By Bill Hauser
One of the 43 species of fishes I discussed in my book, Fishes of the Last Frontier, is the Pacific Halibut, properly known as Hippoglossus stenolepis.
I think every Alaskan knows something about Pacific halibut and most probably find them interesting in some way. They are found in Alaska marine waters in depths of about 20 to 3,600 feet but usually in waters less than 1,000 feet. They occur continuously from the Chukchi Sea along our coast and south to Mexico. A near relative, the Atlantic halibut, is found in that other ocean. They have been reported to achieve 15 feet in length, 850 pounds in weight, and 50 years of age.
Pacific halibut spawn in deep water, as deep as 600 to 1,500 feet. Spawning is between November and March. Pacific halibut are broadcast spawners and the female releases a very large number of very small eggs. The number of eggs depends, of course, on the size of the fish and females grow larger and faster than males. The eggs are slightly buoyant, depending on the salinity of the water, and they drift with the currents. As the eggs slowly rise higher in the water column, the embryo hatches in about 15 days, depending on water temperature.
Here is an interesting fact. When Pacific halibut larvae hatch from the egg, they are only about three-eighths of an inch long, transparent, and swim upright like most other fishes. They have one eye on each side of their heads. They drift passively with the current and they eat tiny planktonic organisms. They ride the currents, rise toward the surface, and move to more shallow waters. As they grow, they become pigmented and the left eye migrates to the right side of the head. By the time they are about six months old and one inch long, they appear as a miniature version of an adult. They are pigmented on the right side while the left side remains white, both eyes are on the right side, and they settle out of the currents to take up life as highly adapted bottom fish.
Males usually reach maturity at about eight years of age and females become mature when they are about 12 years. Females grow faster and longer than males and nearly all Pacific halibut over 100 pounds are females. The largest Pacific halibut was nearly 9 feet long and reported to weigh nearly 800 pounds.
Pacific halibut are carnivorous ambush feeders. They can adjust their skin color for camouflage to match the color of the bottom and, as they settle, they do a quick flutter to create a cloud of silt and sand that settles back onto the fish to improve their camouflage even more. They eat a wide variety of foods, but bigger Pacific halibut become more dependent on other fish. On occasion, Pacific halibut rise from the bottom to feed pelagically.
The International Pacific Halibut Commission (IPHC) has the responsibility for coastwide management of Pacific halibut. The IPHC needs information about the population stability and movement of halibut populations to manage effectively. Some of the more interesting research tools the fishery scientists rely on are electronic tags. These tags are much “smarter” than many other types of tags and, as the ICPH report queries, what makes the Pacific halibut researcher’s heart go pitter patter? Answer: PITs and PATs.
A PIT is a Passive Integrated Transponder tag and a PAT is a Pop-up Archival Transmitting tag. The PIT is much like the microchips that are inserted in animals for identification. Each has a unique identification signal and is inserted under the skin of a Pacific halibut. A scanner device elicits a response from the transponder and the recovery date, size, and location can be compared to release data to determine growth rate and movement. Early results are still somewhat unclear, mainly because of a low rate of returns. Nevertheless, in some areas along the coast, some migrations were substantial.
A PAT tag is even smarter. These store or archive a series of data points that include depth, water temperature, and light. They are attached to the fish externally by a thin wire that is programmed to break on a prescribed date. After it floats to the surface, the PAT transmits the stored data to a satellite along with its final position. Results from these studies confirmed that Pacific halibut migrate to deep water for overwintering and return to shallow water for summer feeding. Other results showed that some Pacific halibut moved very little in a year but others traveled long distances. One was tagged near Unimak Pass in the Aleutian Islands and a year later it was near Grays Harbor, WA. It traveled a minimum of nearly 1,900 miles.
Why is this important? The basic ingredients of any management plan include some sort of an inventory or assessment of the size and distribution of the resource. In addition to the size and distribution of the resource, it is vitally important to understand how and when fish move through various habitats and how discrete the individual populations may be so each population can be managed individually. In other words, the assessment will be made at a particular time and location and the scientist can make a harvest plan to fish in particular areas and seasons for some harvest quota of fish. However, if the fish move to some other location before or during the harvest, the plan will be bogus and under—or over—harvests of particular stocks will occur.
Although these electronic tags have not been deployed for long, they are already providing more insight about halibut migration and behavior than scientists we able to get before. Here are some results from halibut tagging studies.
Although scientists have known for some time that Pacific halibut migrate during winter from shallower water to deeper water to spawn, they have now learned that rather than moving directly offshore, they may move considerable distances adjacent to and parallel with the shore before moving to deep water. Some fish may spend two or three winter months at a depth of around 1,000 feet. Thus, extended fishing seasons may intercept fish that may have been inventoried in different fishing zones.
After about a year, 60 per cent of recaptured Pacific halibut were caught within about 6 nautical miles from their tagging site and 80 per cent within 13 nautical miles. These results suggest seasonal homing and site fidelity rather than broad-scale, general dispersal.
Are Pacific halibut of interest to Alaskans? The answer is obvious. Pacific halibut are harvested in commercial, sport, and subsistence fisheries. Pacific halibut are savored as a food fish and the halibut fishing industries, both sport and commercial, create a huge economic benefit.
(For this FISH TALK, I consulted the Fishbase website, IPHC website and the book, Fishes of Alaska.)
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