The low number of sockeye returning to Lake Washington has been an unsolved mystery for years.
And this summer, the saga continues.
Sockeye travel through Seattle’s Ballard Locks each summer to reach Lake Washington, which separates Seattle from Bellevue and Eastside suburbia. Adult sockeyes are counted as they climb the fish ladder. As of July 31, the count was less than 43,000 – one of the lowest returns on record.
Sockeye have been tallied every summer here since 1972. The counts track the run size, and a sufficient run size determines whether sport or tribal commercial fisheries will occur. The last time runs were large enough for sport fishing was in 2006.
The returns each year fluctuate from as high as 531,000 to less than 22,000. But in the last 20 years, sockeye numbers have dipped dramatically on a more frequent basis.
The lowest sockeye return took place in 2009 when the count was 21,719. The second lowest happened the previous year in 2008 when the count was 33,619.
Similar trends have been seen across the Northwest and Canadian stocks have also had unexplained decreases, says Val Tribble, a biologist for the Washington Department of Fish and Wildlife.
“We don’t know why the numbers of sockeye returning are so low. That’s been up for debate for a number of years,” Tribble says. “We have no real answers.”
The Lake Washington sockeye run is not a native run, Tribble says. Sockeye stock was taken from Baker Lake in the 1930s and ‘40s to establish a run through the Ballard Locks to Lake Washington, where anglers could fish for them.
“The run did well for quite some time. People have a lot of memories of fishing for sockeye in Lake Washington,” Tribble says.
But numbers of sockeye have been in decline in recent years. And the reasons are murky.
Since the 1960s, scientists have been actively investigating what factors influence sockeye returns. According to Tribble, low returns could be related to competition with other species, predator-prey interactions or water temperatures.
“These fish are difficult to study because they are part of a dynamic urban environment,” Tribble says. “We’d like to understand at what life stage they are failing.”
This is the crux of the mystery.
Sockeye could be dying as juveniles in freshwater, or once they reach saltwater, or somewhere during the course of their migration. It’s not clear.
The best time to see salmon traveling through the fish ladder at Seattle’s Ballard Locks is during spawning season, from late June to mid-August.
But one thing researchers know for sure is that salmon prefer chilly waters— exactly the type of weather many people in the Northwest have been grumbling about this summer.
Michael Brett, a University of Washington scientist in environmental engineering, has been studying long-term trends in summer surface-water temperatures. Brett’s research suggests Lake Washington’s surface water temperatures have been rising by as much as 4 degrees over the last 40 years.
From 1960 to 2000, the average surface temp for Lake Washington in late July was 69 degrees, Brett says.
This summer water temperatures are bucking that trend. There was only one day in July that surface water temperatures on Lake Washington reached above 69 degrees. That was on July 27 when surface water temperatures hit 72.5 degrees, according to readings collected by the King County monitoring buoy on Lake Washington.
“The temperatures this year would be favorable for sockeye,” Brett says. But warmer water temperatures a few years ago, Brett says, might have affected the parents of the current generation of sockeye, causing the numbers this year to be lower.
“The hard evidence that high (water) temperatures have hurt the adult Lake Washington sockeye in the past is weak,” says Tom Quinn, a University of Washington aquatics and fishery sciences professor who studies fish behavior, ecology and evolution. “Though it is a lot stronger in other systems such as (British Columbia’s) Fraser River.”
Quinn agrees that the cool temperatures this year are good for the returning adults.
So water temperatures this year may lead to higher sockeye returns a few years from now.
Or it may not.
Nobody can predict for sure.
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