Flatfishes

As the name implies, the flatfishes include the fishes that are flat in the dorsoventral axis (Fig. 3). They have both eyes on one side of the body and are often found lying on the bottom of the ocean covered in layer of sand or mud. All the flatfishes are classified in the order Heterosomata and include the soles of the family Soleidae and the flounders in the family Pleuronectidae. This latter family includes the commercially important species of the true halibuts in the genus Hippoglossus, the turbots in the genus Reinhardtius, and the right-eyed flounders in the genus Platichthys. The commercially important species tend to be right-sided; that is the side that is visible to the observer is the right side of the fish. The left eye migrates, along with appropriate skeletal, muscular and neural rearrangements, so that both eyes end up on the right, uppermost side of the animal. Although these fishes are well known as benthic animals, fish in the juvenile stages are also pelagic. The reading of rings in the otoliths, or ear stones is sometimes used to age flatfishes. Nine commercially important species are discussed here.

Halibut

Although it seems reasonable at this time to regard both the Atlantic and Pacific halibuts as belonging to the same species Hippoglossus hippoglossus, the Pacific halibut is considered in current literature to be a separate species Hippoglossus stenolepis (Schmidt) (3,4). There were declining levels of global commercial catches during the 1960s and 1970s. The large increases in bottom trawling in the 1960s and 1970s were significant contributions to the decline. These global trends were even more exaggerated in some local fisheries such as the Atlantic landing in New England. The average annual landing was 4.83 million kg (eviscerated and heads off) in 1879. In 1930-1939 it was 802,858 kg and by the period 1970-1975, it had fallen to 75,296 kg. Regulation and stock management have curtailed this trend and Pacific stocks, for instance, seem to be recovering.

The chief way in which commercially caught halibut is preserved for storage and transport to market is freezing. The animal is eviscerated and has the head off before the intact trunk is frozen. While earlier methods included salting and chilling with ice, current methods of quick freezing and glazing or vacuum packaging in plastic provides a more reliable and consistent method.

Atlantic Halibut. The Atlantic halibut (Hippoglossus hippoglossus) is a highly prized food fish. The Atlantic hal

Figure3. Halibut (Hippoglossus hippoglossus). Source: Copyright 1990 by B. Guild-Gillespie.

Figure3. Halibut (Hippoglossus hippoglossus). Source: Copyright 1990 by B. Guild-Gillespie.

ibut is commercially taken by otter trawls and longlines, with smaller fish being taken by trawls and the larger fish being caught on longlines. Both sizes are taken by fisheries specifically directed at their catch or as incidental catches when trawling or long-lining for cod, haddock, or other ground fishes. The flesh is marketed fresh or frozen. The high price it commands enables fishermen to take relatively small quantities. In spite of this fact, there is evidence suggesting that certain populations are being overfished. The average length of the mature male and female halibut was 84 cm and 98 cm, respectively, in the period 1959 to 1969. Those lengths dropped to 66 cm and 70 cm, respectively, in the years 1970 to 1979. This reduction in average size at maturity is characteristic of populations subjected to heavy harvesting of the larger individuals.

The Atlantic halibut has a typical appearance of a flatfish. It has a flattened body, lying on its left side with its right side showing. The lower side is characteristically white and the right uppermost side is colored various shades of brown. The lateral line is prominent and curved over the pectoral fins. It is a very large fish, with record weights of over 100 kg. The larger fishes are between 30 and 35 yr old. It is the largest of the flatfishes. The females, in particular, have the highest growth rates as well as growth potential among the flatfishes. Females, therefore, become larger than males.

The Atlantic halibut is found on the east and west shores of the north Atlantic Ocean. The distribution on the east shore spans the Bay of Biscay to the Barents Sea. On the west side, the Atlantic halibut is found off the west coast of Greenland, south along the Canadian coast to the coast of Virginia. It prefers cooler waters between 3 and 9°C, at depths of 200-300 m.

Knowledge about reproduction in the halibut is relatively new. Atlantic halibut spawn between February and March around the distribution in Canada. They spawn in deep waters ranging from about 180 m on the western side and about 1,000 m or deeper in the northeastern Atlantic. The mature female, which weighs about 90 kg, may produce over 2 million eggs. The eggs are about 3 mm in diameter, spherical, and neutrally buoyant. They float during development at depths greater than 54 m but the hatched young sink as they develop. These free-floating eggs are most abundant at temperatures between 4.5 and 7°C and salinities between 33.8 and 35.0 ppt. Incubation takes about 16 days at 6°C. Little else is known about the details of the early life stages of the Atlantic halibut.

