The Pacific halibut, known for its impressive size, thrives on the continental shelf of the North Pacific Ocean and the Bering Sea. The primary fishing grounds for Pacific halibut are concentrated in the Gulf of Alaska and the Bering Sea, extending along the west coast of Canada. While smaller halibut catches are reported in coastal areas of Washington, Oregon, and California, the bulk of the population resides in the 10 regularity management areas.
Pacific halibut exhibit a distinctive diamond-shaped body, characterized by elongation that surpasses most other flatfish species. The width of their body is approximately one-third of their length. Notably, they possess a pronounced arch in the lateral line over the pectoral fin, and their tail takes on a lunate or crescent shape, setting them apart from their flatfish counterparts. Their skin is embedded with small scales, and both eyes are situated on the dark upper side of their body.
The Pacific halibut's coloration adapts to its surroundings, with the dark side mirroring the ocean bottom and the underside appearing lighter, resembling the sky when viewed from below. This remarkable adaptation enables halibut to avoid detection by both prey and predators. In terms of size, they rank among the largest flatfish species, with adults weighing up to 500 lb (230 kg) and growing over 8 ft (2.4 m) in length. (See the diagram below for a visual representation of the Pacific halibut's physical characteristics.)
Spawning among Pacific halibut takes place during the winter months, with the peak activity occurring from December through February. The majority of spawning occurs off the continental shelf's edge in deep waters ranging from 600 to 1,500 ft (183 to 457 m). Male halibut reach sexual maturity between 7 and 8 years of age, while females attain maturity between 8 and 12 years. Females can lay anywhere from 0.5 to 4.0 million eggs annually, depending on their size.
Following fertilization, the eggs hatch after approximately 15 days. During their free-floating larval stage, which lasts up to six months, they are transported by North Pacific currents, covering several hundred miles in a counter-clockwise direction. Notably, the young halibut undergo significant transformations during this stage, including the movement of the left eye to the right side of their body.
Upon reaching the surface, the young halibut are carried to shallower waters by prevailing currents. At around six months of age, they assume their adult form, measuring about 1.4 inches (3.6 cm) in length. In the shallower waters, these young halibut begin their lives as bottom-dwellers, often spending five to seven years in rich nursery grounds like the Bering Sea.
Halibut exhibit migratory behavior, with young individuals generally migrating in a clockwise direction to the east and south of the Gulf of Alaska. While older halibut tend to be less migratory, they continue to move predominantly in a clockwise direction. During winter, mature fish embark on spawning migrations towards deeper waters, traversing multiple areas in some instances. It is worth noting that while small localized spawning populations may exist, a single genetic stock of halibut is known in the North Pacific due to the mixing of larvae and juvenile halibut across the Gulf of Alaska.
The growth rates of Pacific halibut vary depending on their location and habitat conditions, with females generally exhibiting faster growth than males. The oldest recorded female and male halibut were 55 years old. A weight-length relationship, based on data published by the International Pacific Halibut Commission (IPHC), allows us to estimate the weight of halibut based on their length.
For Pacific halibut, the relationship can be expressed as follows:
W = c * L^b
Where:
According to the IPHC data, for Pacific halibut, the constant values are approximately c = 0.00018872 and b = 3.24. Utilizing this relationship, we can estimate the weight of various lengths of Pacific halibut. For example, a 20-inch (51 cm) halibut would weigh approximately 3.0 lb (1.4 kg), a 36-inch (91 cm) halibut would weigh around 20 lb (9.1 kg), and a 58-inch (150 cm) halibut would weigh approximately 100 lb (45 kg).