Sentinel Fisheries - Atlantic Halibut

History

While halibut is now the most highly prized flatfish in the northwest Atlantic, there was a time when it was in low demand and even considered a nuisance fish.

Before the end of the 19th century the Atlantic halibut was not held in high esteem as a food fish by Canadian fishermen. Its exploitation prior to this time was carried out primarily by fishermen from the United States and even there it was not a popular food fish until the first quarter of the 19th century.

In response to the demand from the Boston market in the early 1820s, fishing for halibut began in earnest in the Massachusetts Bay-Cape Cod area. This fishery initially had very high catch rates, but by the 1830s the numbers of halibut caught in these areas declined rapidly. The 1830s thus saw the beginnings of the offshore halibut fishery which initially exploited Georges Bank and the Nantucket shoals until catches there began to dwindle in the 1850s. From there the fishery spread into Canadian waters, initially utilising areas around Browns Bank and gradually moving into other areas.

These very early years of Canadian involvement in the halibut fishery are not well documented but records are available from the early 1900s to the present.

Description

The Atlantic halibut (Hippoglossus hippoglossus) is the largest of the flatfish, growing to a length of 2.5 meters and a weight exceeding 300 kilograms. The halibut belongs to the family Pleuronectidae whose members usually have both eyes on the right side of their bodies with the left side being totally blind. They are strongly compressed (flattened from side to side) and swim with the left side facing the bottom and the right or eyed side facing the surface. The halibut's coloration, which occurs only on the eyed side of the body, varies from greenish brown to very dark chestnut brown. The blind side is usually white in young fish but becomes mottled with grey or even cherry red in older, larger fish. The mouth has a very large gape extending to the midline of the eyes and is armed with numerous sharp curved teeth. Apart from its size, the Atlantic halibut can be distinguished from most other species of flatfish by its concave tail.

Life history

Halibut spawn between February and May in water ranging from 700 to 1000 meters deep over bottom consisting of clay or soft mud. Large mature females may release several million eggs during the spawning season. Estimates of fecundity (egg production) are not numerous but there are reports of a 91 kg female carrying more than 2 million eggs.

Actual details of spawning behaviour have not been observed. The fertilized and developing eggs float freely in the water and are most commonly encountered at depths of 300 to 400 m, although they have been collected from waters as shallow as 50 m and as deep as 700 m.
The eggs hatch in approximately 16 days (at an incubation temperature of 6°C) giving rise to larvae (newly hatched fish) which are between 6 and 7 mm long. Following this initial period of larval growth they begin feeding on small planktonic organisms.

Until the larvae are 16 to 20 mm long they are quite similar to the larvae of most other marine fish, that is, they swim with their bellies toward the bottom and have one eye on each side of their heads. At this point, however, the left eye begins to move over the top of the head towards the right side. This movement continues until the eye is completely relocated on the right side of the head, when the larvae are about 44 mm long. When juveniles reach lengths of 50 mm or more, they appear to adopt the bottom-dwelling existence of the adult fish, swimming with their unpigmented blind side toward the bottom and the coloured, eyed side facing the surface. After they settle to the bottom they begin a slow migration from the shallow waters of the banks to the deeper waters of the continental slopes or fjords.

From the time they settle close to the bottom until they reach sexual maturity, halibut grow from less than 10 cm to lengths of about 1 m. During this period of relatively rapid growth they are known to go through several distinct feeding phases. Fish up to 30 cm feed almost exclusively on worms and crustaceans. From there until they reach approximately 80 cm in length their diet is a mixture of invertebrates and fish. Halibut larger than 80 cm feed almost exclusively on other fish, including redfish, cod, haddock, and lumpfish.

In Canadian waters, female halibut mature at lengths of 70 to 115 cm while males mature at 66-100 cm. These lengths correspond to ages of approximately 10 to 12 years for females and 7 to 11 years for males.

Fishery

During the early years of the fishery, halibut were fished in relatively shallow waters (40 to 135 meters) using handlines. The lines were equipped with a single hook, weights to take it to the bottom, and baited with strips of herring, haddock or cod. The lines, fished over the side of the vessel, would be played along or just above the bottom in an attempt to make the thin strips of bait look like live fish. Once the halibut was hooked it was often a difficult time to get it into the boat, with many fish breaking free.

When the halibut fishery became a more profitable venture, the handline method was replaced by longlining, the latter being a more efficient method of catching halibut. The basic method of halibut longlining has not changed much since its inception in the 1840s and 50s. The longline consists of a heavy groundline with slightly lighter sidelines or 6 4 gangings" (pronounced gainjings) attached at intervals of several meters. Each ganging is equipped with a large halibut hook and strips of bait.

The entire longline, which consists of several sections, may be a mile or more long. When it is set out, each end is anchored and a marker buoy is attached to one end. After the line has fished for several hours (or even days depending on the weather) the anchor at the marked end is lifted and the line hauled up. During the 1800s and early 1900s the setting and hauling of longlines was done by two men fishing in small dories away from the mother vessel and was done by hand. Each fish was hauled up from the bottom, often with a considerable struggle, dispatched with a killing club and stowed in the bottom of the dory.

Present day longliners are equipped with machinery which reduces the amount of labour involved in setting and hauling the lines. Hydraulic winches have taken the place of muscles to haul the fish from the bottom and more durable and stronger synthetic materials have replaced the manila lines. However, the number of longliners now employed in a directed halibut fishery has dwindled. Only a small number of these vessels hailing from a few ports in the Maritimes actively pursue this fishery. Many halibut are now taken as by-catch by vessels fishing with towed nets (otter trawls) for other species.

Reference

Adaptation of document : Atlantic Halibut, by Zwanenburg K., Coll. Underwater World, published by Communications Directorate, Fisheries and Oceans Canada, 8 p. 1991.