The incredible shrinking salmon

Salmon from two rivers in Alaska and Russia have shrunk in size in the Gulf of Alaska.

Scientists at the University of Alaska in Fairbanks used high-resolution digital imaging equipment to look at 2,000 fish scales taken from chum salmon caught on Alaska’s Yukon River and Russia’s Anadyr River over more than 30 years.

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.

By looking closely at the width of the scales, scientists from the U.S. and Russia found chum salmon had decreased in size by about 25 per cent between 1965 and 1997.

While scientists have known for years that the size of Pacific salmon has decreased, the scale study shows exactly when growth slows during the life of the fish.

The study found that the fish grow well in the first year of life, but growth rates decrease when the fish enter the Gulf of Alaska before returning to rivers to spawn.

Fishermen and scientists have suggested competition for ocean food with hatchery fish is causing the Yukon River’s salmon population to decline.

But genes may also play a role in the smaller size of salmon. The preference for larger fish by commercial and sport fishermen could be taking these bigger fish from the gene pool, leaving only the genetically smaller fish to reproduce.