Southern flounder, continued 



will cease feeding below 7.3°C (Prentice 1989). 



Salinity: The southern flounder is euryhaline. Larvae 

 have been found in salinities of 10 to 30%o (Ward et al. 

 1980). Salinities in which juveniles have been col- 

 lected range from 2 to 60%o, but they apparently prefer 

 waters that are 2 to 37%> (Ward et al. 1980). Adult 

 southern flounder have been collected in waters with 

 salinities that range from to 60%o, with a preference 

 for 20 to 30%o (Ward et al. 1980). Adults, while in 

 estuaries, prefer the mixing and tidal fresh zones 

 (Gunter1945). 



Dissolved Oxygen (DO): Deubler and Posner (1963) 

 demonstrated avoidance behavior in juvenile southern 

 flounder when dissolved oxygen levels fell below 3.7 

 mg/l, for temperatures 6.1°, 14.4°, and 25.3° C. 



Migrations and Movements : Adults emigrate from the 

 estuaries to spawn in deeper offshore waters during fall 

 and winter. The migrations coincide with falling water 

 temperatures (Gunter 1945, Kelley 1965, Shepard 

 1986). Males usually leave estuaries for the Gulf 

 earlier than females (Stokes 1 977). Hoese and Moore 

 (1977) report severe "northers" will result in mass 

 emigrations, while moderate to warm winters cause 

 flounders to leave dispersed over longer periods of 

 time. Stokes (1 977) indicates that only those emigrat- 

 ing are gravid. Some juveniles and adults overwinter 

 in the deeper holes and channels of bays and estuaries 

 (Ogren and Brusher 1977, Stokes 1977, Ward et al. 

 1980). Postlarvae and juveniles immigrate into the 

 bays and estuaries from late winter to spring. Williams 

 and Deubler (1968) indicated postlarval immigration 

 correlates with lunar phase. In addition, adults migrate 

 back into estuarine habitats throughout spring and into 

 summer. Juveniles tend to migrate to low salinity 

 water, often going up into river channels (Williams and 

 Deubler 1968, Pineda 1975). Stokes (1977) reported 

 that local movements within and between estuaries 

 rarely exceeded 18 km. 



Reproduction 



Mode : The southern flounder has separate male and 

 female sexes (gonochoristic). Fertilization is external, 

 by broadcast of milt and roe into the water column. The 

 eggs are buoyant, and float at or near the water surface 

 (Arnold et al. 1977, Gilbert 1986). Development is 

 oviparous. 



Spawning : Spawning occurs during late fall and early 

 winter in marine neritic waters (Sabins and Truesdale 

 1974, Reagan and Wingo 1985, Gilbert 1986) with a 

 December peak reported in Louisiana (Shepard 1 986). 

 In laboratory studies, Arnold etal. (1977) reported that 

 males attended females for a period of 3 weeks prior to 

 spawning. At spawning, the females would swim to the 



surface and release eggs which were immediately 

 fertilized by the attending male. Larvae of Paralichthys 

 species are known to occur in the northern Gulf of 

 Mexico from Septemberthrough April, with a peakfrom 

 December to February (Ditty et al. 1988). 



Fecundity : Arnold et al. (1 977) reported that 1 3 spawns 

 from 3 pairs of southern flounder produced a total of 

 120,000 eggs. 



Growth and Development 



Egg Size and Embryonic Development : Eggs are 

 spawned oviparously. Eggs are spherical, with an 

 approximate mean diameter of 0.91 to 0.92 mm, and 

 one oil globule with an approximate diameter of 0.18 

 mm (Henderson-Arzapalo et al. 1988, Powell and 

 Henley 1995). In a laboratory study, spawned eggs 

 hatched in 61 -76 hours at 1 7°C and 28%o (Arnold et al. 

 1977). 



Age and Size of Larvae : Recently-hatched larvae are 

 approximately 2.1 mm notochord length (NL) (Powell 

 and Henley 1 995). Larvae, 40 to 46 days old and 8 to 

 1 1 mm long, begin metamorphosis into the postlarval 

 stage. Transformation is complete by about 50 days 

 (Arnold et al. 1 977). Optimal growth in early postlarvae 

 occurs at high salinities (Deubler 1960); while ad- 

 vanced postlarvae grow better at salinities of 5 to 1 5%o 

 (Stickney and White 1973). In general, at any given 

 size, larval gulf flounder (P. albigutta) are further devel- 

 oped than southern flounder (P. lethostigma) (Powell 

 and Henley 1995). There are differences in pigmenta- 

 tion patterns between the two species, but these may 

 be difficult to discern with field-collected specimens. 



Juvenile Size Range : The minimum size of settled 

 juveniles overlaps that of the postlarvae in some cases 

 (10-15 mm TL). Peters (1 971 ) concluded P. lethostigma 

 grows faster at warm temperatures and low salinities. 

 Size-at-age is highly variable for this species, and age 

 year classes are known to develop bimodal length- 

 frequency distributions (Fitzhugh et al. 1996). This 

 may be the result of faster growth after an ontogenetic 

 shift to piscivory at a size of 70 to 180 mm TL. Size 

 estimated after the first and second year of growth is 

 201 and 250 mm TL for male, 225 and 364 mm TL for 

 female southern flounder (Stokes 1977). Immature 

 fish >170 mm TL have distinctive gonads and matura- 

 tion occurs by the second year in fish ranging from 341 

 to 560 mm TL. Maturity occurred in one study at 243 

 mm TL for females and 1 70 mm TL for males (Shepard 

 1985). 



Age and Size of Adults : Stokes (1977) reported a 3 to 

 5 year life span for this species. Females appear to 

 grow faster, live longer, and attain greater size than 

 males (Stokes 1977). The largest individuals reported 



336 



