Summer Chum Salmon spawn primarily in the lower 500 miles of the Yukon River whereas fall Chum spawn from the Tanana River confluence to the Yukon’s headwaters in Canada. Both are extremely important for subsistence and are managed under the Pacific Salmon Treaty between the U.S. and Canada.
They’re the same species but have separated themselves in time and space and we can tell them apart based on their genetics and life history strategies. Fall Chum tend to spawn in fall or early winter in upwelling areas (springs) that are cold in summer, warm in winter, really well-defined, and are pretty few and far between. Summer Chum are more widely distributed, mostly in the lower to middle Yukon. They stray between rivers and their gene flow is a lot more active. So genetically, it’s really easy to tell a summer chum from a fall chum.
To meet treaty obligations with Canada and ensure enough fish from the different populations reach their spawning grounds, managers need to know the contribution of different stocks caught in the harvest. To this end, fin clips from from Chum Salmon caught in the Pilot Station test fishery are immediately flown into Anchorage. We extract the DNA to determine the genetic profile of each fish and can then compare it to a genetic baseline we developed for Yukon River Chums with the Department of Fisheries and Ocean Canada. With this information and the sonar data we can estimate the relative proportion of specific stocks in the harvest mixture — for example, what percentage are headed for the upper Koyukuk in Alaska verses the Porcupine or White Rivers in Canada. We can turn this information back to state and federal managers in 24–48 hours.
Listen to this episode of Fish of the Week to hear more about Yukon River chum salmon.
