This is Scientific American's 60-second Science, I'm Julia Rosen.Every year, salmon come home to Alaska's frigid rivers to mate, lay their eggs and die.The state's salmon runs are some of the biggest in the world.But over the past few decades, those big salmon runs have featured ever smaller salmon."You talk to people up there who've been fishing for long time,and they're definitely able to tell you that, you know, we just don't see those really large, old salmon that we used to see."Krista Oke, a postdoctoral fellow at the University of Alaska Fairbanks.Oke and colleagues at the University of California, Santa Cruz, and elsewhere analyzed records of fish size going back to the 1950s.They included data on some 12.5 million salmon―each of which had to be measured by someone from the Alaska Department of Fish and Game.And there's no question about it: salmon have shrunk.Sockeye salmon today are 2.1 percent shorter than their ancestors.Chum salmon are 2.4 percent shorter, and coho are 3.3 percent shorter. Chinook, or king, salmon showed the greatest declines at 8 percent.That's an average difference of more than two inches in length. The study is in the journal Nature Communications.The researchers haven't nailed down the exact reasons behind this trend.But their analysis suggests that climate change and competition with wild and hatchery-raised salmon both play a role.They also discovered that much of the change in body size is due to fish returning from the ocean at a younger age now than in the past.
