Mystery of the Incredible Shrinking Halibut

California Halibutby Robert Deen for Anglers Club Magazine

It’s a troubling mystery. Why are Pacific Halibut getting so much smaller?

Halibut weigh about a third of what they did 30 years ago, scientists say. In Alaska’s major port areas, including Homer, Seward, Kodiak, Ninilchik and Whittier, the average size of an 18-year-old female halibut has declined from more than 80 pounds in 1997 to about 40 pounds last year.

Despite the occasional big fish, the average halibut size continues to shrink, according to Scott Meyer, the statewide bottom fish coordinator for the Alaska Department of Fish and Game. “We’re seeing a continued decline in the size of halibut,” he said of a trend that has continued for more than two decades in Alaska.

Pacific HalibutScientists don’t know why, but theories abound, including fishing pressure, competition from arrowtooth flounder (a plentiful fish with no commercial value that inhabits many of the same areas as halibut), and even genetic changes that could be selecting for smaller fish.

Nowhere is the trend plainer – or more painful — than at Alaska’s largest fishing tournament – the Homer Jackpot Halibut Derby. Last year’s winner sounds big at 214 pounds, but it was the smallest in the derby’s 30-year history – 137 pounds less than the derby record caught in 2007. The number of derby tickets sold declined for the second consecutive year, down 31 percent since 2013.

According to the International Pacific Halibut Commission, for the past 15 years or so halibut growth rates have been depressed. Both females and male halibut have the potential to grow rapidly until about age 10, about 2 inches per year for males and 2.5 inches for females. Thereafter, females have the potential to grow even faster, while males slow down. Growth rates for these larger fish over the past 10 years have been more like one inch (or less) per year. This translates into a much smaller fish at any given age.

Sometime around 1980, growth rates started to drop. For example, in the northern Gulf of Alaska, an 11-year-old female halibut weighed about 20 pounds in the 1920’s, nearly 50 pounds in the 1970’s, and now again about 20 pounds. The reasons for both the increase and the decrease are not yet known but may be tied to increased abundance of other species, and availability of food supply.

Fishermen for decades have claimed that Arrowtooth flounder, which grows to about 3 feet in length, are blanketing the bottom of the Gulf and out-competing halibut for food. Their numbers have increased 500 percent over roughly the same time that halibut sizes have dropped.

“People think that potentially arrowtooth are competing with halibut for space and/or prey, which is limiting the growth of Pacific halibut,” Cheryl Barnes, a doctoral student at the University of Alaska, told the Alaska Dispatch News.

Barnes is studying the contents of halibut and arrowtooth stomachs collected from sport anglers. Her study complements others that focus on environmental factors and the impact of fishing.

“Especially size-selective fishing — the idea that we have been removing the larger, faster growing individuals, and it just kind of brings that average size at age down,” she said.

The Mystery of the Incredible Shrinking Pacific Halibut

 


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