Pacific Halibut. The Pacific halibut (Hippoglossus sten-olepis) has had a long commercial history on the Pacific coast of North America. The transacontinental railways across North America facilitated the Pacific halibut catches to be shipped across to eastern destinations and contributed to the near demise of the Pacific populations of this species. At the turn of the century, the total Canadian and U.S. catches was more than 4.54 million kg. Additional technological advancements in fishing vessels and gear increased harvests to a point that by 1930, increases in fishing efficiency no longer yielded greater catches. Through regulation under the International Fisheries Commission, the Pacific halibut stocks have rebuilt over time. The Pacific halibut is commercially caught mainly on hook and line or on other gear that does not involve nets of any kind. Numerous baited hooks are set on lines and rest on the bottom. Those sets can be made by dories from larger mother ships. Longlines of baited hooks can be set and retrieved by the mother ship.

The Pacific halibut is distributed throughout both western and eastern coasts of the Pacific. On the western side, it is found from southern California, north along the western coast to the Bering Sea. It is found off Anadyr, Kuril Islands, and Kamchatka, down to the northern areas of Japan. Like other flatfishes, the Pacific halibut is benthic (living on the bottom) at depths of about 1,100 m.

Reproduction and details of the early life stages in the Pacific halibut are better understood than the Atlantic halibut. Spawning takes place in the winter between November and January from 275 to 412 m. It is not entirely clear whether females spawn every year or whether they spawn every other year. Both may be possible. Pacific halibut are known to migrate as far as 1,600 km to spawn. The mature female is about between 8 and 16 yr old and about 100150 cm long. Males are considerably smaller. The female may produce between 600,000 and 1.5 million spherical eggs of about 3-3.5 cm in diameter, which are neutrally bouyant between 100 and 200 m. The egg surface has a honeycombed appearance due to the presence of many small holes. The eggs as well as the newly hatched young are pelagic for four to five months after spawning. Like the Atlantic halibut, the newly hatched alevins start to sink to deeper waters and are found below about 200 m. The alevins are transparent, except for the eyes, and about 8—15 mm long with very large yolk sacs. The young are still symmetrical at this point. When they reach a size of about 18 mm, the left eye starts to migrate to the right side and the yolk sac is barely evident. The young appear as small adults by the time they are about 30 mm. By the time the young are about three to five months old, they rise in the water column to about 100 m. Currents carry the young inshore and they become benthic fish at about six to seven months in age. As they mature, they gradually move to deeper waters. By the time they are about 5-7 yr old, they are established at depths of about 100 m and available to the commercial fishery.

Greenland Halibut. The Greenland halibut (Reinhard-tius hippoglossoides) is a moderately flat fish that has a rich-tasting flesh. It is also known commonly as the Greenland turbot in the marketplace. Greenland halibut are caught commercially by longlines, gill nets, and otter trawls. It is only moderately flat because both sides of the fish are equally muscled. While it is primarily sold as frozen fillets, it is also sold as a smoked product. It is a moderate-size fish that grows to about 25 kg and 120 cm in Atlantic waters; the Pacific specimens are somewhat smaller. The underside is usually white in a young fish but gray in older fish. The visible side is dark brown to black. It is distinguished by its prominent teeth along a protruding lower jaw.

The Greenland halibut is distributed in cold waters in both the Atlantic and Pacific Oceans. In the northeast Atlantic, it is found from around Iceland and the Greenland

Sea to the Arctic Ocean, and both Barents and Norwegian Seas. From there, it is found southward to the Faroe-Shetland Ridge. In the northwest Atlantic, it is distributed from western Greenland, south to Georges Bank. In the Pacific, it is found from northern California to Japan through the Bering and Okhotsk Seas, Sakhalin, and Kamchatka.

The limited knowledge about the details of spawning in this species is, no doubt, due in part to the fact that it spawns in the winter in the far north and at depths of about 650-1,000 m, at temperatures of about 0-3°C, making observation difficult. Spawning takes place in the winter or early spring, depending on location. The Greenland halibut can live to an age between 15 and 20 yr old. Until they are about 5-7 yr old, the males and the females grow at about the same rate. After that, the females grow larger and at a faster rate. All fish over 90 cm are females. The mature female may produce between 30,000 and 300,000 eggs, depending on size. The eggs are larger than the halibut eggs, measuring about 4-4.5 mm in diameter. The eggs are transparent and the newly hatched young rise to depths of about 30 m below the surface, where they live as pelagic fish until they are about 70 mm long. At that stage, they move to deeper waters but do not become benthic like the halibuts; although they are associated close to the bottom.

