Flatfish Such as Flounder and Sole

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FLATFISH

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There are over 500 species of flatfish. They include flounders, soles, turbot, plaice and halibut. Their unique body features — their flat shape, a pair of eyes on the top of their head — are suited for living on the bottom of the sea on one side. Flatfish such as flounders generally live in saltwater and marine environments in coastal areas. They live a benthic lifestyle on sand and rock bottoms at depths up 80 meters (262 feet). Sometimes they enter estuaries and areas with brackish water.

Newly hatched flatfish look like other fish larvae. When they are around six weeks old one of their eyes — usually the left one — begins migrating across the or head to the other side of the body. Many species feed on thin-shelled mollusks and worms that lie buried in the sediments and can change the color of their upper body so the are well camouflaged on the sea bottom floor.

Halibut are a large flatfish that are highly valued as a food fish. The Atlantic halibut can reach lengths of 2.5 meters and weigh as much as 315 kilograms. Unlike other flatfish they are active swimmers and are often caught far from the sea bottom.

The right-eyed flounder family (Pleuronectida) have both eyes on the right side and lie on the ocean floor on their left side. The left-eyed flounder family (Bothidae ) have both eyes on the left side and lie on the ocean floor on their right side. Left-eye flounders are also distinguished by the presence of spines on the snout and near the eyes.

See Halibut and Turbot Under POPULAR NORTHERN COMMERCIAL FISH: COD, POLLOCK AND HALIBUT ioa.factsanddetails.com

Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems

Flounder

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Flounders are bottom-living fish with two eyes on the top sides of their bodies. They move along the sea floor by undulating their enlarged dorsal and anal fins that fringe their sides. Flounder have pectoral fins but they serve no real purpose. The fish spends most of its time buried in the sand, gazing upwards with its two eyes.

Flounder begin life resembling other fish but a few months after they hatch they go through an extraordinary metamorphosis. They lose their swim bladder and their head twists so the mouth moves sideways and one eye migrates around the body and takes a position on the top of the body next to the other eye.

Some flounder have their eyes on the right side of their head; others have them on their left side. Eyes start out one on each side. As the fish develops one eye migrates to join the other one on the side of the head. A team of researchers led by Tohru Suzuki of Tohuku University in Japan has discovered the gene that controls eye migration and found that a slight brain disorder can determine which side the eyes migrate to. The same gene — pitx2 — also determines the formation of the human heart on the left side of the body.

Larval flounder look transparent (an effect of lighting). They swims around in the open sea. Many are gobbled up by predators. Their eyes begin to migrate 20 to 40 days after birth with the body turning dark on the side on which the eyes appear.

Flounder Physical Characteristics

Flounder are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature) and heterothermic (have a body temperature that fluctuates with the surrounding environment). Males and females may have different shapes. Smaller medium-size species reach lengths of 45 centimeters (18 inches), with their average length being 35 centimeters (14 inches).[Source: Kelsey Otterbein, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web (ADW): flounders are flattened and circular in shape, with both eyes located on one side of their body. In members of the family Bothidae eyes appear on the left side, which is also more brightly colored. Members of this family possess unequal pelvic fins; the fin on the eye-side is longer.

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Flounders are generally brown in color. Sometimes they have rings or spots near the head and fin area. Most flounder adults can rapidly change color, which is used to blend in with the sea bottom. Male and female flounder look similiar. Males occasionally have a longer pectoral fin on the eye-side than females.

Plate fish larvae look very different than adults. Larvae are 5.5 to 39.5 millimeters in length, with each eye on a separate side of the flattened body. Dorsal and anal fins are fully formed in the larval stage, and an elongated ray emerges from the dorsal fin. Larvae are almost free of pigment, making them nearly transparent. The only pigmentation appears as a cluster of melanophores at the base of the elongated ray of the dorsal fin. Larvae also have no teeth. /=\

