46 days, when they were approximately 8  - 11 mm in length. Metamorphosis to the juvenile stage was complete by 50 - 51 days (Reagan and Wingo 1985). Data from Powell and Henley (1995) show that larvae complete metamorphosis when they reach approximately 8.7 - 9.0 mm SL. By this time, the migrating eye has reached the dorsal midline, and the larval stage is complete.

Powell and Henley (1995) examined egg and larval development in both P. lethostigma and P. albigutta. Results from their study show that fins begin to develop when larvae reach approximately 5.4 mm notochord length (NL). The dorsal fin is generally the first to begin development, followed by the caudal, anal, pelvic, and pectoral fins (Powell and Henley 1995). Development of the caudal fin in P. lethostigma can begin when larvae are approximately 5.5 mm NL, but fin rays are not observed until larvae attain 8.2 mm SL. Dorsal fins begin to develop when larvae are in the preflexion stage, at approximately 6.5 mm NL. The dorsal fin is first observed in the head region, with development proceeding posteriorly. By the time larvae reach 8.4 mm standard length (SL) the dorsal fin is fully developed. Following postflexion, when larvae reach approximately 7.3 mm standard length (SL), anal fin rays begin to develop, with the full adult complement of fin rays reached at a body size of 8.4 mm SL. Pelvic fins are first observed on larvae at approximately 8.2 mm SL, and are fully developed by the time larvae attain 9.7 mm SL. Pectoral fins first begin formation when larvae are approximately 8.4 mm SL, and are fully formed when larvae exceed 11.0 mm SL.

Pigmentation is first observed in middle-stage eggs, following blastopore closure. In larvae, pigmentation is more pronounced in the caudal area, being less developed overall in P. lethostigma than in its close relative P. albigutta, the gulf flounder (Powell and Henley 1995).

Larvae spawned offshore in the Atlantic Ocean make their return to estuarine habitats by passive transport on nearshore and tidal currents from November through April, with a peak in recruitment occurring in February (Burke et al. 1991). In the gulf of Mexico, Southern flounder postlarvae are caught along the Gulf of Mexico coast during winter and early spring. At Galveston Island, Texas, southern flounder postlarvae 18 - 34 mm in total length (TL) were captured during February, March, and May. Fish 25 - 51 mm TL were caught in the Mississippi River during the spring. In Aransas Bay, Texas, the peak movement of postlarvae flounders into estuaries is in February, when water temperature is 16.0 - 16.2 °C (Stokes 1977).

IV.  PHYSICAL TOLERANCES

Temperature:
Temperature influences the migration of postlarval and adult southern flounders (Reagan and Wingo 1985). In Louisiana coastal waters, adult southern flounders have been collected at temperatures ranging from 5 - 35 °C.

Salinity:
Southern flounder are highly euryhaline, and withstand fluctuations in salinities ranging from 0- 35 ‰ or more. Data from Smith et al. (1999) indicates that salinity tolerance in this species increases with age.

Adult P. lethostigma make extensive migrations from estuarine habitats in order to spawn in offshore waters. Eggs of P. lethostigma are buoyant at 32 ‰, and sink at 29‰, though data indicate that eggs that sink may still hatch. Experiments conducted by Smith et al. (1999) showed that eggs incubated in water with a salinity between 0-5 ‰ all died within a day. Eggs incubated at 10 ‰ had greater hatching success (82%), but all larvae moved sluggishly and died shortly after hatching. Eggs incubated in water of salinities between 15 - 35 ‰ were normal in appearance and were active. However, larvae reared through metamorphosis to the juvenile stage showed greater mortality at 15‰ salinity than at either 25‰ or 35‰. Further, recently metamorphosed juveniles showed less tolerance to freshwater conditions than did older fish (Smith et al. 1999).

Southern flounder eggs and larvae from wild populations develop in offshore waters, with late stage, premetamorphic larvae (stage 4b - 5), likely returned to estuarine habitats via passive transport on nearshore and tidal currents. Once returned to estuaries, larvae settle on the substratum and metamorphose into juveniles. In one North Carolina study, comparative data from Burke et al. (1991) suggests that settlement in P. lethostigma is influenced by salinity. These authors reported that though larvae of both summer and southern flounder begin to recruit into estuaries during February, southern flounder larvae concentrated on tidal flats near the heads of estuaries where salinity ranged from 9 - 25‰, and the substratum had a low sand content (4 - 50%). Conversely, summer flounder larvae settled more downstream, in the middle reaches of estuaries where salinity ranged from 24 - 35 ‰ and the substratum had a much higher sand content (53 - 95 %).

