A salmon struggles against a dam, symbolizing the loss of genetic diversity.

Vanishing Spring: How Human Interference Threatens Wild Salmon

Lena Kashyap in Climate & Environment August 2025 • 5 min read.

"A new study reveals the rapid loss of adaptive variation in wild salmon populations due to habitat alteration, raising concerns about long-term restoration potential."

Phenotypic variation, the range of observable differences within a species, is crucial for the long-term survival and adaptability of populations. It allows species to withstand environmental changes and enhances their capacity to evolve. However, human activities are significantly diminishing this variation across diverse species, creating an urgent need to understand and address the underlying causes and long-term effects.

Chinook salmon, prized for their cultural and ecological value, exhibit remarkable diversity in their migration patterns. Some migrate as immature adults in the spring (spring-run), while others migrate in the fall as mature adults (fall-run). This diversity allows them to utilize different habitats and cope with varying environmental conditions. However, anthropogenic activities, such as dam construction, are disrupting these migration patterns, leading to dramatic shifts in their populations.

A groundbreaking study published in PNAS (Proceedings of the National Academy of Sciences) investigates the rapid phenotypic changes in wild Chinook salmon caused by dam construction and other human activities. The study reveals a strong genetic link to migration patterns and highlights the potential for irreversible loss of adaptive variation. These findings underscore the importance of conserving and restoring critical adaptive variation to ensure the long-term survival of wild salmon populations.

The Genetic Basis of Salmon Migration: A Single Locus with a Big Impact

A salmon struggles against a dam, symbolizing the loss of genetic diversity.

The study's most striking discovery is the robust association between migration phenotype (spring-run or fall-run) and a single genetic locus, GREB1L. This locus plays a critical role in determining when salmon migrate, with specific alleles (gene variants) linked to either spring or fall migration. The researchers found that the rapid phenotypic shift observed after dam construction was driven by dramatic changes in allele frequency at this locus.

Before dam construction, the spring-run allele was prevalent, indicating a population dominated by spring-migrating salmon. However, after the construction of dams like Lost Creek Dam (LCD) on Oregon’s Rogue River, the fall-run allele became increasingly dominant. This shift occurred because the altered river conditions favored fall-run salmon, leading to a selective pressure against the spring-run phenotype.

Habitat Alteration: Dams and other human activities have altered river flow, temperature, and access to spawning grounds, creating conditions less favorable for spring-run salmon.
Selective Pressure: Fall-run salmon are now better suited to the altered environment, giving them a survival advantage and leading to increased reproductive success.
Genetic Shift: The increased reproductive success of fall-run salmon has led to a higher frequency of the fall-run allele in the population, diminishing the spring-run allele.

To understand the long-term consequences of this genetic shift, the researchers modeled the potential future of the spring-run allele under continued selection pressure. The modeling demonstrated that continued selection against the spring-run phenotype could rapidly lead to the complete loss of the spring-run allele. This loss would severely limit the population's ability to adapt to future environmental changes and reduce its overall resilience.

Why Preserving Adaptive Variation is Key

The study's findings highlight the importance of conserving adaptive variation to ensure species' long-term survival. The loss of the spring-run phenotype in salmon would not only diminish their ecological role but also reduce their ability to adapt to future challenges, such as climate change. This research emphasizes the urgent need for conservation efforts that protect and restore critical adaptive variation before the potential for recovery is lost. It’s a clear call to action: protecting biodiversity isn’t just about numbers, it’s about safeguarding the unique genetic tools that species need to survive.

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This article is based on research published under:

DOI-LINK: 10.1073/pnas.1811559115, Alternate LINK

Title: Anthropogenic Habitat Alteration Leads To Rapid Loss Of Adaptive Variation And Restoration Potential In Wild Salmon Populations

Subject: Multidisciplinary

Journal: Proceedings of the National Academy of Sciences

Publisher: Proceedings of the National Academy of Sciences

Authors: Tasha Q. Thompson, M. Renee Bellinger, Sean M. O’Rourke, Daniel J. Prince, Alexander E. Stevenson, Antonia T. Rodrigues, Matthew R. Sloat, Camilla F. Speller, Dongya Y. Yang, Virginia L. Butler, Michael A. Banks, Michael R. Miller

Published: 2018-12-04

Everything You Need To Know

What is phenotypic variation, and why is it important for wild salmon?

Phenotypic variation refers to the observable differences within a species, such as the different migration patterns of Chinook salmon. For wild salmon, this is crucial for their long-term survival and adaptability. It allows them to cope with environmental changes and enhances their capacity to evolve. In the context of Chinook salmon, spring-run and fall-run migration patterns, which are phenotypic variations, allow the species to utilize different habitats and withstand varying environmental conditions. Diminishing this variation, as observed due to habitat alteration, threatens their resilience and adaptability.

How do dams specifically impact the genetic diversity of Chinook salmon migration patterns?

Dams, such as the Lost Creek Dam (LCD), significantly impact the genetic diversity of Chinook salmon migration patterns by altering river conditions. These alterations, including changes in flow, temperature, and access to spawning grounds, create conditions less favorable for spring-run salmon. This leads to a selective pressure favoring fall-run salmon. The increased success of fall-run salmon then leads to a higher frequency of the fall-run allele at the GREB1L locus, effectively diminishing the prevalence of the spring-run allele. This shift reduces the overall genetic diversity within the salmon population, limiting its ability to adapt to future environmental changes.

What is the role of the GREB1L locus in salmon migration, and how does it relate to the observed genetic shifts?

The GREB1L locus plays a critical role in determining when salmon migrate. The study revealed a strong association between migration phenotype (spring-run or fall-run) and this single genetic locus. Specific alleles (gene variants) at this locus are linked to either spring or fall migration. The observed genetic shifts, specifically the decline in spring-run alleles, were directly linked to changes in the frequency of alleles at the GREB1L locus. After dam construction, altered river conditions favored fall-run salmon. This resulted in increased reproductive success, leading to a higher frequency of the fall-run allele and, consequently, a rapid phenotypic shift.

What are the potential long-term consequences of the loss of the spring-run allele in Chinook salmon, as predicted by the study's modeling?

The study's modeling demonstrated that continued selection against the spring-run phenotype could rapidly lead to the complete loss of the spring-run allele. This loss would severely limit the population's ability to adapt to future environmental changes. Without the spring-run allele, the Chinook salmon population would lose a significant part of its ability to cope with changes, such as climate change or further habitat alteration. The resilience of the population would decrease. This loss emphasizes the urgent need for conservation efforts to protect and restore the critical adaptive variation.

Beyond dams, what other human activities contribute to the loss of adaptive variation in wild salmon, and why is conserving this variation so crucial for their survival?

Besides dam construction, other human activities that contribute to the loss of adaptive variation in wild salmon include habitat destruction and general habitat alteration. These activities alter river flow, temperature, and access to spawning grounds, thereby creating less favorable conditions for certain phenotypes, like spring-run salmon. Conserving adaptive variation is crucial because it ensures the species' long-term survival and adaptability. The loss of variation, specifically the spring-run phenotype, diminishes their ecological role and reduces their ability to adapt to future challenges. Protecting and restoring adaptive variation ensures the unique genetic tools that species need to survive, especially in the face of environmental changes such as climate change.