Flounder Behavior, Perception and Communication

Flounders are generally solitary, diurnal (active mainly during the daytime), motile (move around as opposed to being stationary), territorial (defend an area within the home range). and live a benthic lifestyle on or near the bottom of the sea. Individuals may spend their days and nights in separate areas, but their overall home range is not large. Male flounders of some species inhabit territories anywhere from 50 to 360 square meters, and females inhabit smaller areas within these territories, from 20 to 50 square meters. Although some species and individuals occasionally migrate in order to spawn, this is not typical. [Source: Kelsey Otterbein, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web (ADW) Male flounders have distinct territories that do not overlap. Each male defends its own territory and exhibits defensive behavior when other males enter their claimed area. Males chase off intruders and display aggressive behavior toward other males. Females, however, do not usually display aggressive behavior. Females do not have the same types of territories, but rather have designated subunits that are smaller in area. Daytime territories are not the same as night retirement sites for members of either sex. Males retire closer to shore and females in deeper water, returning to their daytime territories early in the morning. [Source: Kelsey Otterbein, Animal Diversity Web (ADW) /=]

Flounders communicate with vision, touch and chemicals usually detected by smelling. They also use vibrations to communicate and sense using vision, touch, vibrations and chemicals usually detected with smell. They have a lateral line, a sense organ that detects movements and vibrations in the water. The eyes on the top of the adult body are used as visual organs.

Flounder Food, Eating Behavior and Predators

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Flounders serve as prey for larger fish and are predators to smaller fish and marne invertebrates. Adult flounders typically feed on other benthic organisms living on or near the bottom of the sea. They are active predators, mostly eating other fish; however, peacock flounders can also feed on marine invertebrates, such as crustaceans and sometimes octopi. The diet of juveniles commonly includes drift and benthic algae. [Source: Kelsey Otterbein, Animal Diversity Web (ADW) /=]

Plate fish (peacock flounders) have also been observed feeding on French grunts and Caribbean sharpnose puffers. The latter is somewhat toxic, and it has been speculated that peacock flounders have a unique feeding behavior to effectively stun prey.

Predators of adult and larval flounders include large fish, sharks, rays and various species of snappers. When flounders are threatened, they dive into the ocean bottom. They stay hidden, partly covered in sand, until the predator or other disturbance is gone. Living primarily in shallow waters lowers the risk of predation as fewer larger fish live in these areas.

Many flatfishes have the ability to induce changes in their coloration, often in order to match their background, the bottom of the ocean. This behavior is controlled through neurotransmitters, which send signals that mediate changes in the melanophores. Different colors result from different concentrations of pigment granules in different areas. These changes are always reversible.

Flounder Mating and Reproduction

Flounder are oviparous (young are hatched from eggs) and engage in external reproduction in which sperm from the male fertilizes the female’s egg outside her body. They also employ broadcast (group) spawning, the main mode of reproduction in the sea. It involves the release of both eggs and sperm into the water and contact between sperm and egg and fertilization occur externally. (Thresher, 1988) [Source: Kelsey Otterbein, Animal Diversity Web (ADW) /=]

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Flounder often engage in year-round breeding and are polygynous (males having more than one female as a mate at one time). Some species have extended spawning periods and participate in "serial spawning." Some scientists believed spawning is triggered by warming and cooling trends. Some species in the mid-Atlantic travel north during spring and south during autumn to spawn. /=\

According to Animal Diversity Web: Peacock flounders have a "harem" mating system, in which one male mates with multiple females. Several females have sub-territories within a male's territory. On average, one male mates with six females. Males are defensive of their territory and the females within their territory, denying access to other males. /=\

Mating activities usually begin just before dusk. At this time, a male and a female approach each other with the ocular pectoral fin erect. The two fish arch their backs and touch snouts. After this interaction the female swims away, and the male sometimes follows, approaching the female again from the left side. At this point the male pectoral fin is erect and the female pectoral fin moves up and down, possibly signaling willingness to mate. The male then positions himself underneath the female and mating begins. This process consists of a mating rise, during which the female and male rise in the water column together. On average, these rises last about 15 seconds. At the highest point of this rise, usually around two meters above the substrate, gametes from both fish are simultaneously released, producing a cloud of sperm and eggs.

