SRAC Publication No. 726 VI October 2000 PR Species Profile Southern Flounder H.V. Daniels1 Flounder are known for their Natural history Some researchers hypothesize that unique and spectacular transfor- juvenile and young adult flounder mation from a normal-looking fish Range remain in low salinity water to with an eye on each side of the Southern flounder are found in overwinter for the first 2 years of head to one with both eyes on the rivers and estuaries along the life, migrating out to the ocean same side of the head. This meta- Atlantic Coast from North when they reach sexual maturity morphosis occurs while they are Carolina to northern Florida, and at 2 years of age. still larvae and is one of the more from Tampa Bay, Florida along the Appearance fascinating transformations Gulf coast into southern Texas. among fishes. Southern flounder Their distribution is discontinu- Adult flounder are asymmetrical (Paralichthys lethostigma) are in the ous around the southern tip of in appearance. Instead of swim- family Bothidae and the genus Florida, leading some biologists to ming through the water column Paralichthys. Other important cul- wonder if there are two genetical- like other fish, flounder rest on tured paralichthids are the sum- ly separate natural stocks. the bottom with a dark pigmented mer flounder (P. dentatus) and the Southern flounder are found in a side facing upwards and a white, Japanese flounder (P. olivaceous). wide range of salinities; adults unpigmented side facing down. There is considerable interest in have been captured in a range of 0 Both eyes and nostrils are on the the culture of southern flounder to 36 ppt salinity, and it is not upper side of the head. The because of its worldwide market uncommon to catch them by hook mouth is also slightly twisted appeal, high market value, and and line far inland on coastal toward the upper side. ability to grow in fresh or brackish rivers. Larval flounder look like other water. Southern flounder are Life history fish until they reach metamorpho- found throughout the southeast- sis. During metamorphosis, which ern U.S. and it might be possible Adult southern flounder migrate begins about 30 to 40 days post- to culture them over a large geo- offshore during the fall to spawn hatch, the right eye slowly graphic area. Because these floun- in marine waters. The spawning migrates to the left side of the der appear to grow well in low season begins in December in the head, the jaw twists slightly, and salinity water, growout operations northern extreme of their natural the fish changes from a side-to- could be located farther from range, and in late January to side swimming motion to an up- high-priced coastal areas, reduc- February in the southern extreme. and-down motion. When meta- ing the fixed costs associated with Adults return to estuaries and morphosis is complete in about 2 flounder farming. However, rivers immediately after spawn- to 3 weeks, the fish resemble research on southern flounder cul- ing. Larval flounder feed on zoo- adults and thereafter rest on the ture only began about 5 years ago. plankton in offshore waters for 30 bottom most of the time. Most of this work has concentrat- to 60 days; then metamorphosis Feeding habits in the wild ed on the hatchery phase of pro- begins and the larvae are washed duction. Definitive information is through inlets into estuaries. After Flounder feed by ambushing not yet available on growout metamorphosis, juvenile southern passing prey in a rapid upward methods or the economic feasibili- flounder begin migrating up the lunge, accompanied by a vacuum ty of southern flounder culture. rivers. action of the mouth, to capture and ingest the food in one swift 1North Carolina State University water for only a short time and can be transported in specially constructed live wells placed on the deck of a boat. Some researchers have obtained brood- stock caught by hook and line, from either commercial or recre- ational fishers, but this method is stressful to the fish and may lead to mortality or poor reproductive performance. Artificial spawning Because southern flounder spawn during fall and winter, the envi- ronmental conditions required to induce spawning are a short pho- toperiod of 9 to 10 hours and a water temperature of about 61 oF (16 oC). Photothermal condition- ing should be started several months in advance of the planned spawning date. Daylength and temperature should be reduced gradually and reach target levels at least 2 weeks before spawning to allow the females sufficient time to begin the process of egg development. Broodstock main- tained under these conditions can continue producing eggs for sev- eral months. Most researchers