Yellowtail Flounder. The yellowtail flounder (Limanda ferrunginea) is valued as a tasty product that is often sold fresh but that may also be sold frozen. It is either fished directly with otter trawls or as an incidental catch on longlines in the American plaice fishery. While it is fished commercially, it is usually just the largest of the population that gets caught because of its small mouth. While it has the lateral line that curves over the pectoral fins, like the halibuts, the yellowtail flounder has a small mouth and the body size is much smaller. An average adult caught in the commercial fishery is about 30 cm long. This fish gets its name from the yellow markings anterior to the tail and at the bases of dorsal and anal fins. It has a white underside and the color of the visible right side may range from a reddish brown to an olive green.

The yellowtail flounder has a relatively limited distribution, compared to other flatfishes. It is found only in the western North Atlantic Ocean, from southern Labrador down to the Chesapeake Bay. The time of spawning depends on location, but mainly occurs in the spring to summer months. The female produces a large number of small eggs. Females that measure 40-45 cm may produce between 1 and 2 million eggs that are slightly less than 1 mm in diameter. The eggs and milt are deposited on, or near the bottom. Fertilization occurs there and the bouyant eggs then float to near the surface and drift during incubation. The incubation time is about five days at about 10°C. The newly hatched alevins are about 2-3.5 mm long and grow to about 11.5-16 mm before they metamorphose from the anatomically symmetrical young to a smaller version of the adult form. The yellowtail flounder may live to 12 yr of age, and individuals have been recorded up to about 60 cm. The typical fish caught commercially is between 4 and 10 yr old, depending on the location of the catch. The females grow at significantly faster rates than the males.

American Plaice. The American plaice (Hippoglossoides platessoides) is a white-fleshed fish that is highly enjoyed as a food fish. It is a very important commercial fish that constitutes about one-half of the commercially caught flatfish species in Canada. Most of the northwest Atlantic fishery is Canadian. American plaice is commercially caught with otter trawls, gill nets, and with the Danish seine. Most of the commercial fishing takes place along the Labrador Shelf, Grand Bank, along the southern Gulf of St. Lawrence, and on the banks of the Scotian Shelf. The flesh is normally sold as frozen fillets. It is colored white on the underside and reddish to grayish brown on the visible right side. The maximum body size rarely reaches over 60 cm.

The American plaice is found on both sides of the North Atlantic Ocean. It is distributed from Iceland, south to the English Channel, on the eastern side and from Baffin Island, south to Rhode Island on the western side. It is normally found between about 75 and 275 m of water, preferring temperatures around freezing. It is benthic and prefers the fine sand or mud bottom. It has a seasonal migratory pattern, which brings it to deeper waters in the winter and to shallower waters in the spring.

Spawning take place between early April and June, depending on location. Spawning takes place at depths of about 90-180 m, at temperatures between 0 and 2.5°C. The mature female produces many small eggs. A 40-cm-long female, 8 yr old, may produce between 250,000 and 300,000 eggs, while a 70-cm-long female has been recorded to produce 1.5 million eggs. Males mature earlier at 4-5 yr old. The eggs measure about 1.5-2.8 mm in diameter. They are bouyant, without oil droplets and float near the surface during incubation. They, therefore, drift widely. At surface temperatures of about 5°C, the young alevins of about 46 mm, hatch in about 11-14 days. They transform from the symmetrical young to a small fish resembling the adult form at about 18-34 mm in length. They can live to be 25 yr old, and the record size is 81.2 cm long and 6.3 kg dressed.

Petrale Sole. The petrale sole (Eopsetta jordani) is a large flatfish that is prized as an excellent food fish, especially along the west coast of North America. The livers of this fish are a rich source of vitamin A. It is distributed from Bering Sea, south to Baja California. Like other flatfishes, it migrates seasonally and is found at depths of around 70-130 m throughout most of the year, except during the winter, when it moves to deeper waters, around 300-460 m. The petrale sole is white on the underside and olive brown on the visible right side. It has a large mouth and lacks free spines at the origin of the dorsal fin.

The oldest female petrale sole recorded was 25 yr old, whereas the eldest male was 19 yr old. The average spawning female is about 44 cm and the male is about 38 cm. The female produces a large number of relatively large eggs, the number depending on size. Records show a 42-cm female produced 400,000 eggs and a 57-cm female produced 1.2 million eggs. The eggs measure about 1.3 mm in diameter. Spawning takes place between late winter and early spring. Spawning takes place at depths of about 350 m and the bouyant eggs float to shallower depths where they are carried by prevailing currents during incubation. The eggs hatch into 3 mm alevins in about 8-9 days at 7°C and the yolk sac is absorbed within a further 10 days. Samples that measured 22 mm in length showed metamorphosis complete. Other evidence suggests that they settle to their adult benthic existence by the time they are 1-2 yr old.

