Gulf Coast Flounders

This page describes the two common flounders on the Mississippi coast - the southern flounder and the Gulf flounder. The focus in on the more common southern flounder, with information specific to the Gulf flounder added where there are substantial differences.

Southern Flounder

Southern Flounder, illustration by Diane Rome Peebles
Copyright © Diane Rome Peebles

Gulf Flounder

Southern Flounder, illustration by Diane Rome Peebles
Copyright © Diane Rome Peebles

Similar Species

Only rarely will inshore anglers in Mississippi encounter anything but southern or Gulf flounders.

The broad flounder, Paralichthys squamilentus, is similar but is rarely found in inshore waters. Adults prefer depths of 100-220 meters. Juveniles are found in shallower water, but seldom in estuaries or bays. As the name implies, the broad flounder's body is wider, with the body width being half the length or greater.

Another 15 species of lefteye flounders are found in the northern Gulf of Mexico, but they are unlikely to be confused with the southern, gulf, or broad flounder.

Description

The southern flounder and Gulf flounder and other flatfish are compressed laterally and spend most of their life lying on the bottom or swimming along the bottom on their side. They have prominent eyes and a large mouth with large, sharp, pointed teeth.

Southern Flounder, Photo by Jim Franks, GCRL
Southern Flounder, Photo by Jim Franks, GCRL   Click for larger image

 

Southern flounder and Gulf flounder are known as left-eye flounders; the eyes of the adults are always on the fish's left side. The opposite is true for other species of flounders, halibut, and all the species of soles, whose eyes are on the right side.

The left or "up" side of the southern and Gulf flounders is light olive brown to dark brown or nearly black, with many blotches and spots of darker and lighter color. The eyeless "down" size side is white or dusky. Gulf flounders are distinguished by three large dark eye-like spots, arranged in a triangle with a pair of spots about midway on the length of the fish and a third closer to the tail. Southern flounders may also have scattered large spots but they are much more diffuse and gradually disappear as the fish grows older. The scales are small. As described below, flounders can modify their coloration and patterning to match the bottom.

Flounder lying in ambush and partially buried on a sandy bottom.
Flounder lying in ambush and partially buried on a sandy bottom. Click for a larger version.

Southern flounders are larger and live longer than Gulf flounders. Female southern flounders typically grow to about 28", while typical female Gulf flounders reach only about 18". Males of both species are smaller, typically reaching only 10 to 14" in length. After their first year of life, males spend most of their lives offshore, seldom venturing into estuaries and bays. Thus, the majority of flounders caught by anglers in inshore waters are females.

Flounders have small body cavities and lack a swim bladder, making it easier for them to maintain their position on the bottom.

Occurrence

The range of the southern flounder and the Gulf flounder overlap, and both species are common in Mississippi. Southern flounder occur from North Carolina, across the Gulf of Mexico, and southward into Mexico, with the exception of both coasts of southern Florida. The Gulf flounder is found in a continuous range from North Carolina, along both coasts of Florida, across the Gulf of Mexico to Texas.

Flounders are found in a wide range of salinity and water temperature, from shallow, low-salinity estuaries to nearshore and shallow offshore waters to depths of 200 feet. Southern flounders commonly enter fresh water and have been found in rivers 100 miles from the coast. They prefer soft sediment bottoms and are found throughout the estuaries and in the mouths of bays, bayous, and channels, often around rock jetties, piers, and pilings.

Female southern flounders remain in brackish waters most of the year, only moving offshore to spawn in fall and winter. Most adult males remain offshore year round.

Gulf flounders seem to prefer sandy bottoms, and typically stay further offshore as adults.

Life Cycle & Reproduction

For flounders on the northern Gulf coast, spawning occurs offshore in water depths of 60 to 200 feet between November and January, with a peak in December. Females typically spawn every three to seven days over a period of about two months, typically producing 17,000 to 100,000 eggs in each session. Larger females spawn more frequently and have larger clutches of eggs.