currently work- ing with southern flounder brood- stock use some form of hormone intervention to promote final egg maturation and spawning. Cellulose/cholesterol implants Figure 1. Southern flounder life cycle. containing a synthetic analogue of gonad releasing hormone (GnRHa) are placed into the mus- motion. Wild flounder consume batches of about 100,000 eggs per cle about midway between the mainly shrimp and small fish. kg (45,000 eggs per pound) over dorsal fin and the lateral line. A After feeding, they immediately several days. Although the num- dosage of 50 to 100 micrograms/ glide back down to the bottom. ber of eggs released per female at kg is used on female flounder Cultured flounder feed in the any one time is relatively low with maximum oocyte diameters same way as wild fish, but can be compared to other types of fish of 500 micrometers. Eligible trained to come to the water sur- with the same weight, total egg females will have a marked face to eat dry, pelleted feed. production is similar if all egg batches are combined. Eggs are swelling in the abdominal area. Reproduction in the wild about 1 mm in diameter, nearly Females with smaller egg diame- ters cannot be forced to matura- Adults migrate out of rivers and transparent with a single oil tion with hormone implants. estuaries in late fall and spawn in droplet, and highly buoyant. Generally, eggs will reach final the warmer offshore waters of the maturation and ovulation about Gulf stream from November to Culture techniques 48 hours after implantation, and February. Fertilized eggs have can be easily stripped and mixed been found at depths of 100 to Broodstock procurement with sperm from running males. 650 feet (30 to 200 meters). When Because adult flounder migrate at Eggs are released from the ventral southern flounders reach sexual certain times of the year to spawn, or blind side. Sperm is released maturity at 2 years of age, the they can be easily caught in from the dorsal or eyed side. males weigh 300 to 400 g (250 pound nets at the mouths of inlets Viable eggs float high in the water mm, 10 inches) and the females or along the shoreline of estuaries. column but not all viable eggs are weigh 800 to 1,000 g (350 mm, 14 High quality fish can be obtained fertilized. The fertilization rate of inches). Females spawn small this way as the fish are out of floating eggs can be determined at 6 hours post-fertilization. At this Larviculture meters and have an orange or red- time the embryos are in a multi- dish color similar to the Artemia. Eggs hatch after a 55-hour incuba- cell stage that is easily identified Once they are successfully tion period at 63 oF (17 oC). at 100 x magnification on a com- weaned, flounder are graded by Recently hatched larvae do not pound microscope. size to reduce cannibalism and have fins, eyes or mouths, but stocked into nursery tanks. At this Researchers have used strip- develop them during the 5 days time, the fish are about 1 inch spawning to produce fertilized before first-feeding. Larvae are (2.5 cm) long. eggs for larvae culture. Although stocked before first-feeding at 75 strip-spawning requires handling to 115/gallon (20 to 30/L) in 250- Growth rates the fish and is stressful to them, to 1,000-gallon (1- to 4-cubic Little is known about southern this method has the advantage of meter) tanks. Most culturists add flounder growout rates and feed producing a sufficient number of algae to the tank water at a densi- conversion values, but it has been eggs in a short period of time, ty of 570,000 cells/gallon (150,000 determined that southern floun- which is more convenient for cells/L). By first-feeding the yolk der females grow approximately stocking larviculture tanks. is completely absorbed, but the oil three times faster than males. As Recently, tank spawning without drop will remain for several days. with other flatfish, the sex of the hormone intervention has pro- Rotifers are fed at about 5/mL fish is not determined until after duced a significant number of fer- until about 15 to 20 days post- metamorphosis; the precise time tilized eggs, but this method has hatch, when they begin to eat is not known for southern floun- not yet reached the level of relia- Artemia nauplii (Fig. 2). Artemia der. In Japanese flounder, the sex bility needed for commercial-scale are fed at 1/mL initially, then up of the fish can be influenced by larval rearing. Tank spawning is to 5/mL through metamorphosis temperature. The optimum tem- clearly the least stressful method until the start of weaning at day perature for producing the highest on the fish, as handling and anes- 55 post-hatch. Metamorphosis percentage of females is approxi- thetizing are eliminated.