See All Key Ideas<\/p>\n
<\/p>\n
The eastern Baltic cod has shrunk dramatically in size in recent decades due to rapid evolution \u2014 changes at the genetic level \u2014 caused by decades of intensive fishing, a new study says. It\u2019s one of the first studies to show that a marine species has evolved in response to fishing pressure.<\/p>\n
Sampled Eastern Baltic cod, which are a distinct subpopulation of Atlantic cod (Gadus morhua), lost nearly half of their length from 1996 until 2019, according to the study, which was published in the journal Science Advances on June 25. The data collected for the study also showed that the fish lost nearly four-fifths of their weight over the same period.<\/p>\n
\u201cIf you just see the data, it\u2019s crazy,\u201d the study\u2019s first author, Kwi Young Han, told Mongabay. Han is a postdoctoral researcher in marine evolutionary ecology at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany; the study emerged from her Ph.D. work.<\/p>\n
Han\u2019s Ph.D. supervisor and co-author on this paper, Thorsten Reusch, said the findings had disturbing implications.<\/p>\n
\u201cWhen the largest individuals are consistently removed from the population over many years, smaller, faster-maturing fish gain an evolutionary advantage,\u201d Reusch said in a press release<\/a>. \u201cWhat we are observing is evolution in action, driven by human activity. This is scientifically fascinating, but ecologically deeply concerning.\u201d<\/p>\n While scientists have been aware that the length of eastern Baltic cod has decreased in recent decades, this study quantifies that change. The more novel aspect of the study, however, is in connecting the size data with genetic analysis.<\/p>\n Loosely related work showing some evidence of fisheries-induced evolution has been done by other researchers, including a 2023 study<\/a> on Atlantic cod, a 2022 study<\/a> on Atlantic salmon (Salmo salar), and a 2019 study<\/a> on lab-raised silversides.<\/p>\n David Conover, professor emeritus in biology at the University of Oregon in the U.S. and a co-author of the 2019 study, told Mongabay that the new GEOMAR-led study, which he wasn\u2019t involved with, presents \u201cprobably the strongest\u201d evidence so far that \u201cfishing can cause evolutionary changes in wild populations.\u201d Conover, the former head of the U.S. National Science Foundation\u2019s Division of Ocean Sciences, called it a \u201cvery impressive paper.\u201d<\/p>\n One reason it\u2019s convincing, Conover said, is that the eastern Baltic cod serves as a good study subject for fisheries-induced evolution because it doesn\u2019t mix much with other populations, such as Western Baltic cod, so any genetic changes are unlikely to be due to migration.<\/p>\n GEOMAR researchers collected samples of eastern Baltic cod every year between 1996 and 2019, and analyzed their otoliths \u2014 small stones in individuals\u2019 ears \u2014 to determine age. For other fish species, such analysis can be done visually, much like counting tree rings; but for eastern Baltic cod, reliable age measurement required a new chemical technique developed by Karin H\u00fcssy, a biologist at the Technical University of Denmark and co-author of the study.<\/p>\n Getting precise ages for the sampled individuals allowed the researchers to compare sizes of same-aged cod across the 24-year study period. Their findings revealed that during this time, the median length of mature eastern Baltic cod fell from 52.1 to 32.1 centimeters (20.5 to 12.6 inches), and the theoretical length of the longest possible cod dropped from 115 to 53.9 cm (45.3 to 21.2 in). The median weight fell from 1,356\u202fto 272\u202fgrams (48 to 10 ounces), according to Han.<\/p>\n Yet changes in characteristics like length and weight don\u2019t necessarily mean that a species has evolved. In theory, that could happen due to environmental factors like changes in oxygen levels or temperatures; in such cases, fishes\u2019 hardwiring isn\u2019t changing, but their bodies are acclimating to their environment, in what\u2019s known as \u201cphenotypic plasticity.\u201d<\/p>\n To find out if there was a change to the underlying DNA, Han and her team thus turned to genetic analysis. For Atlantic cod, growth is \u201cpolygenic,\u201d meaning it\u2019s a trait produced by a number of parts of the genome interacting with one another. Over the study period, gene patterns consistent with smaller fish became more common in the sampled fish, indicating natural selection at work.