Once towering giants of the Baltic Sea, cod have historically dominated the marine ecosystem with their impressive size and abundance. At their peak, these fish reached lengths surpassing one meter and weighed up to an astonishing 40 kilograms, forming a cornerstone of the Baltic fisheries alongside the once plentiful herring. However, in contemporary times, the Eastern Baltic cod has undergone a dramatic transformation; now, a fully grown specimen can be cupped within two hands, a tangible symbol of the species’ severe decline and “shrinking.” This stark reduction in both population size and individual growth has culminated in a fishing ban imposed since 2019, reflecting a dire need to protect the beleaguered stock.
The profound diminishment of cod is not a mere consequence of natural fluctuations but a direct outcome of pervasive human pressures entwined with environmental shifts. A groundbreaking study led by researchers at the GEOMAR Helmholtz Centre for Ocean Research Kiel has, for the first time, traced the evolutionary genomics behind this phenotypic change. Their findings, published in the journal Science Advances, reveal how decades of intense, selective fishing have left a distinct and measurable imprint on the genetic architecture of this fully marine fish population. These insights place the Eastern Baltic cod not just at an ecological crossroads but at the forefront of understanding anthropogenically induced evolution in marine species.
Dr Kwi Young Han, a marine evolutionary ecologist and lead author of the study, highlights the gravitas of these findings. "Selective overexploitation has instigated directional changes in the genome of Eastern Baltic cod," she notes, drawing a direct line from fishing practices to evolutionary outcomes. The study illustrates a significant decline in the average size of cod, linked explicitly to reduced growth rates. This genomic evidence effectively positions cod as a paradigm for how human activity can accelerate evolutionary modifications with stark ecological and economic ramifications.
The researchers focused on identifying specific genetic variants associated with key life history traits, particularly those governing growth and reproductive timing. Their analysis detected directional selection operating on genomic regions rich in genes known to influence somatic growth and fecundity. Additionally, the team observed selection acting on a chromosomal inversion—a structural genomic rearrangement often implicated in environmental adaptation. The convergence of these genomic signatures substantiates the hypothesis that fishing pressure has not only culled large individuals but also sculpted the genetic landscape of the population, favoring alleles conducive to slower growth.
Central to this remarkable study was the utilization of an innovative biological archive: the otoliths, or ear stones, of cod collected over a 25-year period from 1996 to 2019 in the Bornholm Basin, the last remaining spawning ground in the Baltic Sea. Otoliths function similarly to tree rings, retaining records of annual growth increments. By extracting DNA from these tiny, mineralized structures, the team effectively engaged in genetic "time travel," comparing historical genetic makeup to contemporary samples. This novel approach allowed for an unprecedented longitudinal analysis of both phenotypic and genotypic shifts within the wild fish population under sustained fishing pressure.
The combined application of chemical otolith microchemistry and high-resolution DNA sequencing unveiled a striking association between growth rates and genome architecture. Fast-growing cod, which historically contributed significantly to the gene pool, have all but vanished under harvesting regimes that preferentially remove the largest fish. The survival advantage now tilts towards cod with slower growth trajectories who mature at smaller sizes, evading fishing nets but potentially compromising their reproductive output and ecological function.
Prof. Dr Thorsten Reusch, head of the Marine Ecology Research Division at GEOMAR and Dr Han’s doctoral supervisor, underscores the evolutionary dynamics at play: “When fishing consistently removes the largest individuals, evolutionary forces shape the population towards smaller, faster-maturing fish.” This phenomenon, often termed fisheries-induced evolution, illustrates how anthropogenic selection pressures can drive rapid genetic change in marine populations, revealing evolution as a force directly modulated by human exploitation. While scientifically captivating, these changes raise significant concerns regarding the resilience and recovery potential of the stock.
From an ecological perspective, the consequences of these evolutionary changes are ominous. The loss of genetic variants associated with faster growth and delayed maturation reduces the adaptive capacity of the cod population, making it more vulnerable to future environmental fluctuations such as rising temperatures, hypoxia, and salinity changes endemic to the Baltic Sea. Smaller, genetically less diverse populations are known to recover more slowly from perturbations and are more susceptible to collapse, suggesting that the cod’s ability to rebound, even post-fishing ban, remains uncertain and likely protracted.
Supporting this foreboding outlook, recent length assessments from the 2025 research cruise aboard the vessel ALKOR indicate no meaningful recovery in cod body size despite the fishing moratorium. This stagnation suggests that evolutionary change wrought by human exploitation endures beyond immediate regulatory interventions, highlighting the critical timescales required for genetic resilience and population restoration. The study’s findings therefore carry an imperative for fisheries management to incorporate evolutionary principles and adopt conservation strategies sensitive to generational and genomic timescales.
The Eastern Baltic cod represents a unique evolutionary lineage within the Atlantic cod species complex, having diverged approximately 7,000 to 8,000 years ago following post-glacial shifts that formed the Baltic Sea. This population exhibits distinctive biological and genetic adaptations to the Baltic’s brackish, low-salinity waters, which experience pronounced seasonal and chemical extremes. Unlike other Atlantic cod populations inhabiting the North Sea or the Western Baltic, the Eastern Baltic cod’s genomic identity reflects a history of adaptation to these harsh and fluctuating environments.
Since the mid-1990s, however, its spawning stock biomass—particularly for fish exceeding 35 centimeters in length—has plummeted precipitously. Drastic environmental degradation, including widespread oxygen depletion and increased carbon dioxide concentrations, further compounds the plight of this stock. Two major spawning areas have disappeared, leaving the Bornholm Basin as the solitary reproductive refuge. Accompanying this spatial contraction, life history traits such as size at maturity have shrunk to the lowest recorded levels, with L50 values dropping below 20 centimeters. The confluence of these factors precipitated the closure of targeted cod fishing in 2019, yet this moratorium has yet to foster a significant population resurgence.
The implications of this work extend beyond the specific case of Baltic cod to a broader understanding of how fisheries can impose a powerful, directional form of artificial selection on wild fish populations. It challenges the traditional views of fisheries management that often overlook evolutionary responses, underscoring the need for integrated approaches that maintain genetic diversity and evolutionary potential. Sustainable fisheries, therefore, represent not only an economic and ecological imperative but are vital for conserving biodiversity at the molecular level.
As Dr Han emphasizes, “Our study demonstrates that human influence reaches deep into the genetic fabric of wild populations, making it essential to consider these impacts in future management policies.” Her words resonate as a clarion call for the scientific community, policymakers, and the fishing industry alike to recognize that preserving genetic resources is integral to the long-term viability of marine ecosystems and food security.
Subject of Research: Animal tissue samples
Article Title: Genomic Evidence of Fisheries Induced Evolution in Eastern Baltic cod
News Publication Date: 25-Jun-2025
Image Credits: Photo: Thorsten Reusch, GEOMAR
Keywords: Evolutionary genetics, Genetics, Population genetics, Ecology, Evolutionary processes, Artificial selection, Evolutionary developmental biology, Genomic analysis, Genomic regions, Aquatic animals, Fish, Marine fishes, Marine biology, Marine life
