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Pacific Halibut (Hippoglossus stenolepis): Stock Status and Biology

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

Description and Scientific Name

Pacific halibut belong to a family of flatfish named Pleuronectidae. The scientific name for Pacific halibut is Hippoglossus stenolepis, a name derived from the Greek hippos (horse), glossa (tongue), steno (narrow), and lepis (scale). The name was first proposed in 1904 by Russian scientist P. J. Schmidt, who distinguished Pacific halibut from its Atlantic counterpart (Hippoglossus hippoglossus) by anatomical differences such as the shape of the scales, length of the pectoral fin, and the shape of the body. Since the identification was made, it has been debated as to whether the two are indeed separate species, although genetic work has confirmed the validity of the two different species

Most fishes are torpedo-shaped and symmetrical, often with heavily pigmented backs and light, white bellies. Flatfish are compressed laterally and, except in the larval stages, have both eyes on one side of the head. Pacific halibut usually are dextral, that is, both eyes are on the right side. On the eyed side, pigmentation varies from olive to dark brown or black with lighter, irregular blotches that are similar to the color pattern of the ocean floor. This protective coloration makes the fish less conspicuous to predators and prey. The left or blind side is white with occasional blotching and faces the ocean bottom

Pacific halibut are more elongate than most other flatfishes. The average width of the body is about one-third its length. The mouth is relatively large, extending to below the lower eye, and nearly symmetrical. The small, smooth scales are well buried in the skin and the lateral line has a pronounced arch above the pectoral fin. The tail or caudal fin is crescent-shaped or lunate (Fig. 1)

Reproduction and development

Reproductive output in Pacific halibut varies with sex, age, and size of the fish. Females grow more quickly, but mature more slowly than males. Most males are mature by the time they are eight years old, whereas the average age of maturity for females is about 12 years; these average ages have remained invariant despite the large decrease in size-at-age over the past 20 years. From November to March, mature Pacific halibut concentrate annually on spawning grounds along the edge of the continental shelf at depths from 600 to over 1,600 feet (~200 to > 500 m). The major spawning sites in British Columbia include Cape St. James, Langara Island (Whaleback), and Frederick Island and in Alaska include Yakutat, Cape Suckling – Yakataga (“W” grounds), Portlock Bank, and Chirikof Island. Other reported spawning locations include Goose Islands, Hecate Strait, and Rose Spit in British Columbia and Cape Ommaney, Cape Spencer, and Cape St. Elias in Alaska. Spawning concentrations also occur in the Bering Sea and Aleutian Islands. In addition to these major grounds, there is evidence that spawning is widespread and occurs in many areas, although not in as dense concentrations as those mentioned above. Evidence to support this conclusion is based on the widespread distribution of sexually mature halibut during the winter months as indicated by the IPHC’s studies using satellite tags, historical surveys, and information from commercial fishing operations

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