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Summer flounder, Paralichthys dentatus.  Image courtesy NOAA, Chesapeake Bay Office of Fisheries.

Common Name:
The preferred common name for this species is summer flounder; however, there are many other common names used in reference to this species: fluke, plaice fish, plaice, plaise, splaice, chicken halibut, brail, turbot, flatfish, long-toothed flounder, flounder of New York and common flounder.

Synonymy:
None

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

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Recorded range for this species is from Maine to Cape Canaveral, Florida (Powell and Henley 1995).

IRL Distribution:
P. dentatus can be common throughout the Indian River Lagoon, however, it is most common north of Cape Canaveral.

III.  LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Powell (1974) showed that the annual growth cycle in P. dentatus begins in the spring and ends in the fall as the water temperature reached approximately 7 °C threshold. Flounder in North Carolina were 111 - 219 mm (4.4 - 8.6 inches) TL at the end of their first season. Maximum sizes of males collected from New York were about 600 mm (23.6 inches) TL and 2200 g (5.9 pounds), while females reached 800 mm (31.5 inches) and 5500 g (14.7 pounds) (Powell 1974).

Summer flounder may live about 10 years (Rogers and Van Den Avyle 1983).

Growth rates published for flounder collected outside the South Atlantic Bight were summarized by Smith et al. (1981).

Abundance:
P. dentatus 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 the fall and winter, though some remain in estuaries year-round. Summer flounder begin a spawning migration as they near peak gonadal development. Often the oldest, largest fish migrate first each year (Morse 1981). P. dentatus spawns during late fall, winter, or early spring on or near the bottom in shelf waters ranging from 30 - 200 m deep (Rogers and Van Den Avyle 1983). Evidence indicates that P. dentatus is a serial spawner, continuously releasing mature eggs throughout the spawning season (Morse 1981; Rogers and Van Den Avyle 1983). Evidence from Wilk et al. (1980) shows that summer flounder from Middle Atlantic stocks tend to use the same spawning grounds and wintering areas each year. This pattern has not been proven for stocks in the South Atlantic Bight.

In the Middle Atlantic Bight, the spawning cycle is strongly correlated with the cooling of coastal waters throughout fall and winter. Thus, spawning along the Atlantic coast begins along a North-South gradient, with summer flounder in the northernmost region beginning to spawn in September, and those in the southern region spawning in early spring. In the area between Virginia and North Carolina, the major spawning period for P. dentatus is from November to late January and early February (Smith 1973). From Cape Hatteras through Florida, P. dentatus begin spawning in late November to early December, and are spent by early spring.

Embryology:
Larvae spawned offshore make their return to estuarine habitats by passive transport on nearshore and tidal currents from November through April in North Carolina, with a peak in recruitment occurring in February (Burke et al. 1991).

IV.  PHYSICAL TOLERANCES

Temperature:
Data for the Middle Atlantic Bight and Cape Hatteras areas indicate that adults spawn when bottom water temperatures are 12 - 19 °C. However, most eggs are collected when temperatures are between 18 – 19 °C. Temperatures between 5-21 °C promoted faster development of embryos and yolk-sac larvae. During development, temperatures below 11 °C were lethal to larvae, but at higher temperatures, all larvae were approximately the same length by the time the yolk-sac had been absorbed (Grimes et al. 1989).

Temperature has a pronounced effect on growth efficiency feeding rate, and assimilation efficiency of juveniles held under laboratory conditions (Grimes et al 1989).

Salinity:
Laboratory studies show that growth rates in P. dentatus increase with increasing salinity. Maximum growth rate occurred at salinities greater than 10 ‰, which correlates with typical environmental salinity levels during the period when young summer flounder are most abundant in estuaries (Rogers and Van Den Avyle 1983).

Summer flounder eggs and larvae from wild populations develop in offshore waters, with late stage, premetamorphic larvae (stage 4b to 5), likely returning 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, data from Burke et al. (1991) suggest that settlement in P. dentatus is influenced by substratum type. In a comparison with larvae of P. lethostigma, the southern flounder, the authors reported that though larvae of summer flounder and southern flounder both recruit into estuaries during the same period, southern flounder larvae concentrate on tidal flats near the heads of estuaries where salinity ranges from 9 - 25‰, and the substratum has a low sand content (4 - 50%). Conversely, summer flounder larvae settle more downstream, in the low to middle reaches of estuaries where salinity ranges from 24 - 35 ‰ and the substratum has a much higher sand content (53 - 95%). Burke et al. (1991) concluded that southern flounder settlement was more highly correlated with salinity, while summer flounder settlement was more highly correlated with substratum type.

Other Physical Tolerances:
     Dissolved Oxygen:
     Effects of dissolved oxygen concentration on P. dentatus has not been
     investigated, but studies of the southern flounder, P. lethostigma, a close
     relative to P. dentatus, indicate that this species is likely to prefer water with
     dissolved oxygen concentrations exceeding 5.3 mg/l (Deubler and Posner
     1963; Rogers and Van Den Avyle 1983).

