The case of the shrinking Chinook

The last time a 50-pound Chinook topped the Golden North Salmon Derby standings, the year was 1990. Roger Drapeaux’ 55 pounds, fish took home the top prize.

A derby winner hasn’t topped 50 pounds since then, and if a trend described this week in a scientific paper published in the journal “Fish and Fisheries” continues, a Chinook that size won’t top the Golden North’s leaderboard anytime soon: the paper’s authors say king salmon on the West Coast are both smaller and returning to spawn at younger ages than they did in the late 1970s.

It’s a trend that previous research had identified, but the paper, released on Feb. 27, shows that it’s occurring in a range that extends almost to California.

The trend is twofold: returning Chinook salmon are both smaller and younger than they used to be.

“Not only are the biggest fish disappearing, but the ones that do return, their sizes are also getting smaller,” said one of the study’s four co-authors, University of Washington Professor Daniel Schindler in a Friday interview with the Empire.

The research took data from 85 populations of hatchery and wild Chinook salmon stretching from California to Alaska. Taken together, it shows a marked difference in both the age of Chinook salmon and their length over time.

The size trend was strongest in Alaska, where some Chinook populations of the same age measured about 10 percent shorter today than decades ago.

But that’s only half the trend. The proportion of older salmon is also much smaller in Alaska and coastwide, the researchers found.

Chinook most commonly return to their home rivers after three years in the ocean, but they can stay in the ocean for a few years longer than that. Salmon that used to spend five years in the ocean — the likely age range of Drapeaux’ derby winner — accounted for about 5 percent of Alaska Chinook in the late 1970s. Now, the weighty and sought after “5-ocean” fish, as they’re known, make up less than half a percent of Alaska Chinook.

“They have become incredibly rare,” Schindler said.

Four ocean fish, or fish that have spent four years in the ocean, declined coastwide as a proportion of salmon populations. Three ocean fish have actually increased as a proportion of Chinook populations, most likely, Schindler said, because salmon that might have previously stayed in the ocean for an extra year or two are now returning earlier.

If you put both findings together, it means the average size of a Chinook salmon is much smaller than it used to be.

It’s not clear yet what this trend could mean for Chinook populations. Schindler said there’s some research that Chinook populations could undergo a “homogenization” and return at the same age. How salmon make use of their spawning habitat will possibly change if the trend continues. Older, larger and stronger salmon can lay eggs in the main stem of a river, where only they can move larger gravel sizes to create nests for their eggs. This is harder for smaller, 3-ocean Chinook, which lay their eggs on the smaller, shallower tributaries lining the main stems.

That habitat is vulnerable to drying out and flooding, something larger fish don’t have to deal with in the main stems of rivers.

“They’re pushed to the margins of smaller tributaries where they’re more susceptible to flooding, drying out,” study co-author and ADFG biologist Bert Lewis said in a Friday interview with the Empire.

Larger, older female fish hold more eggs than smaller females. Their dwindling numbers could mean the same number of salmon produce smaller offspring in smaller numbers.

“The implications of that on overall population productivity are something we’re considering,” Lewis said.

Researchers don’t know exactly why salmon are getting smaller and returning at a younger age, but they say the coastwide trend points to a cause outside the local or regional level. Fish on the Alaska’s Copper and Yukon rivers, where size and age trends are strong, show a similar trend as Chinook in Washington state, so overfishing in any one location isn’t likely to blame.

Climate and variations on ocean conditions also don’t jump out as likely causes, Schindler added, because those patterns have varied year-to-year and don’t square with the trend they’re seeing in size and age data.

One potential cause, Schindler said, could be an increase in predator populations, particularly orca, or killer whales, a Chinook predator which has increased in number during the same timeframe the study looked at. Protections for orca have increased since the 1970s, and the intelligent pack hunters are well-known to target larger Chinook.

But any speculation on what’s shrinking Chinook in the northeast Pacific is premature, Schindler said. The next step is to use computer models and build a case as to what the most compelling cause is.


• Contact reporter Kevin Gullufsen at kgullufsen@juneauempire.com. Follow him on Twitter at @KevinGullufsen.


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