Halibut are related to flounders and other flatfish. Pacific halibut are the largest members of the Family Pleuronectidae. They are found near the continental shelf in the northern Pacific Ocean and range from California north to the Chukchi Sea and from the Gulf of Anadyr, Russia south to Hokkaido, Japan. Halibut live on or near the bottom of the ocean and prefer water temperatures ranging from 37.4 to 46.4 degrees Fahrenheit (3⁰ to 8⁰C).
Halibut and their relatives are flattened laterally and swim sideways with both eyes on one side of the body. They have diamond-shaped bodies and are more elongated than most flat fishes. The width of the fish is approximately one-third its length. A halibut’s scales are small and are embedded in the skin, making the fish feel smooth to the touch. The top side of a halibut’s body is gray to olive- brown or nearly black and is mottled with numerous spots, allowing the fish to blend in with a sandy or muddy bottom and providing it camouflage from predators and prey. The bottom side of a halibut is white. The eyes of a halibut are on the dark side of the fish. Nearly all halibut are right-eyed which means the eyes are on the upper, dark side or the right side of the fish. Approximately one in 20,000 halibut is left-eyed with the eyes and dark pigment on the left side of the body. The dorsal fin extends from near the eyes to the base of the tail, and the anal fin begins just behind the anus and ends at nearly the same point opposite the dorsal fin. The mouth extends to the middle of the lower eye, and the tail is broad and symmetrical and lacks a fork. The lateral line arches high over the pectoral fin and is a characteristic that easily distinguishes a halibut from an arrowtooth flounder, a species that looks much like a halibut but has a nearly straight lateral line. Pacific halibut can reach 8 ft. (2.4 m) in length and weigh more than 500 lbs. (230 kg).
Most male halibut are sexually mature at age eight, while females begin to mature when they are 12 years old. They reproduce at depths of 300 to 1500 ft. (91 -457 m), and spawning takes place in the winter from November through March. Males randomly release sperm while females release eggs, and fertilization happens by chance. A female halibut may release a few thousand to four million eggs, depending on the size of the fish. Fertilized eggs hatch in approximately 15 days, and the larvae drift with the deep ocean currents. In the Gulf of Alaska, the larvae drift in a counter clockwise direction along the coast. As the larvae mature, they rise in the water column until they ride the surface currents to shallower coastal waters. When they hatch, larvae swim in an upright position with eyes on both sides of their head. When they are approximately an inch long, the left eye migrates over the snout to the right side of the head, and the color on the left side fades to white. When they are six months old, halibut settle onto the sea floor, where the dark coloration on the side with their eyes helps camouflage them.
Young Pacific halibut are very migratory and migrate in a clockwise direction throughout the Gulf of Alaska. As they age, halibut tend to become less migratory, but mature fish do migrate to deeper water in the winter to spawn and to shallower water in the summer to feed.
Halibut feed on plankton during their first year, and juveniles between the ages of one and three years old eat euphausiids (krill) and small fish. As they grow, halibut become more dependent on fish, and larger halibut eat herring, sand lance, capelin cod, pollock, sablefish, rockfish, flounders, and smaller halibut. They also eat octopus, clams, and crabs. Halibut usually sit on the bottom, but they will swim up in the water column to feed on salmon. A halibut will eat nearly any fish or organism it can catch.
Female halibut grow faster and reach a much larger size than male halibut. Males rarely grow larger than three feet in length (1 m) and weigh a maximum of 60 lbs. (27 kg), while females may reach over 6 ft. (2 m) in length and weigh over 500 lbs. (230 kg). Halibut growth rates vary depending on location, food availability and other conditions. As they grow longer, their weight increases, but the relationship between length and weight is not linear. The relationship between total length (L, in inches) and weight (W, in pounds) for all species of fish can be expressed by the equation: W=cLb. The constant “b” is close to 3.0 for all species of fish, but the constant “c” varies among species. For halibut, c = 0.00018872 and b = 3.24. By applying this equation, a 58-inch-long (150 cm) halibut weighs approximately 100 lbs. (45 kg).
According to scientific research, the size of Pacific halibut at a particular age has changed over time. The average length and weight of halibut in every age class increased from the 1920s to the 1970s but has decreased since then. For example, 12-year-old halibut are now three-quarters the length and one-half the weight they were in the 1980s. The reasons for these changes in size over time are unknown, but possible causes include competition with other species or with other halibut, climate effects on growth or survival, or effects of fishing and size limits.
The oldest recorded male and female halibut were 55 years old. Except for man, adult halibut have few natural predators. They are sometimes eaten by marine mammals and sharks, but they are rarely eaten by other fish.
Next week, I will describe commercial and sport fishing for halibut in Alaska, and I will attempt to explain how halibut is managed.
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