Catching Chinook before they grow up is shrinking the fish, and the future of the run

New modeling from Wild Fish Conservancy and Raincoast Conservation Foundation finds that ocean fisheries routinely harvest immature Chinook salmon, and that fishing closer to the rivers could rebuild bigger fish, healthier runs, and more food for Southern Resident killer whales.

Most Chinook salmon hauled from the northeast Pacific are caught at sea, mixed together with other stocks and, it turns out, mixed together by age. A study published this spring in Scientific Reports makes the case that coastal ocean fisheries are not just “mixed-stock” but “mixed-maturation”: they routinely catch Chinook that are still years away from spawning, alongside the mature adults headed home. The consequence, the authors argue, is a slow shrinking of the fish themselves, and of the runs that follow.

The work, led by aquatic ecologist Dr. Nick Gayeski of Wild Fish Conservancy with co-authors at Raincoast Conservation Foundation, uses a detailed computer model of fall Chinook to compare two ways of fishing: catching salmon out in the ocean, and catching them in or near their spawning rivers. The contrast is stark, and it points toward a practice older than any modern fleet.

What a “mixed-maturation” fishery actually does

Chinook have the most complicated life history of any Pacific salmon. Males mature anywhere from age two to six, females from three to six, so at any moment the ocean holds fish of many ages swimming together. Ocean-type (“fall”) Chinook from British Columbia and the U.S. coast rear for years on the continental shelf before returning to spawn. Marine sport, troll, and net fisheries that intercept them there inevitably scoop up immature fish that have not finished growing.

Harvesting those young fish does something subtler than simply removing them. Over time it tilts the genetic odds in favor of salmon that mature early and small, because the slow-maturing fish are more likely to be caught before they ever reproduce. The model treats this as fisheries-induced selection (a shift in gene frequencies, not a rewrite of the genome), which means it can happen within decades, and, encouragingly, can reverse if harvest pressure eases.

Why big, old salmon matter so much

The losses don’t stop at the dock. Larger, older female Chinook are reproductive powerhouses: they carry more eggs and larger eggs, and they can dig into the coarse, high-quality gravels that smaller fish can’t reach. A run dominated by young spawners leans on the least fecund fish in the population.

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.

Dr. Nick Gayeski, lead author, Wild Fish Conservancy

A diverse spread of ages is also a population’s insurance policy. Ecologists call it the portfolio effect: when a run carries many age classes, a bad year for one cohort is buffered by the others. Strip out the older fish and that buffer thins, leaving the population more exposed to drought, marine heat, and the other shocks of a changing ocean. In the model, ocean harvest measurably lowered Chinook productivity (the population’s intrinsic ability to replace itself) while river-based fishing kept it close to its unfished level.

These fisheries are generally catching salmon before they have the chance to fully grow. Over time, this can lead to younger, smaller Chinook salmon with ripple effects for fisheries, wildlife and future salmon runs.

Misty MacDuffee, Raincoast Conservation Foundation

Those ripples reach beyond the salmon. Big Chinook are the preferred prey of the endangered Southern Resident killer whales, and they anchor Indigenous food systems and commercial and recreational fisheries up and down the coast. Shrinking the average fish quietly shrinks all of it.

The fix is old: fish closer to the rivers

The study’s solution is what researchers call a terminal fishery: harvesting fish in or near their home rivers, where almost everything swimming past is a mature adult on its final journey. Catching salmon in rivers is, of course, a practice thousands of years old, and the model suggests it carries a modern payoff.

Counterintuitively, fishing for fewer, larger fish produced more weight at the dock. In the model, a terminal troll fishery landed about a third more total weight than its ocean counterpart, because each adult had been allowed to keep growing rather than being taken half-grown. Larger Chinook also tend to command higher prices per pound, so the same run can yield a bigger, more valuable catch while leaving a healthier population behind. Those benefits, the authors note, flow to river-based and near-terminal communities rather than to the offshore fleet.

From the boat to the gravel

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Why it counts the catch by weight, not by fish

One of the study’s quieter arguments is about arithmetic. Fisheries are often managed by the number of fish landed, but counting heads treats a half-grown two-year-old as equal to a 33-pound spawner. By measuring its target (maximum sustainable yield) in pounds rather than numbers, the model captures what age-blind counting misses: that hauling in immature fish overstates how much a run can really sustain.

The fine print

The authors are candid about the model’s limits. It follows a single, deliberately robust population under a steady “average” ocean, rather than the messy real world of variable survival and many interacting stocks. It also assumes immature fish are exactly as catchable as adults of the same size; where that isn’t true, the effects would be softer. And by treating all eggs as equal, the model may actually understate the advantage of keeping big old females around. The headline contrast between ocean and river fishing, the authors argue, holds regardless.

Why this lands now

The timing is pointed. Managers and policymakers are wrestling with declining Chinook, the survival of Southern Resident orcas that depend on them, and the looming renegotiation of the Pacific Salmon Treaty that governs harvest across the coast. Maximum-sustainable-yield reference points already sit at the heart of Canada’s Wild Salmon Policy and U.S. management under the Magnuson-Stevens Act. This research offers a concrete lever: shift effort toward terminal fisheries, and the same coast could grow bigger fish, steadier runs, and a better shot at rebuilding the salmon already in trouble.

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The research

This article reports on open-access research published in Scientific Reports: Gayeski N, Swanson D, MacDuffee M, Rosenberger A. (2026). Mixed-maturation fisheries compromise productivity and resilience of Chinook salmon. Scientific Reports 16:17768.

Further reading from the authors’ organizations: the Wild Fish Conservancy and Raincoast Conservation Foundation joint press release and backgrounder on how harvesting immature Chinook can shrink size and reduce future returns.

Photography by Tavish Campbell, Wild Fish Conservancy, Astoria Archives, S. Rae, and Coast to Coast Photography, provided by Wild Fish Conservancy for use with this story. All photographs are credited to their respective authors. SEVENSEAS Media thanks Wild Fish Conservancy and Raincoast Conservation Foundation for sharing.