Rock Sole. The rock sole (Lepidopsetta bilineata) is the most commonly used food fish of the smaller flatfishes. There have been several subspecies identified in the Pacific Ocean. At 15 yr of age, the females may reach a length of 60 cm, and the males, 50 cm. It is recognized by a canal formed along the lateral line that arches, typically, over the pectoral fin. It is a right-sided flatfish. Unlike other flatfishes, it tends to inhabit shallower waters. It is commercially taken at depths up to about 200 m, although it is scarce from about 100 m and deeper. Like other flatfishes, however, it does migrate to even shallower waters during the summer. Its distribution extends to both sides of the Pacific Ocean from southern California, up along the North American west coast to the Bering and Okhotsk seas and around to Korea and the Sea of Japan.

Observation off the North American west coast show that spawnings takes place between February and April. The fecundity of females measuring 35 cm and 46 cm in length were estimated at 400,000 and 1.3 million eggs, respectively. The eggs are pigmented yellowish orange and are adhesive. They measure about 1 mm in diameter. Incubation is a function of temperature and may take between 6 and 25 days, corresponding to a temperature range of about 8 to 3°C, respectively. Hatched alevins are about 5 mm long and yolk absorption takes about 10-14 days, influenced again on ambient temperature. Fully metamorphosed young have been observed at about 20 mm in length. The oldest recorded female was 25 yr old, and the eldest male on record was 15 yr old.

Dover Sole. The Dover sole (Microstomas pacificus) is highly prized for its quality flesh and excellent keeping qualities in the frozen state. It was originally dismissed as a viable commercial species because of its softness and sliminess. It may be uniformly brown on the visible right side and the underside may range in color from a light to a dark gray. It is characterized by having a lateral line canal that is almost straight as well as the excessive production of slime. While the body is also extraordinarily flaccid, it is known as a very hardy fish. The Dover sole is distributed from northern Baja California, up to the Bering Sea, found mainly on soft substrates. The average body size is about 70 cm.

Spawning in California takes place from November to February. Mature females are approximately 45 cm and males mature at about 40 cm. The female produces a wide range of eggs depending on size. Samples from Oregon showed a 42.5-cm female produced 52,000 eggs, while another 57.5-cm female had 266,000 eggs. The eggs are large, measure 2-2.6 mm in diameter, and have a wrinkled surface. After hatching, the young remain bilaterally sym metrical for several months. Flatfishes usually metamorphose and settle to a near benthic existence at about 20 mm. Dover sole specimens up to 100 mm in length have been observed in a pelagic life stage.

English Sole. The English sole (Parophrys vetulus) is characterized by its pointed head and, like the Dover sole, a lateral line canal that is almost straight. It is a moderate-size flatfish that grows to about 60 cm as a female and up to 50 cm as a male. It is colored a uniform brown on the uppermost right side and pale yellow to white on the underside. It has had a long history of being a commercially important species in North America. Its particular iodine flavor, found in some inshore populations, has identified its place in the marketplace. It is typically fished commercially at depths shallower than about 130 m, although its distribution extends to about 300 m.

Spawning off British Columbia occurs between January and March. The range in fecundity is extreme. Records show that a 30-cm female produced 150,000 eggs, and one measuring 44 cm produced 1.9 million eggs. The size of the maturing male and female are rather similar. Mature females measure about 30 cm, whereas the males measure about 26 cm. The eggs are small and measure slightly less than 1 mm in diameter. The eggs float due to the presence of oil droplets of various sizes in the yolk of the egg, start to sink just before hatching. The surface of the egg is covered with small wrinkles and pores. Observations in California show that the incubation time is about 90 days and the new alevins are about 2.8 mm long. Due to the oil droplets, the alevins hang upside down until the yolk is absorbed, in about 10 days, at which time they can swim. They remain pelagic for 6-10 weeks, after which they metamorphose to their adult form and seek the bottom. Young English sole are found in shallow waters but as they mature, the larger fish move to deeper waters. Like the other flatfishes, it tends to inhabit deeper waters in the winter and seek the shallower waters closer to shore in the spring. Another rather peculiar characteristic of this flatfish is that it can migrate long distances. There are records of English sole traveling over 1,000 km, between Vancouver Island and California.

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