While the orcas of Puget Sound are sliding toward extinction, orcas farther north have been expanding their numbers. Their burgeoning hunger for big fish may be causing the killer whales’ main prey, chinook salmon, to shrink up and down the West Coast.

Chinook salmon are also known as kings: the biggest of all salmon. They used to grow so enormous that it’s hard to believe the old photos now. Fishermen stand next to chinooks almost as tall as they are, sometimes weighing 100 pounds or more.

“This has been a season of unusually large fish, and many weighing from 60 to 70 pounds have been taken,” the Oregonian reported in 1895.

"It's not impossible that we see individuals of that size today, but it's much, much rarer," University of Washington research scientist Jan Ohlberger said on Monday, more than a century later.

Ohlberger has been tracking the downsizing of salmon in recent decades, but salmon have been shrinking in numbers and in size for a long time. A century’s worth of dam building, overfishing, habitat loss and replacement by hatchery fish cut the average chinook in half, size-wise, studies in the 1980s and 1990s found.

The dam building and fishing have tailed off, but chinooks have been shrinking even faster in the past 15 years, according to a new paper by Ohlberger and colleagues in the journal Fish and Fisheries. Older and bigger fish are mostly gone.

Few fish are making it to old age, which for a chinook salmon means spending five or six years in the ocean after a year or two in freshwater. “The older fish, which normally come back after five years in the ocean, they come back earlier and earlier,” Ohlberger said.

The trend is clear, the reasons less so.

Two species eat more chinook salmon than any others: orcas and humans.