The fertilized eggs float to the surface and hatch in about two days, producing a larva about one tenth of an inch long. The translucent larvae drift with the currents and feed on plankton. At this stage they resemble most fish, with one eye on each side of their head. They swim upright with a side-to-side motion of the tail. As the larva mature they are transported by currents and tides into the shallow water of bays and estuaries.

Metamorphosis and Eye Migration

The stages of the southern flounder from early larva through settlement as a juvenile. Source: Southern Regional Aquaculture Center Publication No. 726, Species Profile, Southern Flounder, H.V. Daniels. N.C. State University.
The stages of the southern flounder from early larva through settlement as a juvenile. Source: Southern Regional Aquaculture Center Publication No. 726, Species Profile, Southern Flounder, H.V. Daniels. N.C. State University.

Near the end of their larval period, 30 to 60 days after hatching and at a length of 1/3 to 1/2 inch, the larval flounder settle to the bottom and begin a strange and complex metamorphosis. The skeletal system, muscles, and nerves of the head gradually rearrange themselves and the right eye moves to the left side of the head. The body also adapts to allow the fish to lie on the bottom and swim with its left side up using an up-and-down motion. The two sides of the fish change color to suit its bottom dwelling lifestyle, dark on the upper side and white on the lower side. The transition is completed in two to three weeks when the fish is a ¾ inch to one inch juvenile.

In flounders and other flatfish, the sex of the individual fish is not determined until after the metamorphosis is complete.

Development from Juvenile to Adult

The young fish enter the bays during late winter and early spring, seeking shallow grassy areas near the Gulf passes. As growth continues, they move farther into the bays and estuaries. Some even enter coastal rivers.

Typical Length of Female
Southern Flounders
Age
(years)
Typical length
(inches)
1 10
2 16.7
3 18.8
4 19.6
5 20
6 20.4
7 20.5
Source: Flounder Facts, Jerald Horst, 2003, Sea Grant Louisiana

Small southern flounder grow rapidly and may reach 12 inches in length by the end of their first year. Females grow as much as three faster than males. Female flounders grow rapidly for the first two years; then their growth slows. Approximate average lengths at each age for females are shown in the table. Males seldom exceed 16 inches in length.

Southern flounders reach maturity at an age of about two years, though the age at maturity is reported to vary with location along the Gulf coast. Female southern flounders live for about seven years, while almost no males live past three years old. Gulf flounders have shorter lifespans, about three years, though some studies suggest longer lives.

During the first year after hatching, both male and female southern flounders remain in shallow, brackish estuaries, and even in fresher water. They gradually move to deeper waters as they age, but remain within inshore estuaries during the first year. In the late fall, they congregate in large numbers in the lower portions of the estuaries near the Gulf of Mexico. As water temperatures cool, the fish move offshore into the Gulf to spawn, often in mass movements triggered by the arrival of a cold front. The return to inshore waters during the later winter and spring is much more gradual.

While flounders tend to remain in a small area while in inshore waters and often return to the same home area, the spawning migrations of individual flounders can cover large distances. In tagging studies on the coast of Georgia and the Carolinas, tagged southern flounders have been observed to move 300 to 400 miles.

After their first year of life, male southern flounders spend most of their lives offshore, seldom venturing into estuaries and bays. Gulf flounders, especially males, seldom venture into inshore waters. Thus, the majority of flounders caught by anglers in inshore waters are female southern flounders.

Diet

Southern flounders and Gulf flounders are wonderfully adapted ambush predators that lie in wait on the bottom, hidden by extremely effective camouflage. They are able to change the coloration and patterning of their upper side to match the surrounding bottom. This is done with cells called chromatophores in the skin of their upper sides that are capable of moving a dark brown or black pigment called melanin. It is the same mechanism that allows squid and octopuses to camouflage themselves to match their surroundings.

