Chinook salmon on the West Coast are getting younger and smaller as the species' kings disappear. The kings are the largest and oldest Chinook salmon, and each year, there are fewer and fewer.

"Chinook are known for being the largest Pacific salmon and they are highly valued because they are so large," Jan Ohlberger, a research scientist at the University of Washington, said in a news release. "The largest fish are disappearing, and that affects subsistence and recreational fisheries that target these individuals."

Ohlberger and his colleagues analyzed four decades of salmon hatch and wild population data. The findings, published in the journal Fish and Fisheries this week, showed fish are getting smaller. Salmon in Alaska and Washington have been affected the most by the disappearance of large specimens.

Chinook salmon measuring nearly four feet in length used to be common, but no more. The research echoes the anecdotal evidence of fishermen in the region.

Though born in freshwater river and streams, Chinook salmon develop their large size out in the oceans. While California Chinook salmon tend to occupy waters off the California coast, Oregon and Washington salmon migrate northward to the Gulf of Alaska. Alaskan salmon travel to the Bering Sea.

After a few years at sea, the fish return to the place of their birth, swimming upstream to spawn and perish. The latest research suggests fewer large, old fish are returning during the annual spawning run.

Because the trend is significant among all of the Chinook populations along the West Coast, scientists believe something is driving the loss of kings of a large scale.

"There is something about the larger ocean environment that is driving these patterns," Ohlberger said. "I think fishing is part of the story, but it's definitely not sufficient to explain all of the patterns we see. Many populations are exploited at lower rates than they were 20 to 30 years ago."

The loss of large specimens is more than a peculiar anomaly. It's a potential problem. Smaller fish can't carry and lay as many eggs as larger fish. The loss of larger fish is likely to lead to a less reproductive success for the species.

Though conservationists are concerned, scientists suggest fishing pressure on the species has relaxed in recent decades and that the latest losses could be driven by natural pressure, like orcas and other marine predators. Orcas especially tend to target the biggest, oldest fish.

"When you have predators and prey interacting in a real ecosystem, everything can't flourish all the time," said Daniel Schindler, a professor of aquatic and fishery sciences at UW. "These observations challenge our thinking about what we expect the structure and composition of our ecosystems to be."

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