<\/p>\n Though fishing pressure itself was not a variable in the study, the authors believe it caused the evolutionary pressure. The trawling companies that targeted eastern Baltic cod were only permitted to catch individuals 35 cm (14 in) or longer due to regulations designed to protect juveniles, which included minimum mesh sizes. Trawling was done heavily for decades but was banned in 2019.<\/p>\n \u201cThe study is a wonderful example of genomic time travel,\u201d Malin Pinsky, an associate professor of biology at the University of California, Santa Cruz, and co-author of the 2023 paper, told Mongabay in an email.<\/p>\n \u201cThe broad message that humans are altering the course of evolution resonates far beyond Baltic cod,\u201d Pinsky, who wasn\u2019t involved in this study, added.<\/p>\n Conover agreed that the study suggests that fisheries-induced evolution could be happening more broadly, saying, \u201cThat\u2019s why this got published in a journal like Science Advances,\u201d which is a leading journal known for publishing high-impact research.<\/p>\n Conover said the shrinking of eastern Baltic cod was \u201calarming,\u201d and that reduced biomass and fish size is something that fisheries managers should seek to avoid. They have the ability to do so using advanced modeling techniques, but without proof that the models are necessary \u2014 that fishing is having an evolutionary impact \u2014 managers will be reluctant to adopt the models or change their ways, he said.<\/p>\n \u201cThat\u2019s why a paper like this is so important,\u201d Conover said. \u201cIt\u2019s going to up the dialogue and increase conversations in the management community.\u201d<\/p>\n Banner image:<\/strong> An Atlantic cod (Gadus morhua). Image by Dylan.B via iNaturalist<\/a> (CC BY-NC 4.0<\/a>).<\/p>\n Sweeping cuts and deregulation imperil U.S. fisheries, experts warn<\/a><\/p>\n<\/blockquote>\n Citations:<\/strong><\/p>\n Han, K. Y., Brennan, R. S., Monk, C. T., Jentoft, S., Helmerson, C., Dierking, J., \u2026 Reusch, T. B. (2025). Genomic evidence of fisheries-induced evolution in Eastern Baltic cod. Science Advances, 11(26), eadr9889. doi:10.1126\/sciadv.adr9889<\/a><\/p>\n Reid, B. N., Star, B., & Pinsky, M. L. (2023). Detecting parallel polygenic adaptation to novel evolutionary pressure in wild populations: A case study in Atlantic cod (Gadus morhua). Philosophical Transactions of the Royal Society B: Biological Sciences, 378(1881). doi:10.1098\/rstb.2022.0190<\/a><\/p>\n Czorlich, Y., Aykanat, T., Erkinaro, J., Orell, P., & Primmer, C. R. (2022). Rapid evolution in salmon life history induced by direct and indirect effects of fishing. Science, 376(6591), 420-423. doi:10.1126\/science.abg5980<\/a><\/p>\n Therkildsen, N. O., Wilder, A. P., Conover, D. O., Munch, S. B., Baumann, H., & Palumbi, S. R. (2019). Contrasting genomic shifts underlie parallel phenotypic evolution in response to fishing. Science, 365(6452), 487-490. doi:10.1126\/science.aaw7271<\/a><\/p>\n![\"Map](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/07\/4-2-scaled.jpg\")
<\/a>(Left) A map of the Baltic Sea showing Bornholm Basin, the main spawning site for eastern Baltic cod. The fish historically also spawned in the Gotland Basin and Gdansk Deep, but no longer do at a large scale. Eastern Baltic cod are a distinct subpopulation of Atlantic cod (Gadus morhua). (Right) A graph showing the distribution of lengths of sampled eastern Baltic cod collected by GEOMAR researchers during five years from 1996 until 2009. The dashed lines show the median length for a given year. The median length fell from 52.1 centimeters (20.5 inches) in 1996 to 32.1 cm (12.6 in) in 2019. Image by Han et al.<\/a>, 2025 (CC BY 4.0<\/a>).<\/p>\n![\"(Left)](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/07\/Eero-Aro-and-Kwi-Young-Han-.jpg\")
<\/a>(Left) Eero Aro, late Finnish fisheries biologist, sits with eastern Baltic cod in a photograph from March 1987, when the fish were much larger than they are today. (Right) GEOMAR researcher Kwi Young Han holds a full-grown eastern Baltic cod in May 2025, following decades of shrinking due to intensive fishing. Images courtesy of (left) Jesper Bay\/Danish Institute for Fisheries and Marine Research and (right) Thorsten Reusch\/GEOMAR.
\n![\"Researchers](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/07\/otoliths.jpg\")
<\/a>Researchers studied tiny ear stones called otoliths to determine the ages of eastern Baltic cod that they\u2019d collected. Otoliths contain annual markings similar to tree rings. Image courtesy of Sarah Uphoff\/GEOMAR.<\/p>\n\n