The hidden secrets of Chinook salmon’s long, hard winter
Photo credits: Brandon Deepwell
Moody skies, a damp chill, and persistent drizzle meet Pacific Salmon Foundation (PSF) biologists Will Duguid and Katie Innes as they steer their small 20-foot boat, Primno, out of the Comox marina. It’s a typical December day on Vancouver Island. But for the crew, the lack of wind this morning represents a window of opportunity.
“If it’s calm, we’ll go out,” explains Duguid. “Whatever the weather – snowing, raining, minus ten, we get out there and get the samples we need.”
Duguid and Innes lead PSF’s Winter Ecology study – research testing the leading hypothesis that juvenile Chinook salmon’s first winter at sea is a period of increased mortality, mainly due to starvation. Early findings from their research suggest that might not be the case.
To assess the theory, the team collects field samples at the most challenging time of year for marine research. Stormy weather often shuts down operations for weeks at a time, but the team has still completed 192 winter field days over the past six years.
“One year, the marina froze up, and we had to ice break our way out of there. We didn’t have a heater, and some of the chemicals we use for sampling actually crystallized. We realized that we needed a heater – not so much for ourselves, but to keep the chemicals from freezing,” says Duguid.
Sampling is conducted by microtrolling, an innovative technique that uses recreational fishing gear but is adapted to catch juvenile fish without harming them.
Once a fish is caught, it is thoroughly assessed. The team checks for injuries, parasites, and disease, collects stomach contents, and takes gill samples for genetic analysis. They also insert a tag as part of a wider PSF study investigating salmon survival bottlenecks in the Strait of Georgia.
Crucially, the fish is then released unharmed, minimizing the impact researchers have on at-risk Chinook salmon. All these data are sent to laboratories for analysis, which will later provide a big-picture view of how specific salmon populations are doing over the winter.
Innovative genetic analysis
Back in the lab, those tiny gill samples collected during winter field days unlock some of the study’s most powerful insights. The tissue is about the size of a grain of salt but reveals unprecedented amounts of information on an individual fish’s life.
In December 2025, PSF’s Will Bugg published a peer-reviewed study that demonstrated that gill samples when combined with Fit-Chips – cutting-edge genetic technology – can determine whether a fish is starving.
“The Fit-Chip is a molecular tool that detects the different stressors and pathogens that a fish may be experiencing,” says Bugg, who leads this research at PSF. “It’s really unique as we can analyze thousands of fish very quickly and make population-level assessments without harming them.”
PSF and partners have been using Fit-Chip technology in other research projects as well, underscoring its versatility.
For example, Bugg is also using the technology to monitor juvenile salmon swimming by decommissioned open-net salmon farms in the Discovery Islands and to investigate the health of endangered West Coast Vancouver Island Chinook salmon.
“It’s an incredible tool – we can measure whether a fish is experiencing disease, thermal stress, and low oxygen, just from a tiny tissue sample.”
Getting fat on herring
As the fish sampled in the Winter Ecology study return to their spawning grounds over the coming years, researchers will gain a clearer understanding of how winter conditions affect salmon survival. Findings could inform future decisions around hatchery release timing and fisheries management, depending on the strength of juvenile salmon populations.
The good news is that in the years and regions the team has examined, fish don’t appear to be starving.
“There are some indications that sometimes food is limited, but it doesn’t appear that starvation is the primary driver of mortality in the winter,” says Innes.
However, ocean conditions fluctuate and can quickly affect juvenile salmon survival. When populations crashed in the early 2000s, poor summer growth leading to high winter mortality was suspected, but no one was out there monitoring the fish through winter. The next time ocean conditions change and populations decline, PSF’s data will make it far easier to discern whether winter is really a critical period for salmon survival.
What has been clear throughout the study is the importance of Pacific herring as a food source for salmon: results indicate that juvenile salmon that eat plenty of herring over winter are thriving.
“Fish that are eating herring have fuller stomachs and are getting a much better meal than those feeding on other prey,” adds Innes.
Restoring a symbol of British Columbia
Chinook salmon are considered by many to be the most iconic species of Pacific salmon.
Ecologically, they are critical to the Pacific Northwest marine ecosystem. They are also deeply woven into the culture, food systems, and traditions of many First Nations. And for recreational anglers fishing up and down B.C.’s coast, Chinook remain the most prized catch.
Duguid and Innes are hopeful that lifting the lid on an under-researched period of Chinook salmon’s life history will strengthen conservation management of these fish going forward.
There are already promising signs for the populations that they are studying on the east coast of Vancouver Island.
PSF’s State of Salmon Report indicates that Chinook populations in the East Vancouver Island & Mainland Inlets are up 236 per cent compared to long term averages, although many of these runs are supplemented by hatchery fish. But in other regions like the Skeena and the Yukon, Chinook are struggling.
“Coming back home after a day on the water in winter is awesome,” says Duguid as he ties the final knot on another day of winter fieldwork.
“Chinook salmon are critically important. If we don’t understand what’s going on in the ocean, we’re not going to be well-placed to make the correct decisions to protect these fish.”
The Winter Ecology study is a collaboration between Pacific Salmon Foundation, the Juanes Lab at the University of Victoria and BC Conservation Foundation. The study is part of the broader Bottlenecks to Survival project, funded by the BC Salmon Restoration and Innovation Fund.
