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Harvesting Immature Chinook is Shrinking Salmon and Reducing Future Returns, New Research Finds by Staff Raincoast Conservation Foundation, June 11, 2026

Study shows that millennia-old practice of fishing in rivers could restore salmon size

A new study by researchers at the Wild Fish Conservancy and Raincoast Conservation Foundation has found that marine salmon fisheries along the Pacific coast typically harvest Chinook too early in their life cycle, before they reach their full size as adults. The implications from this are far reaching for fisheries, future Chinook runs, and the ecosystem.

The study, published this week in the journal Scientific Reports, examined the effects of "mixed- maturation fisheries," where both immature and mature Chinook salmon are harvested together in coastal marine waters. This modern fishing practice may be contributing to widespread declines in the age and size of Chinook salmon across the northeast Pacific.

"These fisheries are generally catching salmon before they have the chance to fully grow," said co-author Misty MacDuffee of Raincoast Conservation Foundation. "Over time, this can lead to younger, smaller Chinook salmon with ripple effects for fisheries, wildlife and future salmon runs."

Using an innovative model of Chinook salmon that rear in coastal waters, the study found that immature fish can make up to 60% of the Chinook catch in ocean fisheries. In contrast, fisheries that occur in or near spawning rivers -- known as terminal fisheries -- allow more large, older Chinook to reach their spawning grounds.

The study also found that terminal fisheries can produce a greater harvest weight from landing fewer fish. By targeting only mature adults and allowing immature Chinook to continue growing in the ocean, these fisheries yield larger salmon that not only contribute more weight to the catch, but typically command higher prices per-pound.

"Large Chinook salmon also play an outsized role in both reproduction and the ecosystem," said lead author Dr. Nick Gayeski. "Older females produce more eggs and larger eggs, and larger Chinook are better able to use high quality spawning habitats not accessible to smaller fish."

The research provides solutions amid growing concern over declining Chinook abundance and shrinking body size across the Pacific coast, trends that have implications for Indigenous food systems, commercial and recreational fisheries, ecosystem resilience, and recovery of Southern Resident killer whales.

Mixed-maturation fisheries compromise productivity and resilience of Chinook salmon

Abstract

Most ocean-type Chinook salmon (Oncorhynchus tshawytscha) in the northeast Pacific Ocean are harvested in mixed-stock marine fisheries. Here, multiple populations with varying abundance and productivities are encountered. In addition to being mixed-stock, many of these fisheries encounter both mature and immature Chinook salmon. Hence, these fisheries are better described as mixed-stock and "mixed-maturation" fisheries.

Harvest of immature fish can skew the age composition of Chinook salmon populations towards younger, and hence smaller, individuals. This shift reduces the proportion of older and larger Chinook salmon that contribute disproportionately to the productivity of their populations. We developed a deterministic individual-based demographic-genetic model of fall Chinook salmon to evaluate the effects of fisheries that harvest mature and immature Chinook salmon. We then compared these effects on age, size and weight to terminal fisheries that harvest only mature fish, evaluating the benefits to both landed catch and Chinook salmon rebuilding.

In our modeled scenarios of mixed-maturation fisheries, we found that immature Chinook salmon can comprise up to 59% of the total numbers of fish caught, and 47% of the total weight of the catch. If instead, these Chinook salmon were not harvested until they reached terminal fisheries as mature adults, they would contribute greater biomass to landed catches. Unharvested and terminally harvested populations retain a more archetypal age and sex structure than contemporary fall Chinook salmon subject to marine mixed-maturation fisheries.

Lastly, when harvested at rates that achieve maximum sustainable yield, terminal fisheries allow a greater proportion of larger, older Chinook salmon reach their spawning grounds, thereby increasing the productivity of their populations. The benefits of terminal fisheries would accrue to river-based fishers and communities, and other near terminal locations (such as inlets or estuaries). They would also support rebuilding of depleted Chinook salmon runs.

Continue reading on Nature Scientific Reports