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This is Scientific American 62nd Science. I'm Julia Rosen.

Every year, salmon come home to Alaska's frigid rivers to mate, lay their eggs, and die.

The state 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 know, you talk to people up there who's been fishing for a 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 Oak, a postdoctoral fellow at the University of Alaska Fairbanks.

Oak 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,

a fish and game.

And there's no question about it.

...