Hippoglossus stenolepis, the Pacific halibut, is a species of righteye flounder. This very large species of flatfish is native to the North Pacific and is fished by commercial fisheries, sport fishers, and subsistence fishers.
Pacific halibut have diamond-shaped bodies. They are more elongated than most flatfishes, their width being about one-third of their length. They have a high arch in the lateral line over the pectoral fin, and a lunate, or crescent-shaped tail, which is different from other flatfishes. Small scales are embedded in the skin. Halibut have both eyes on their dark upper sides. The color on the dark side varies, but tends to assume the coloration of the ocean bottom. The underside is lighter, appearing more like the sky from below. This color adaptation allows halibut to avoid detection by both prey and predator. They are one of the largest flatfish (only surpassed by the closely related Atlantic halibut), and can weigh up to 500 lb (230 kg) and grow to over 8 ft (2.4 m) long.
The Pacific halibut is found on the continental shelf of the North Pacific Ocean and Bering Sea. Fishing for the Pacific halibut is mostly concentrated in the Gulf of Alaska and the Bering Sea, off the west coast of Canada. Small halibut catches are reported in coastal Washington, Oregon, and California. Pacific halibut is broken up into 10 regularity management areas.
Halibut are demersal, living on or near the bottom of the water and prefer water temperatures ranging from 3 to 8 °C (37.4 to 46.4 °F). Pacific halibut belong to the family Pleuronectidae. From November to March, mature halibut concentrate annually on spawning grounds along the edge of the continental shelf at depths from 183 to 457 m (600 to 1,500 ft).
Halibut are strong swimmers and are able to migrate long distances. Halibut of all ages and sizes are involved in a predominantly clockwise (northwest to southeast) migration from their settlement areas (western part of the Gulf of Alaska and Bering Sea), and reproductive fish also make regular seasonal migrations from more shallow feeding grounds in summer to deeper spawning grounds in winter.
Being strong swimmers, halibut are able to eat a large variety of fishes, including cod, turbot, and pollock, and some invertebrates, such as octopus, crab, and shrimp. Sometimes, halibut leave the ocean bottom to feed on pelagic fish, such as salmon, sand lance, and herring, and even seal remains have been found in their stomachs.
Spawning takes place during the winter, with the peak of activity occurring from December through February. Most spawning takes place off the edge of the continental shelf in deep waters of 600 to 1,500 ft (183 to 457 m). Male halibut become sexually mature at 7–8 years of age, while females attain sexual maturity at 8–12 years. Females lay 0.5 to 4.0 million eggs annually, depending on the size of the fish.
Fertilized eggs hatch after about 15 days. Free-floating larvae float for up to six months and can be transported several hundred miles counter-clockwise by North Pacific currents. During the free-floating stage, many changes take place in the young halibut, including the movement of the left eye to the right side of the fish. During this time, the young halibut rise to the surface and are carried to shallower water by prevailing currents. At six months, the halibut have their adult form and are about 1.4 in (3.6 cm) long. In the shallower water, young halibut then begin life as bottom-dwellers. Most young halibut ultimately spend from five to seven years in rich, shallow nursery grounds such as the Bering Sea.
Young halibut are highly migratory and generally migrate in a clockwise direction east and south throughout the Gulf of Alaska. Halibut in older age classes tend to be less migratory, but continue to move predominately on a clockwise direction. Mature fish are also involved in winter spawning migrations towards deeper waters, migrating across several areas in some instances. Small, localized spawning populations may occur in deep waters such as in Chatham Strait in northern Southeast Alaska. Because of the free-floating nature of larvae and subsequent mixing of juvenile halibut from throughout the Gulf of Alaska, though, only one genetic stock of halibut is known in the North Pacific.
Halibut growth rates vary depending on locations and habitat conditions, but females grow faster than males. The oldest recorded female and male were 55 years old. The largest recorded sport-caught halibut was 459 lb (208 kg) near Unalaska, Alaska, in 1996.
