Water temperatures, streamflows and even the prevalence of other species of salmon were all found to influence the reproductive productivity of chinook salmon from the Canadian part of the Yukon River.
This is from a recently-published study that delved into the positive and negative effects environmental conditions have on the Yukon River chinook. It is set against recent declines in salmon returns to the area that the researchers write amounts to a crisis for thousands of Indigenous and non-Indigenous people in the river’s drainage.
The study in question, titled multiple environmental drivers across life stages influence Yukon River chinook salmon productivity, was published in the Canadian Journal of Fisheries and Aquatic Sciences in December 2023. Alyssa Murdoch, who works out of Carleton University, was one of the researchers behind the study.
“No one had really looked at all of these different factors on the Canadian side of the border before, and it just seemed like it was kind of high time to do that, considering, you know, the recent years and just the growing salmon crisis that was going on,” Murdoch said.
Murdoch and her five co-authors looked at a host of different potential environmental factors that might influence the chinooks’ productivity. These were selected based on previous evidence with a focus on past studies of the chinook in Alaska and the Yukon River basin. Productivity is defined as the number of salmon produced per spawner, not just the number of fry, but the number of adults that return to spawn themselves.
The study considered data from as early as 1985 and as recent as the salmon that spawned in 2012 and whose offspring returned by about 2018.
The pool of evidence out there was the starting point for a study of the correlations between environmental variables and the productivity of the Yukon River chinook population. Some variables suggested a positive impact on chinook productivity while others were negative.
Warmer temperatures at sea during the winter showed a positive correlation with productivity. Murdoch said this may be because during the first winter at sea the salmon face a lot of mortality due to their small size.
“The idea here was that warmer winters may actually allow them to increase their feeding and their growth and then reduce this potential for what we’re calling this size-selective marine mortality,” Murdoch said.
Earlier ice out in the interior also seems to be positive, likely because it allows the salmon smolts to reach productive feeding areas in coastal parts of the Bering Sea earlier and so they grow larger faster, aiding their survival in the long run.
Increased spring snowpack and warmer winters in the Yukon were seen as a benefit. Murdoch said the possible reason for this is cooler water later into the migration and maybe even the early spawning period and more consistent streamflows.
“The idea is that it could be potentially beneficial for the salmon because they’re going to want to seek more consistent, kind of less flashy streamflows, like areas where they have more stable habitats to move through and then to spawn,” Murdoch said.
The study looked at winter air temperatures as a measure of the winter’s length and severity. Murdoch said the thick ice present over streams in long hard winters can limit habitat. Shorter and less-severe winters could positively impact egg survival the winter following spawning.
Warmer temperatures during the upstream migration is a negative factor for salmon survival and productive breeding that Murdoch said is getting more and more attention as very warm summers become more common in the Yukon and Alaska.
The temperatures are seen as a negative impact due to physiological stress placed on the fish that could delay or impede them from getting to their spawning areas. Murdoch said that the stress can even have carryover effects from the spawning salmon to their offspring, putting them at a disadvantage and affecting their viability.
High abundance of pink salmon showed a negative relationship with productivity for the Yukon-origin chinook. Murdoch said this might indicate direct or indirect competition between the fish. The relationship with the productivity of the chinook seems to be confined to the pinks as no similar relationship was found with the chum salmon.
Warmer wetter habitat for juvenile chinooks prior to their entry into the ocean could be contributing to productivity declines. Murdoch said a possible explanation is that the young salmon can be vulnerable to high streamflows which can impede their ability to effectively eat, grow and survive during their first summer in fresh water. Research from the Tanana region of Alaska suggests the negative impacts of high streamflows on salmon are driven by the increased energy demands on fish feeding in fast water and increased sedimentation that make it more difficult for them to find quality prey.
Murdoch collaborated on the study with Whitehorse-based biologist Chrystal Mantyka-Pringle, Steven Cooke at Carleton, federal Department of Fisheries and Oceans researcher Brendan Connors, Nick LaPointe of the Canadian Wildlife Federation and Joanna Mills Flemming from Dalhousie University.
Along with the main study, the researchers released a summary that included some commentary on how their work could inform salmon management: Suggestions include precautionary ecosystem-based management, the protection and restoration of freshwater habitats for juvenile salmon and transboundary coordination on planning for salmon recovery.
“We did not find a single ‘smoking gun’ that is responsible for the salmon crisis. Instead, we revealed a host of potential environmental factors that may be influencing salmon, both positively and negatively, over their many habitats from the headwaters of the Yukon River out to the Bering Sea,” the summary reads.
Tangible management steps pitched in the summary include increasing the number of fish allowed to escape harvest and return to spawn in years when the salmon are under elevated environmental stress, efforts to protect juveniles from high streamflows and the creation of mathematical models to help understand changes in the number of spawning salmon required to sustain the populations.
Looking ahead to the future, Murdoch said differences in outcomes for salmon in the upper and lower Yukon River would be an interesting question to study. Murdoch also hopes that someone, possibly the same team that worked on the recent study, will be able to expand the work to cover more recent years as new data becomes available.
Contact Jim Elliot at jim.elliot@yukon-news.com