30-year decline in chinook size is statewide
The iconic Alaska chinook salmon has unequaled world-renown for big fish tales.
The largest fish on record, a 97-pounder pulled from the Kenai River in 1985, changed the focus of the Alaska fishing economy, bringing in waves of fish tourists and seeding the politically influential guided angler industry.
Since then, the chinook, or king, salmon across the state have been getting smaller, researchers have found.
Alaska Department of Fish and Game biologists Bert Lewis, W. Stewart Grant, Richard Branner, and Toshihide Hamazaki authored a paper released in June detailing a gradual and inexplicable decline in 10 Alaska chinook salmon stock sizes from the 1980s through 2010, due to an array of factors from ocean current patterns to size-selective harvest methods.
The study used information from gillnet and weir count samples from the 1980s through 2010 of 10 rivers in Alaska, including the Kenai and Copper rivers, the Yukon and Kuskokwim rivers, and Southeast’s Unuk River.
“We found that on average these fish have become smaller over the last 30 years (6 generations), because of a decline in the predominate age at maturity and because of a decrease in age-specific length,” the report concludes.
Kings spend a year in the river before heading out to the ocean, where they typically spend between two and five winters before heading back to their spawning grounds.
In the 1980s, most of the kings swimming back to spawning grounds were “four-ocean fish,” salmon that spent four years in ocean. Over the last 30 years, the predominant maturity age has switched to three-ocean fish, which are smaller.
In all 10 of the examined stocks, the percentage of four-ocean fish declined; by 2012, two- and three-ocean king salmon were the dominant spawners.
Beyond younger and smaller salmon, the size-at-age has diminished in nine of the 10 four-ocean fish stocks and five of 10 of the three-ocean fish stocks. Depending on the stock in question, average nose-to-fork length decreased as much as 150 millimeters in the last 30 years.
Lewis first noticed the trend in Copper River data, then expanded the scope of the study to find out if it was a statewide pattern.
“This started off looking at just the Copper,” said Lewis. “We started asking around for other data sets, and it turns out there are data sets are consistent. We ended up finding 10 stocks across the state that had adequate data sets, and as we looked at it, all these stocks were behaving the same.”
Initially, Lewis thought the numbers were tainted by the methodology. The age and length statistics he first looked at were from commercial gillnets, which target specific salmon sizes. When the team added data from sonar weirs, however, the numbers stayed consistent.
“When you throw in a weir, which accounts every fish, when you put them together, and they’re all behaving the same, the reduction is the same, that seems to suggest that the size selectivity of the gillnet data is not a driving factor,” said Lewis.
The study finds correlations between size decline and a host of factors, but not direct causative links. There are too many factors involved to nail down a main culprit. Lewis said even with an unlimited budget — an ADFG wish, now that the Chinook Research Initiative has been dialed back due to Alaska’s fiscal troubles — it would be impossible to find a single cause in a such a large, dynamic marine system.
Lewis said larger salmon are being slowly culled from the gene pool by size-selective harvest methods, as fishermen target and keep the heavier fish.
“Chinook salmon in Alaska have been targeted in size-selective commercial, sport, and subsistence fisheries for over 100 years,” the report states. “Even when chinook salmon are not targeted directly, incidental harvest in other fisheries can influence phenotypic distributions in chinook salmon populations.”
The data however, is consistent across the fisheries where size-selective harvests occur and areas without them, meaning the harvest method alone isn’t the primary driver in size declines.
Environmental factors also play a large, if unquantifiable, role.
Warmer waters tend to foster faster maturity and smaller size, both through changes in food patterns and changes in metabolism; Pacific salmon in more southern, warmer West Coast waters typically return at younger ages than their Alaska brethren. The North Pacific went through a water-warming shift in ocean currents beginning in 1977.
Larger numbers of salmon could be contributing to the size and maturity age declines as well. Increased hatchery production of pink, chum, and sockeye salmon since the 1970s has produced an all-time high of Pacific salmon numbers that correlates directly with the chinook studies, though Lewis again cautions that the findings are not causative.
The decline in size-at-maturity and overall size has implications for the overall health of chinook. Simply, bigger fish are more fertile and more likely to survive. Larger females carry larger numbers of eggs, and larger eggs altogether, which have higher survival rates.
Despite the notorious blob of warm surface water in the Gulf of Alaska, scientists are expecting a reversal of the 1977 shift in Pacific Decadal Oscillation that led to warmer waters. If colder temperatures rebound, Lewis said, there could be a corresponding growth in size and number of chinook, which declined in number starting in 2007 due to unknown factors.
DJ Summers can be reached at firstname.lastname@example.org.