Once the couple returns from the rise, the male "checks" to make sure mating was successful, and the pair separates quickly, swimming away from each other in opposite directions. Not all mating rises are successful, and the process of "checking" is thus important. The exact purpose of the mating rise in these flounders unknown; possible reasons for rising include better dispersal of gametes and predator avoidance.

Potential mates communicate through touch, using the ocular pectoral fin. Some kind of signal is sent between organisms during the “checking period” but it is unknown whether this is a visual signal or a chemical (pheromonal) signal. Spawning at dusk reduces the chance of predation during mating, because there are not as many potential predators in the water column at this time. The upward mating rise may also confuse predators.

Flounder Offspring and Development

Females produce eggs in large batches, and many eggs are fertilized at once. This strategy is used by many families of fish, including bothids, soles, and tonguefish. Because flounders practice broadcast spawning, and many small eggs are produced at once, it is presumed that there is no parental involvement in parenting. The survival rate for individual zygotes or larvae is very low. [Source: Kelsey Otterbein, Animal Diversity Web (ADW) /=]

The flounder life cycle is characterized by metamorphosis — a process of development in which individuals change in shape or structure as they grow. Most flounders do not reach sexual maturity directly after metamorphosis, but rather spend time as juveniles. This time varies among individuals, particularly among individuals living in different areas with different resources.

According to Animal Diversity Web (ADW): Peacock flounders have three stages of life: egg, larval, and adult. Larvae and adults differ in coloration, body shape, and symmetry. As larvae become adults, body depth increases and shape becomes more circular. The swim bladder present in larvae disappears, and adults develop teeth. Another considerable change in morphology is the migration of the right eye to the left side of the body. As this migration occurs, peacock flounders become asymmetrical and blind on the right side. Eye migration occurs through a slit formed during separation of the dorsal fin from the cranium. This method is distinctive of this species; other closely related species utilize a hole in their head for this process. /=\

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Changes in pigmentation also occur during metamorphosis of peacock flounders. Similar to those in related flatfishes, larval melanophores (pigment-cells) are present on both sides of the body, but are not abundant. During metamorphosis, melanophores disappear from the blind side and are present only on the eye-side. The final pattern is determined by differentiation of adult pigment cells. /=\

Sex differentiation of peacock flounders is controlled by the endocrine system. Hermaphrodites are rare. Levels of various sex steroid hormones have direct effect on germ cell development, leading to the formation of different gonads, either male or female sex organs and associated secondary sex characteristics. /=\

Arrowtooth Flounder

Arrowtooth flounder (Scientific name: Atheresthes stomias) are a relatively large, reddish-brownish colored flatfish with a large mouth. Also known as flounder, arrowtooth halibut, turbot and paltus, they’re members of the Pleuronectidae right-eyed flounder family and have light tan-colored fins. [Source: NOAA]

Arrowtooth flounder grow slowly and can live up to 27 years. Males can reach 61 centimeters (2 feet) in length, and females grow a bit larger, up to almost 92 centimeters (3 feet). Males sexually mature when they reach 3 to 7 years old, and females are able to reproduce when they reach 4 to 8 years old.

Arrowtooth flounder spawn multiple times during a spawning season, releasing eggs that are then fertilized externally. Spawning season varies by location: Off the West Coast it is from late fall through early spring. In the Gulf of Alaska it is during spring and summer. Off the coast of Alaska it occurs during fall and winter. Arrowtooth flounder eggs hatch in deep water (below 400 meters) and rise up the water column as they develop, then settle to the ocean bottom during the summer and fall. Larvae eat copepods, a type of small crustacean.

Juveniles and adults feed on crustaceans (mainly pink shrimp and krill) and fish (mainly cod, herring, and pollock). A variety of fish and marine mammals prey on arrowtooth flounder, including skates, sharks, shortspine thornyhead, halibut, orcas, other toothed whales, and harbor seals. In the Gulf of Alaska, arrowtooth flounder are an important part of the diet of Steller sea lions..

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In U.S. waters, arrowtooth flounder are found from Northern California through the Bering Sea. Juvenile and adult arrowtooth flounder live on the ocean floor. They’re most commonly found on sand or sandy gravel habitat and occasionally over low-relief rock-sponge bottoms. During the summer, arrowtooth flounder feed in shallow water on the continental shelf. They migrate to deep water over the continental slope to spawn in the winter..

