General Description

Pacific halibut are the largest flatfish in Family Pleuronectidae. Halibut and other flatfish are flattened laterally, and swim sideways, with one side facing down and the other facing up. The upper side is typically gray to brown, or nearly black, with mottling and numerous spots to blend in with a sandy or muddy bottom. The underside is typically white. Virtually all halibut are right-eyed, meaning both eyes are found on the upper, dark side of the body. Left-eyed halibut are rare; one report suggested a ratio of about 1 in 20,000. In these fish, the eyes and dark pigment are on the left side of the body, and the fish swims with the right (white) side facing down. The dorsal fin is continuous from near the eyes to the base of the tail, and the anal fin extends from just behind the anus to the same point on the other side. The mouth extends to the middle of the lower eye or beyond, and is nearly symmetrical. The scales are quite small and buried in the skin, making the skin appear smooth. The tail is broad, symmetrical, and lacks a distinct fork. The lateral line is strongly arched over the pectoral fin. The maximum reported size is over 8 feet in length and over 500 pounds.

Similar Species

Inexperienced anglers occasionally confuse Pacific halibut with arrowtooth flounder. Unlike halibut, arrowtooth flounder have coarse scales and prominent, needle-like teeth. The lateral line of arrowtooth flounder is barely curved over the pectoral fin. When cooked, arrowtooth flounder turn mushy and are generally considered inedible.

Life History

Reproduction and Development

Most male halibut are sexually mature by about 8 years of age, while half of the females are mature by about age 12. Most halibut spawn during the period November through March, at depths of 300 to 1,500 feet. Female halibut release anywhere from a few thousand to several million eggs, depending on the size of the fish. Eggs are fertilized externally by the males. About 15 days later, the eggs hatch and the larvae drift with deep ocean currents. As the larvae mature, they move higher in the water column and ride the surface currents to shallower, more nourishing coastal waters. In the Gulf of Alaska, the eggs and larvae are carried generally westward with the Alaska Coastal Current and may be transported hundreds of miles from the spawning ground.

Halibut larvae start life in an upright position like other fish, with an eye on each side of the head. The left eye moves to the right side of the head when the larvae are about one inch long. At the same time, the coloration on the left side of the body fades. The fish end up with both eyes on the pigmented (olive to dark brown), or right, or upper side of the body, while their underside is white. By the age of 6 months, young halibut settle to the bottom in shallow nearshore areas.

Halibut feed on plankton during their first year of life. Young halibut (1 to 3 years old) feed on euphausiids (small shrimp-like crustaceans) and small fish. As halibut grow, fish make up a larger part of their diet. Larger halibut eat other fish, such as herring, sand lance, capelin, smelt, pollock, sablefish, cod, and rockfish. They also consume octopus, crabs, and clams.

Growth

Female halibut grow faster and reach larger sizes than male halibut. The growth rate of halibut has changed over time. The growth rate was highest in the 1980s and lowest in the 1920s and 2000s. By the 2000s, 12-year-old halibut were about three-quarters the length and about one-half the weight they were in the 1980s. The growth rate is believed to decrease due to competition among halibut or between halibut and other species, such as arrowtooth flounder, that have a similar diet.

Movements

Juvenile and some adult halibut migrate generally eastward and southward, into the Gulf of Alaska coastal current, countering the westward drift of eggs and larvae. Halibut tagged in the Bering Sea have been caught as far south as the coast of Oregon, a migration of over 2,000 miles. Because of the extensive movements of juvenile and adult halibut, the entire eastern Pacific population is treated as a single stock for purposes of assessment. Research is continuing to determine if there are spawning sub-stocks of varying productivity.

Halibut also move seasonally between shallow waters and deep waters. Mature fish move to deeper offshore areas in the fall to spawn, and return to nearshore feeding areas in early summer. It is not yet clear