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Home > Biological Info > Southern Flounder

Biological Info: Southern Flounder

Southern Flounder

Scientific Name:
Paralichthys lethostigma
Common Names:
Flounder, Doormat
Range & Habitat:
This common fish is found Gulfwide, on mud, and to a lesser degree, sand bottoms, from shallow, low-salinity estuaries to nearshore and shallow offshore waters. Southern flounders commonly enter fresh water and have been found 100 miles up rivers.
Identification & Biology:
The body is very compressed laterally and right side is white and eyeless. The left side has both eyes and is olive brown in color with dark and white spots. Flounders, like other fish, hatch with one eye on each side of their head. Movement of the right eye to the left side of the head begins when the fish is a to ½ inch in length and is complete when the fish is ¾ inch to 1 inch in length. At this same time, the left side develops its dark color and the right side turns white.

Male and female southern flounder are almost like two different species of fish. Males grow slower and have a short life span, almost never living over 3 years old or growing over 14 inches long. Females live longer and can grow to 28 inches. Also, after their first year of life, males live mostly in offshore waters. This means that most of the inshore catch consists of females. Southern flounder, both male and female, spend their first year after hatching in shallow, low-salinity estuary and even river waters. As they grow, they tend to use slightly deeper waters, but still within inshore estuaries. As temperatures cool in the fall, mature southern flounders move to the lower portions of the estuaries near the Gulf of Mexico, where they stage in large numbers. Between mid-October and mid-November they begin a mass migration into Gulf offshore waters to spawn.

This migration can be slow if water temperatures cool gradually, or it can happen all at once with the passage of a strong cold front. Between this period and February/March, very few large southern flounders are found inshore. Once offshore, spawning activities take place between November and January, with a peak in December. During the two months each female spawns, they will spawn every 3 to 7 days, producing an average of 44,225 to 62,473 eggs per spawn. After spawning, the females move back into the estuaries between January and April to start the cycle over again. The seasonal spawning movement cycle is the key to understanding southern flounder biology. Tagging studies in several southeastern states indicate that between spawning migrations, southern flounders move only short distances, usually within the same bay system. The spawning migration, however, reshuffles the deck and some fish move considerable distances. A Georgia study showed maximum movement of 334 miles. South Carolina research showed 243 miles. One North Carolina study had a southern flounder move 257 miles and another 444 miles. In the last study, a fish moved 387 miles in 131 days, which is an average of 3 miles a day.

Southern flounder are well adapted for ambushing quick-moving prey such as fish or shrimp. Their flattened shape allows them to become nearly invisible on the bottom. Their brain has large optic lobes to serve their large eyes, and they have large mouths and strong teeth. Typically, they remain motionless on the bottom and wait for their prey to come within striking distance before attacking. While waiting, flounder show rapid eye movements as they track their prey. Research indicates that flounders will eat from 4 to 8 percent of their body weight in food each day. Feeding activity is heaviest at water temperatures of 61 to 77º F and during the 3-day period following a first quarter moon and the 3-day period before a new moon.

Southern flounders eat a wide variety of food items, including shrimp, mullet, anchovies, croakers, and menhaden (pogies). One research project in Texas reported southern flounders to be the dominant fish predator on brown shrimp during the spring in Galveston Bay. The researcher also noted an increase in the predation rate on brown shrimp in muddy water. This may have been due to murky water giving the flounder a feeding advantage or to a change in shrimp behavior. When southern flounder feed on fish, they seem to prefer smaller fish. Unlike most predatory fish which eat larger fish as they get larger, flounder just eat higher numbers of small fish.
Up to 3 feet in length and 20 pounds, but most fish caught are 1-5 pounds.
Food Value:
Excellent; very lean, white flesh.
More Info:

Flounder Facts (fact sheet)

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