Arrowtooth Flounder Fishing

In 2021, the commercial landings in the U.S. of arrowtooth flounder totaled 16 million kilograms (36 million pounds) and were valued at $3.8 million, according to the NOAA Fisheries commercial fishing landings database. Commercial interest in arrowtooth flounder has grown in recent years. Although arrowtooth flounder are a low-value fish, fishermen have been retaining more of the arrowtooth they catch — up to about 80 percent in Alaska. Catches have been higher because arrowtooth flounder are more abundant, resulting in higher incidental catch in other fisheries, in addition to increased marketing efforts for arrowtooth fish meal and surimi. [Source: NOAA]

Bottom trawl gear is used to catch arrowtooth flounder. In general, arrowtooth flounder live on sandy or sand/gravel habitats. These soft bottom habitats are usually more resilient than other habitats to trawling impacts. Arrowtooth flounder are included in these fishery management plans because of their importance to the ecosystem (it’s a very abundant flatfish and an important part of the food chain as both predator and prey).

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U.S. wild-caught arrowtooth flounder is a smart seafood choice because it is sustainably managed and responsibly harvested under U.S. regulations. There are above target population levels. The fishing rate is at recommended levels. Area closures and gear restrictions protect habitats affected by some types of fishing gear used to harvest arrowtooth flounder. Regulations are in place to minimize bycatch.

Population Status: There are three stocks of arrowtooth flounder: 1) Bering Sea-Aleutian Islands, 2) Gulf of Alaska, and 3) Pacific coast. According to the most recent stock assessments all three stocks are not overfished (2020 stock assessment) and not subject to overfishing based on 2021 catch data.

Arrowtooth Flounder Fishing

NOAA Fisheries and the North Pacific Fishery Management Council manage the arrowtooth flounder fishery in Alaska. The species are managed under the Gulf of Alaska and Bering Sea/Aleutian Islands Groundfish Fishery Management Plans, which limit on the total amount of arrowtooth flounder that can be harvested each year. Annual harvests have consistently been below this level. [Source: NOAA]

NOAA Fisheries and the Pacific Fishery Management Council manage the arrowtooth flounder fishery on the West Coast. The species are managed under the Pacific Coast Groundfish Fishery Management Plan, which: 1) places limits on the number of permits and fishermen allowed.Limits on the minimum size of fish that may be harvested; 2) puts limits on how much may be harvested in one fishing trip; 3) specifies certain seasons and areas that are closed to fishing; and 4) implements gear restrictions help reduce bycatch and impacts on habitat.

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A trawl rationalization catch share program includes catch limits based on population information for each fish stock and divided into shares that are allocated to individual fishermen or groups. These fishermen can decide how and when to catch their share — preferably when weather, markets, and business conditions are most favorable, allowing the fishery the flexibility to be more environmentally responsible, safer, more efficient, and more valuable.

In Alaska, NOAA Fisheries scientists and the flatfish fishing industry collaborated to develop changes to fishing gear that would reduce effects of flatfish trawling on seafloor habitats of the central Gulf of Alaska and the eastern Bering Sea shelf. The modified gear they developed — Bering Sea flatfish gear — not only reduced impacts to sea floor habitat and the animals living there but also reduced the fishery’s impacts on crabs. In the central Gulf of Alaska and Bering Sea areas, flatfish fishermen are now required to use this modified fishing gear.

Halibut are sometimes incidentally caught, but there is a limit on the number of halibut that can be incidentally caught in the fishery. When this limit is reached, the directed fishery is closed. Some rockfish are still unintentionally caught, but management caps the amount of rockfish that can be incidentally caught in the fishery. To protect sensitive fish habitat off the West Coast, gear restrictions limit where flatfish fishermen can fish.