Burke et al. (1991) also reported that juvenile southern flounder moved further upstream to more riverine environments later in the spring, while juvenile summer flounder tended to remain in higher salinity tidal areas near Spartina saltmarshes. Catch comparisons showed that approximately equal numbers of southern flounder were caught in sandy versus muddy substrate types in low salinity waters; while summer flounder were most abundant in sandy substrata in higher salinity waters. Burke et al. (1991) concluded that southern flounder settlement is more highly correlated with salinity, while summer flounder settlement is more highly correlated with substratum type.

Other Physical Tolerances:
     Dissolved Oxygen:
     P. lethostigma tolerates low dissolved oxygen concentrations (less than 5
     mg/l). However, in a laboratory study, postlarval southern flounder attempted
     avoidance when dissolved oxygen concentrations in culture vessels fell below
     3.7 mg/l (Reagan and Wingo 1985).

V.  COMMUNITY ECOLOGY

Trophic Mode:
Southern flounder are carnivorous fishes that are generally considered to be top or near-top predators. Larvae reared under laboratory conditions begin feeding on rotifers 4 - 6 days posthatch. By 8 - 13 days posthatch, larvae begin to feed on newly hatched Artemia nauplii (Denson and Smith 1997).

Small southern flounders consume a wide variety of invertebrate prey, but upon reaching approximately 20 mm total length (TL), they become primarily piscivorous (Reagan and Wingo 1985). Based on differences in morphology and behavior between summer flounder and southern flounder in North Carolina, Burke (1995) compared prey distribution and feeding ecology between the 2 species following metamorphosis to the juvenile stage. Southern flounder juveniles have generally larger mouths, larger, inwardly curved teeth, and fewer, heavier gill rakers than do summer flounder. They also tend to remain still on the bottom, waiting for prey to come within striking distance (Minello et al. 1987; Burke 1995). Small southern flounder primarily consumed amphipods and mysid shrimp, followed by copepods, insects, fish and invertebrate parts. In this study, summer flounder 20 - 60 mm SL consumed spionid polychaete worms, followed by clam siphons, mysid shrimp, calanoid copepods, the blue crab, Callinectes sapidus, and small fishes. From this data, Burke (1995) concluded that post-settlement differences in feeding habits developed between the 2 species, with southern flounder shifting to more mobile prey which could be attacked from below, and summer flounder continuing to feed upon benthic prey organisms.

The adult diet of P. lethostigma consists primarily of fish, but is augmented by crustaceans depending upon regional location. In Louisiana, adult southern flounders eat shrimp and fish; though, Fox and White (1969) reported that the primary prey species for southern flounder was striped mullet (Mugil cephalus). Also included in the diet are fat sleepers (Dormitator maculatus) and anchovies (Anchoa spp.). Larger flounders (150 mm long) ate primarily anchovies, menhaden (Brevoortia spp.), sciaenids, and mullet (Reagan and Wingo 1985).

Habitats:
Adult P. lethostigma spend the warmer months in coastal embayments and riverine habitats in the upper reaches of estuaries (Rogers and Van Den Avyle 1983). Many adults migrate to offshore spawning grounds during late fall and winter, though some do remain in estuaries year-round. Larvae spawned offshore make their return to estuarine habitats by passive transport on nearshore and tidal currents. In a study conducted in North Carolina, Burke et al. (1991) reported that peak recruitment into estuaries by late stage (stage 4b and 5), premetamorphic larvae occurred in February, though larvae were collected from late November through mid-April. These larvae settled into tidal mudflats near the head of the estuary, however, in spring, southern flounder apparently migrated upstream into riverine habitats. Spring through summer, southern flounder prefer the silt and mud substrata of coastal bays and river systems, and become most common in the upper reaches of estuaries, sometimes entering freshwater (Burke et al. 1991; Smith et al. 1999).

