The number of chinook salmon that reached the Whitehorse fish ladder this year hit a 40-year low — and it's not clear why.
"They're the lowest since 1977," said Elizabeth MacDonald, executive director of the Yukon Salmon Sub-Committee, a public advisory body.
"That's not significant just because of the length of time since that low run, but also because in 1977 there was no hatchery contribution — so there were actually less salmon ... available to make that journey."
MacDonald says it's puzzling because plenty of spawning chinook were counted downriver in Alaska, earlier in the season.
"There were ... almost 220,000 chinook returning to the river, and this would be both U.S.- and Canadian-origin chinook, which is quite a bit higher than we've seen in recent times," she said.
"But that number just didn't materialize at the border or at Eagle sonar [station]."
Just 282 chinook passed through the Whitehorse fish ladder. That's compared to 690 last year, and more than 1,200 in 2017.
Alaska harvest may be a factor
Yukon River chinook runs have been in general decline in recent decades, and that's prompted Indigenous communities in Yukon and Alaska to restrict their harvests in recent years — but there's been tension over how many fish are still harvested.
In the past, some Yukon First Nations have blamed over-fishing in Alaska for reducing the number that reach spawning grounds in Yukon.
"I will say that in past years, that relationship was quite confrontational. But in recent years, like in the last five, 10 years, things have definitely been improving," MacDonald said.
She says the Alaskan harvest may have been a factor this year, but it's too soon to say. Harvest numbers won't be known until later this year.
"They were very conservative this year at the beginning this season, and then as the [return] numbers started to increase, they relaxed their fishing regulations and allowed for more harvest," she said.
MacDonald says there were also fewer summer chum salmon in the river this year. They typically migrate at the same time as chinooks, so MacDonald says people fishing for chinook often net a lot of chum. This year, they may have been catching more chinook.
Warmer water may be another factor. There were record-high temperatures in some Alaskan waters this year, MacDonald says.
"We did see some large pre-spawn mortality die-offs in a tributary of the Yukon River — the Koyukuk in Alaska. This was for summer chum, and not chinook — but we expect that that higher water temperature also affected the chinook migrating through."
She said once biologists have more data, including harvest numbers, they'll be able to determine "their best guesses" for why so few chinook made it into Yukon this year.
"There were a lot of interesting things happening in the river this year, so it has been quite unique. So it's hard to find out the exact cause of it," she said.
"We really don't want to see numbers come this low."