Yukon River's winter waters teeming with chinook salmon

While chinook salmon tend to get the most attention during their epic spawning runs in the summer, Yukon's waters are teeming with the fish year-round — including during the winter.

In fact, that's when Yukoners are surrounded by the highest number of chinook, according to a Whitehorse fish biologist, although they're hidden under the ice and much smaller than the bright-red giants most people are used to seeing. 

"I guess the most important part of their habitat in the winter is water that remains liquid," Oliver Barker, a senior aquatic scientist with Fisheries and Oceans Canada, told CBC. 

"It's mostly a case of sticking around and surviving until spring." 

An extended freshwater stay

Yukon River chinook salmon will spend about a year and half in freshwater before making the journey to the ocean, which is much longer than their counterparts further down south. While some southern chinook juveniles will leave for saltwater their first summer, Yukon River chinook actually spend two winters in the watershed — the first, immediately after hatching, and the second as juveniles that will begin migrating in the spring. 

Newly-hatched chinook, which emerge from that year's eggs in the early winter, are called alevin and spend the colder months biding their time in the gravel. (It's unclear how many chinook salmon hatch every year, but a female salmon can lay between 5,000 to 10,000 eggs). 

They eventually emerge as hungry little fry who spend the rest of their time in freshwater snapping up as many small invertebrates as possible, finding spots where the water doesn't freeze and staying out of the way of predatory fish like pike. 

The young fish can travel "quite a long way" to find ideal habitats, Baker said, and can reside in smaller streams not typically associated with fully-grown salmon. 

"We have found juvenile Chinook even hundreds of kilometres downstream from where they were hatched, but this isn't part of their migration to the ocean —  this is them finding ideal habitat for growth and survival," he explained. 

"... We've also found them tens of kilometres upstream of where they were hatched, so you can imagine these feisty little fish pushing their way upriver trying to find ideal places that have the right amount of flow, the right kind of temperature and abundant food so that they can grow big enough to survive their migration to the ocean." 

By the time the ice breaks up in the spring, Chinook who have made it through their second winter are only about the length of a finger but prepared to make the more than 3,000-kilometre journey to the Bering Sea. While there is a degree of the "mad rush" seen with the adult spawning migration, there are a few differences for young salmon heading for saltwater — they're swimming with the current instead of against it, and are also eating along the way. 

The migration typically peaks in mid-June. About 95 per cent of the chinook salmon are now in the "smolt" life stage, having hit the ocean by mid-July, where they'll spend several years before returning home. 

The migration exception

Not all juvenile chinook salmon make the journey, though — there are rare cases where male fish will actually spend their entire lives in freshwater, from hatching to spawning. 

Barker said those chinook are unofficially known as "sneakers," because they only grow to about the size of a hand and can't attract egg-laying females on their own when spawning season finally rolls around. 

 "They have to rely on the big ocean-going males to find females and convince them to spawn, and then they'll dart in underneath them, hoping not to be detected and release milt to fertilize those eggs," he explained. 

The unusual situation is an evolutionary gamble of sorts. On one hand, the longer a fish lives, the bigger it can get. Being a larger fish means being able to carry more eggs or milt, and having the ability to dig a better nest in the gravel. On the other hand, the longer anything is alive, the higher the chance it'll die before it can reproduce. 

"This is an especially big gamble for fish like salmon that only spawn once — you don't get a do-over, you've got that one shot," Barker said. "So having a component of the population where the adults reproduce at a very small size like these sneaker males is (the species) hedging their bets against high predation or high mortality rate."

The phenomenon, he added, appears to be exclusive to male salmon; a tiny female salmon wouldn't be able to carry enough eggs to make the strategy worth it. 

Difficult to study

Although there's interest in the early stages of the Chinook salmon life cycle, Barker said that practically, it can be a "pretty difficult" stage to actively study.

"Streams are hard to access in the winter, and in particular, small fish in streams are particularly hard to access and it can be difficult to handle them as well," he said. 

"You're taking fish out of cold water into much colder air, and so there's trouble keeping those fish from freezing." 

While Fisheries and Oceans Canada has done monitoring and survey work in the past, juvenile chinook salmon aren't tracked nearly as closely as their adult counterparts; some parts of their world remain a mystery.