In a comparative study, Burke et al. (1991) reported that larvae of both southern and summer flounder recruit into estuaries during the same period, and for a time, show considerable overlap in distribution within an estuary. However, segregation occurs quickly (Burke et al. 1991; Burke 1995). Premetamorphic larvae of southern flounder tend to concentrate on tidal flats in the upper reaches of estuaries where salinity ranges from 9 - 25 ‰, and the substratum consists of 4 - 45 % sand. Conversely, premetamorphic larvae from summer flounder generally move into silt and mudflat areas in the lower and middle reaches of estuaries where salinity ranges from 24 - 35 ‰ and the substratum consists of 50 - 95 % sand (Burke et al. 1991). Capture data following segregation of the 2 species within the Newport River Estuary, North Carolina, showed that summer flounder were most common on sand flats than on mudflats in the lower estuary, while there was little difference in capture rates among southern flounder in sandy vs. muddy substrates in the upper reaches of the estuary. Burke et al. (1991) concluded that settlement in P. dentatus is most likely influenced by substratum type, while that of P. lethostigma is influenced by salinity.

Activity Time:
A tank study showed that southern flounder tend to be more active at night than during the day (Reagan and Wingo 1985).

VI.  SPECIAL STATUS

Special Status:
Fisheries.

Fisheries Importance:
The southern flounder is a valuable sport and commercial fish along the Gulf coast of the United States. Most of the commercial catch in the Gulf of Mexico is incidental to the catch by shrimp trawlers. There has been significant interest in utilizing southern flounder as an aquaculture species. Studies in the southeastern U.S. and in the Gulf of Mexico are currently underway to improve spawning techniques and develop larval rearing methods for southern flounder in order to improve its attractiveness as an aquaculture product (Jenkins and Smith 1999; Smith et al. 1999).

Flounders of all species are harvested annually from waters in and around the Indian River Lagoon, and are especially prized by recreational anglers.  However, the commercial fishery is not of particularly high value.  For the years 1987 - 2001, 1.7 million pounds of flounders were harvested, with a dollar value of over $3.1 million reported in the 5 county area encompassing the IRL (Volusia, Brevard, Indian River, St. Lucie and Martin Counties).   This ranks flounders nineteenth in commercial value within the IRL, and twenty-ninth in pounds harvested.   

Figure 1 below shows the dollar value of the flounder fishery to IRL counties by year.  Note that all species of flounders were combined in the data presented.  As shown, commercial catch ranged from a low of $77,149 in 1987 to a high of over $350,927 in 1999.  Volusia County annually accounts for the largest percentage of the flounder catch with 83% in total (Figure 2),  followed distantly by Brevard County, which accounts for 8% of the total.  Indian River, St. Lucie and Martin Counties account for 3%, 4% and 2% of the total respectively.  Note that the fishery's value brings in $125,000 - $300,000 annually to Volusia County businesses, while in all other IRL counties, the dollar value is typically less than $25,000.

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

 Figure 2.  Breakdown of total  flounder dollar value by county for the years 1987 - 2001.

 

	Volusia	Brevard	Indian River	St. Lucie	Martin	Total
	Value	Value	Value	Value	Value	Value
YEAR	($)	($)	($)	($)	($)	to IRL
1987	$52,332	$16,478	$462	$4,740	$3,137	$77,149
1988	$125,679	$24,502	$773	$4,896	$6,499	$162,349
1989	$159,271	$0	$0	$5,312	$0	$164,583
1990	$135,210	$20,461	$1,113	$10,947	$5,396	$173,127
1991	$168,724	$20,692	$2,446	$22,654	$6,410	$220,926
1992	$117,085	$16,988	$2,813	$15,816	$4,620	$157,322
1993	$182,403	$20,647	$1,574	$11,826	$7,469	$223,919
1994	$202,828	$15,739	$6,091	$5,041	$5,984	$235,683
1995	$238,435	$14,654	$6,773	$5,227	$4,412	$269,501
1996	$137,805	$7,207	$7,347	$638	$2,625	$155,622
1997	$194,655	$20,528	$19,640	$7,254	$2,787	$244,864
1998	$145,311	$16,449	$13,133	$14,215	$5,855	$194,963
1999	$306,281	$25,090	$9,182	$8,772	$1,602	$350,927
2000	$265,389	$19,629	$3,097	$13,248	$3,244	$304,607
2001	$148,233	$14,655	$9,103	$4,789	$1,256	$178,036
Cumulative Totals:	$2,579,641	$253,719	$83,547	$135,375	$61,296	$3,113,578

Table 1.  Total dollar value of flounders to IRL counties between 1987 -2001.