     Environmental Contaminants:
     Toxic levels of most contaminants to summer flounder have not been quantified
     (Rogers and Van Den Avyle 1983). However, Hall et al. (1978) showed that
     arsenic, copper, and zinc residues were somewhat high in summer flounder
     collected in the South Atlantic Bight. Mean and maximum values for mercury
     concentrations were below U.S. Food and Drug Administration action levels.
     Low levels of some polynuclear aromatic hydrocarbons were detected in
     summer flounder collected from the Baltimore Canyon in the Middle Atlantic
     Bight (Brown and Pancirov 1979; Rogers and Van Den Avyle 1983).

V.  COMMUNITY ECOLOGY

Trophic Mode:
Adult P. dentatus are visual feeders that are adept at feeding on the bottom or in the water column (Olla et al. 1972). They are generally regarded as top or near-top predators. Though the feeding ecology of this species is not fully documented, larvae and postlarvae are thought to initially feed on zooplankton. In a Pamlico Sound study, juveniles longer than 80 mm were found to initially consume mysid shrimp. As they grew, they fed on progressively larger prey items, shifting their diets from mysids to small fish and other crustaceans. As these fish reached adulthood, the diet was again shifted toward larger fish (Rogers and Van Den Avyle 1983). Data from the South Atlantic Bight indicate that adults in estuaries and shelf waters north of Cape Hatteras feed primarily on fish and large invertebrates (Rogers and Van Den Avyle 1983).

Burke (1995), based on differences in morphology and behavior between summer flounder and southern flounder in North Carolina, compared prey distribution and feeding ecology between the 2 species following metamorphosis to the juvenile stage. Summer flounder juveniles have generally smaller mouths, smaller, teeth, and lighter, more numerous gill rakers than do southern flounder. Feeding in the summer flounder was always preceded by active searching behavior (Burke 1995), while southern flounder tended to remain still on the bottom and wait for prey to come within striking distance (Minello et al. 1987; Burke 1995). 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. In contrast, southern flounder primarily consumed amphipods and mysid shrimp, followed by copepods, insects, fish and invertebrate parts. Burke 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, while summer flounder continued to feed upon benthic prey organisms.

Habitats:
Adult summer flounder spend the warmer months in nearshore shelf waters and coastal embayments (Rogers and Van Den Avyle 1983). Summer flounder eggs and larvae from wild populations develop in offshore waters, with late stage, premetamorphic larvae (stage 4b to 5), likely returning to estuarine habitats via passive transport on nearshore and tidal currents. Once returned to estuaries, larvae settle to the substratum and metamorphose into juveniles.

In a comparative study, Burke et al. (1991) reported that larvae of both summer flounder and southern flounder recruit into estuaries during the same period, and for a time, show considerable overlap in distribution within an estuary (Burke 1995). 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. Premetamorphic larvae from summer flounder generally move into the 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). 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. 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 vs. 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. Data from Rogers and Van Den Avyle 1983 are in agreement with Burke et al. (1991) study, showing that juvenile summer flounder occur more frequently over sandy substrata than mud or silt bottoms in Pamlico Sound, North Carolina. They further reported that during daylight hours, summer flounder tended to occupy areas in estuaries that have submerged vegetation.

Activity Time:
Laboratory studies and field collections indicate that summer flounder are active primarily during daylight hours (Rogers and Van Den Avyle 1983).

VI.  SPECIAL STATUS

Special Status:
Fisheries.

Fisheries Importance:
         
          COMMERCIAL FISHERY:
P. dentatus is an important commercial and recreational fish along much of the Atlantic seaboard, but the commercial fishery is not substantial in the southernmost extent of its range (between Cape Hatteras, North Carolina and Florida). The estuaries of Pamlico Sound, North Carolina are believed to be a major nursery ground for juvenile summer flounder from the Middle Atlantic Bight (Cape Hatteras northward to Cape Cod) and South Atlantic stocks, but this is uncertain because dispersal patterns are not well understood (Rogers and Van Den Avyle 1983).

In 1982 the Atlantic States Marine Fisheries Commission prepared a Fishery Management Plan for summer flounder which included recommendations for the best management practices for this species. Beneficial practices included regulating the annual catch and setting size limits for harvest; regulating commercial gear types; maintaining and protecting wetland habitat areas; controlling sedimentation; and controlling sources of thermal, chemical and physical pollution (Rogers and Van Den Avyle 1983). Among the adverse management practices cited in this plan were draining wetlands, marshes, ponds and lakes; dredging; wastewater assimilation and disposal; flow withdrawal of water supply; shoreline development; and construction of migration barriers (Atlantic States Marine Fisheries Commission 1987).

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.