American author Mark Kurlansky attached the subtitle "The biography of the fish that changed the world" to his 1997 book Cod. Following the Vikings of Northern Europe, the Basque people of Western Europe went to sea with salted cod. Salted cod was an essential food source for long-distance voyages. The Basque expanded their cod fishing grounds to the Americas, discovering the New World before Columbus.
Cod opened up the Age of Exploration for humanity, but in return, it faced extinction. Not only did their population decline, but their size also shrank. In the past, cod the size of a four-year-old child was common, but now they are small enough to fit in a hand. It is not simply that larger cod were frequently caught. Scientists have uncovered that human overfishing altered the body size of cod. To survive in the hands of humans, cod evolved to reduce their size.
◇Reduced to one-fifth of its weight in a quarter of a century
Professor Thorsten Reusch and Dr. Kwi Young Han of the GEOMAR Helmholtz Centre for Ocean Research Kiel reported on the 25th in the international journal Science Advances that "due to excessive fishing, the cod population in the Baltic Sea not only decreased but also its body length was halved."
In the past, cod and herring were the backbone of Baltic Sea fisheries. However, due to overfishing and a drastic decline in their population, cod fishing has been banned in the Baltic Sea since 2019. The same goes for North America. Canada had already prohibited cod fishing in the Grand Banks off the coast of Newfoundland in 1992. The U.S. implemented a harsh catch limit system in the Georges Bank area of Massachusetts starting in 1994. All of these were once the locations where the most cod were caught globally.
The research team analyzed the otoliths of 152 cod captured in the Bornholm Basin of the Baltic Sea from 1996 to 2019. Otoliths are hard calcareous stones located in the inner ear, responsible for a fish's sense of balance. They grow like tree rings, adding one layer each year. This allows us to understand what changes have occurred in cod during that time.
If humans had stopped overfishing, wouldn't cod have rebounded? Unfortunately, even if their population recovers, they will not be the cod they once were. The average length of adult cod during the study period decreased from 40 cm to 20 cm. The average weight also fell drastically to 272 grams in 2019, only one-fifth of the 1356 grams in 1996.
Dr. Kwi Young Han noted, "Excessive fishing has altered the genome of Eastern Baltic cod." Analysis showed that genetic mutations which contribute to larger body size have decreased over time. Fast-growing cod have nearly disappeared, while smaller cod that grow slowly but can still reproduce have increased in number.
This is a result of smaller cod being naturally selected to survive outside the nets prepared by humans. Professor Reusch stated, "When the largest individuals in a population are continuously removed, smaller and faster-maturing fish gain an evolutionary advantage," and added, "Evolution driven by human activity is scientifically intriguing, but it is ecologically a very concerning situation."
◇Overfishing coupled with warming has created a crisis
The research results explain why, despite the comprehensive fishing ban implemented after the collapse of the Baltic cod stock in 2019, cod of the former size have not returned. Even if humans do not catch them, the large cod have already become genetically extinct.
Professor Stefano Mariani of Liverpool John Moores University evaluated the findings, stating that "this is a landmark result showing that human activity can accelerate evolution," and emphasized, "Observing genetic changes in fish populations is just as important as tracking population numbers." However, he pointed out that the reduction in cod size cannot be explained solely by genetics, as environmental factors likely played a significant role.
In fact, Dr. Andrew Pershing of the Gulf of Maine Research Institute reported in the international journal Science in 2015 that despite the subsequent bans on fishing in the 1990s in significant cod fishing areas, the numbers had hardly increased. He found that the rapid increase in water temperature in the Gulf of Maine due to global warming significantly reduced cod survival rates. It was a case of being hit by added misfortunes following overfishing.
From 1984 to 2004, the water temperature in the Gulf of Maine rose by 0.03 degrees Celsius annually, three times the global average increase in sea temperatures. However, from 2004 to 2013, the rise in temperature skyrocketed to seven times the global average. The increase in temperature eliminated small fish, which are the food for juvenile cod. Cod fry began to seek cold water earlier than before and ventured into the open sea, instantly making them targets for predators. In such a situation, their numbers cannot possibly increase.
Due to focusing solely on catch limits without considering the effects of rising temperatures, the researchers noted that this has led to a counterproductive outcome, legally eliminating the remaining few cod instead of protecting them. Humans have inflicted a double burden on cod through prolonged overfishing combined with global warming.
◇Urban squirrels and rats have changed their faces
If cod in the sea have reduced their size because of humans, squirrels and foxes have altered their faces to live in cities. A research team led by Dr. Anderson Feijó of the American Museum of Natural History announced on the 26th in the international journal Integrative and Comparative Biology that "the appearance of rodents residing in Chicago has changed in accordance with urbanization over the past 125 years."
The research team measured the skull shapes of Eastern chipmunks (scientific name Tamias striatus) and Eastern meadow voles (Microtus pennsylvanicus) collected from the Chicago area from 1898 to 2023. As Chicago became urbanized, the skulls of the squirrels grew larger, but their teeth became shorter. The researchers explained this as a change in feeding habits.
As urbanization progressed, squirrels were able to easily obtain calories from food discarded by humans. Consequently, their body size increased. Meanwhile, strong teeth that helped extract calories from nuts and seeds in nature became less necessary.
The meadow voles did not show significant changes in skull size. This indicates that while avoiding humans, the food sources did not increase for them even as urbanization occurred. However, the auditory bullae in the ears of meadow voles became smaller in urban areas. The team explained this was an evolutionary response to reduce urban noise.
◇The elongated snout of foxes is a thing of the past in cities
The red foxes in the UK have also transformed in the city. The trademark elongated snout is now hard to find in urban areas. A research team led by Professor Kevin Parsons at the University of Glasgow noted in 2020 in the Philosophical Transactions of the Royal Society B that "urban red foxes have smaller skull shapes overall compared to their wild counterparts to adapt to metropolitan living."
Professor Parsons' research team examined 274 fox skulls stored at the National Museum of Scotland, London Natural History Museum, and other locations. Urban foxes have short and flat snouts. The researchers speculated that a shorter snout would be beneficial when sticking their heads into garbage bins to sniff. They explained that the long snouts of rural foxes allow for quick closure to catch fleeing prey in the wild.
In the sea and on land, animals are each finding ways to survive. However, as ecosystems have already changed, even if they survive, they will not have their original forms. When will large, true cod be caught at sea again, and when will squirrels with sharp front teeth and foxes with long snouts be able to thrive on land?
References
Science Advances(2025), DOI: https://doi.org/10.1126/sciadv.adr9889
Integrative and Comparative Biology(2025), DOI: https://doi.org/10.1093/icb/icaf081
Proceedings of The Royal Society B(2020), DOI: https://doi.org/10.1098/rspb.2020.0763
Science(2015). DOI: https://doi.org/10.1126/science.aac9819