Flathead Sole

Flathead sole (Scientific name: Hippoglossoides elassodon) have an oval-shaped, compressed body. Also known as sole, flounder, flathead flounder, halibut-like flounder, they are flatfish, with both of their eyes located on the right side of their head. Their upper side is dark olive brown to reddish gray-brown, sometimes with dusky blotches, and their underside is white. Their dorsal and anal fins have dusky blotches. [Source: NOAA]

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Flathead sole grow up to 55 centimeters (1.8 feet) and can live at least 34 years. They are able to reproduce at 2 to 3 years old in the southern part of their range, but not until 6 years old in the northern part. Flathead sole generally feed on bottom-dwelling invertebrates such as crustaceans, mollusks, and brittle stars, as well as fish and squid. Pacific cod, halibut, Alaska pollock, and arrowtooth flounder prey on flathead sole..

Flathead sole spawn from February through April in deeper waters on the continental shelf. Depending on their size, females release 72,000 to 600,000 eggs. Eggs are large and are fertilized externally. Eggs hatch in nine to 20 days, depending on water temperature.

Flathead sole are found from Alaska south along the west coast of North America to northern California. Young flathead sole live in shallow estuaries, bays, and nearshore coastal areas along the Pacific coast. Adults live on mixed muddy and sandy ocean habitats in depths less than 1,000 feet. They migrate from winter spawning grounds along the outer continental shelf to feeding grounds in shallower water in the spring..

Flathead Sole Fishing

In 2021, the commercial landings in the U.S. of flathead sole totaled 10 million kilograms (22 million pounds) and were valued at approximately $3 million dollars, according to the NOAA Fisheries commercial fishing landings database. Almost all commercial harvest of flathead sole comes from Alaska, mainly the Bering Sea and Aleutian Islands. [Source: NOAA]

Flathead sole are primarily caught with bottom trawls, and some are caught with pelagic trawls. Gulf of Alaska and Bering Sea/Aleutian Islands fishermen use modified trawl gear that reduces the impact of trawling on animals living on the sea floor, including crabs. Halibut, salmon, and crab are incidentally caught in the groundfish fishery in Alaska.

There are limits on how much halibut, herring, and crab can be caught incidentally. If this limit is reached, an area or the entire fishery is closed for the remainder of the season. In Alaska and on the West Coast, NOAA Fisheries and the regional fishery management councils have implemented large closed areas to protect sensitive rocky, cold-water coral and sponge habitats from bottom trawls. Recreational fishermen may fish for flathead sole: 1) Only hook-and-line and spear gear is allowed. 2) Bag limits on the number of fish that can be caught vary by state.

U.S. wild-caught flathead sole is a smart seafood choice because it is sustainably managed and responsibly harvested under U.S. regulations. There are above target population levels in Alaska. The fishing rate is at recommended levels. Area closures and gear restrictions protect habitats affected by bottom trawls used to harvest flathead sole. Regulations are in place to minimize bycatch. Population Status:

There are three stocks of flathead sole: 1) Gulf of Alaska, 2) one stock contained in the Bering Sea/Aleutian Islands Flathead Sole Complex, and 3) one stock contained in the Other Flatfish Complex along the Pacific coast. According to the most recent stock assessments: The Gulf of Alaska coast stock is not overfished (2017 stock assessment) and not subject to overfishing based on 2021 catch data. The other two stocks have not been assessed but are not overfished based on 2020 catch data. [Source: NOAA]

NOAA Fisheries and the North Pacific Fishery Management Council manage this fishery in Alaska. The species are managed under the Fishery Management Plans for Groundfish of the Bering Sea and Aleutian Islands and Gulf of Alaska. Fishermen must have a permit to participate in the fishery, and the number of available permits is limited to control the amount of fishing. Managers determine how much flathead sole can be caught each year based on assessments conducted in the Bering Sea/Aleutian Islands and Gulf of Alaska.

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In the Bering Sea/Aleutian Islands a percentage of the allowable catch is allocated to the community development quota program, which benefits fishery-dependent communities in Western Alaska. The rest is allocated under a catch share program to the trawl catcher/processor sector based on historic harvest and future harvest needs to improve retention and utilization of fishery resources by the trawl fleet. In the Gulf of Alaska, total allowable catch is allocated by regulatory area. Catch is monitored through record keeping, reporting requirements, and observer monitoring.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated April 2023

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