Gulf flounders blending into the bottom near an off-shore reef as well as a southern flounder partially covered with sediment. Source South Carolina Department of Natural Resources.
Gulf flounders blending into the bottom near an off-shore reef as well as a southern flounder partially covered with sediment. Source South Carolina Department of Natural Resources.

Their flattened shape allows them to become nearly invisible on the bottom. For futher camouflage, flounders flex and flutter their dorsal and anal fins to create a slight depression in the bottom and lift sand, sediment, and bits of shell and debris to cover themselves. Often, only the fish's eyes protrude above the bottom, moving rapidly as they watch for potential prey. When an unwary fish or shrimp comes within striking distance, the flounder erupts from the bottom and gulps it's prey in an instant.

Adult flounders often feed in shallow water at night. The abandoned depressions where flounder had lain in wait for prey, called "beds", are often clearly visible hours later, sometimes even on dry sand after tides have falled.

Southern Flounder Eating Bull Minnow
Source: Low Country Estuarium

Soon after hatching, flounder larvae eat mostly plankton. As juveniles, their diet consists mainly of fish spawn, crustaceans, polychaetes, and small fish. At a length of about six inches, they shift to the adult diet of fish and shrimp. Adults favor schooling fish such as menhaden, anchovies, pinfish, grunts, pigfish, croakers, and mullet. A research project in Mississippi showed that southern flounders’ stomachs most frequently contained fish, with one-third also containing shrimp. It has been observed that as flounders get larger they don’t eat larger fish like most other predatory fish, but simply eat more small fish.

Adult flounders consume from four to eight percent of their body weight daily. Feeding activity is heaviest at water temperatures of 61 to 77ºF and during the three days following a first quarter moon and the three days before a new moon.

Predators

As larvae and juveniles, flounders are preyed upon by a variety of fish. As adults, their camouflage and bottom dwelling lifestyle provide effective protection from most potential predators. Dolphins and sharks are known to feed on adult flounders.

Fishing for Flounders

Flounders are popular sportfish and food fish throughout their range. On the northern Gulf coast, flounders are popularly regarded as one of the "big three" of sportfish, along with red drum (redfish) and spotted seatrout (speckled trout). Large flounders are referred by proud anglers as "doormats."

Flounders are caught not only on conventional hook and line tackle and techiques but by gigging. Fishers walk or pole a small boat along the shore in very shallow water and shine a bright light onto the bottom to aid in spotting flounders, which are then speared with a gig. Successful gigging requires relatively clear water and a calm night with little wind to ruffle the surface. Keen eyesight and concentration are needed to discern the faint outline or the eyes of a camouflaged and buried flounder.

Most of the commercial catch of flounder occurs as incidental catch in shrimp trawls, though scattered individual commercial fishermen using gigs or rod and reel also fish for the market.

Flounder on the Table

Broiled flounder

Broiled flounder, often stuffed with crabmeat, is one of the classic seafood dishes of the northern Gulf coast. Flounders are also enjoyed fried, baked, sauteed, and raw as sashimi. Flounders have mild flavored white flesh, and may be cooked as whole fish or as filets.

Conservation Status

The status of both the southern flounder and the Gulf flounder is listed as "Least Concern" on the IUCN Red List, though the population status of the Gulf flounder is described as decreasing.

Aquaculture

Flounders offer significant potential for aquaculture, due to their attractiveness as a table fish, their fast growth rate, and their tolerance for wide ranges of water temperature and salinity. Research is being conducted with southern flounder in Texas and North Carolina. An aquaculture program in New Hampshire with the similar winter flounder is being pursued for stock enhancement.

Resources and References

 

Flounder Videos

Each link opens in a new window. Close the new window to return here. Some of these videos show flounder species other than the southern flounder or Gulf flounder, but do illustrate typical flounder behavior.

Gyotaku print of a 21" southern flounder from Chandeleur Island, Louisiana

Gyotaku print of a 21" southern flounder from Chandeleur Island