ANCHORAGE - Salmon from two rivers in Alaska and Russia have shrunk in size in what scientists say could be a decades-old fight for food in the Gulf of Alaska.
Scientists at the University of Alaska Fairbanks used high-resolution digital imaging equipment to look at about 2,000 fish scales taken from chum salmon caught on Alaska's Yukon River and Russia's Anadyr River over more than 30 years.
The National Marine Fisheries Service, the Alaska Department of Fish and Game and the Russian Pacific Fisheries Research Center joined in the study.
By looking closely at the width of the scales, scientists determined that chum salmon decreased in size by about 25 percent between 1965 and 1997, said William Smoker, director of the School of Fisheries at the university and lead investigator for the three-year, $262,000 federally-funded study.
While scientists have known for years that the size of Pacific salmon has decreased, the scale study reveals just when growth slowed during the life of the fish, Smoker said Monday.
It was the first time that scientists looked at growth rates of salmon from both sides of the Bering Sea. The Anadyr River, which also empties into the northern Bering Sea, is almost directly across the Bering Sea from the Yukon River.
The study found that the fish grew well in the first year of life, but growth rates decreased when the fish foraged for food in the Gulf of Alaska before returning to rivers to spawn.
"These fish have grown more slowly as young adults at age 2, 3 and 4. What we know about their life histories, those are the years that are spent in the North Pacific Ocean," Smoker said.
Scientists used a digital camera and a dissecting microscope to analyze acetate images of scales from the archives at the Alaska Department of Fish and Game. The scales were taken from Yukon River chum salmon captured at test sites on the lower Yukon River. The Alaska collection dates back to 1960. The Russian collection is older.
The impressions were made by pressing the scales' bony ridges onto sheets of acetate.
Fishermen and scientists have speculated that competition with hatchery fish for ocean food caused the crash in Yukon River salmon populations in recent years.
Smoker, however, doubts that the nearly 5 billion hatchery salmon released each year into the North Pacific Ocean are entirely to blame. Those fish are released from hatcheries in the United States, Russia, Canada and Japan.
"The hatchery programs have not been significant until the late 1970s and early 1980s," he said. "This trend toward a smaller body size and growth has older origins."
Genes could be responsible for why salmon are getting smaller, said Buel D. Rodgers, an instructor of medicine at Johns Hopkins University's School of Medicine who has been studying growth rates in fish.
Rodgers said the preference for larger fish by commercial and sport fishermen could be removing those bigger fish from the gene pool, leaving only the genetically smaller fish to reproduce.
"Fishermen always want to catch the big fish," he said. "If you are always selecting out the big ones ... what you are left with is genetically smaller fish."
But UAF's Smoker doubts genes are the whole answer. He points to the downward size trend in other salmon populations, including chum salmon caught in fish traps in Japan and pink salmon caught by purse seiners near Juneau. Neither harvest method is selective about fish size, but both yielded smaller fish.
"This size trend has occurred in a lot of different populations of salmon all across the ocean," Smoker said.
Barbara Belknap, executive director of the Alaska Seafood Marketing Institute in Juneau, said the study's findings are perplexing, given that the average size of harvested chums in Alaska is increasing.
The average size of chums this year was 8.40 pounds, the fifth straight year when the average topped 8 pounds, she said, citing state statistics.
"To say it is overfishing and competition for food when the fish are getting larger on average doesn't make sense," she said.