Cod used to be giants. With their impressive size — over a metre in length and weighing up to 40 kilograms — and abundance, they, alongside herring, were the backbone of the Baltic fishery. Today, a fully grown cod would fit neatly on a dinner plate. That is, if fishing them were still permitted: due to the collapse of the stock, a ban on targeted cod fishing has been in place since 2019.
The shrinking of the cod population, in terms of both number and in size, is the result of human influence. In their new study, scientists from the GEOMAR Helmholtz Centre for Ocean Research Kiel have demonstrated for the first time that decades of intense fishing, combined with environmental change, have profoundly affected the genetic make-up of a fully marine species. Their findings are published today in the journal Science Advances.
"Selective overexploitation has altered the genome of Eastern Baltic cod," explains Dr Kwi Young Han, first author of the study and a biologist who completed her PhD in the Marine Evolutionary Ecology group at GEOMAR about this topic. "We see this in the significant decline in average size, which we could link to reduced growth rates. For the first time in a fully marine species, we have provided evidence of evolutionary changes in the genomes of a fish population subjected to intense exploitation, which has pushed the population to the brink of collapse."
Specifically, the researchers identified genetic variants associated with body growth that showed signs of directional selection — that is to say, they became systematically more or less frequent over time. These regions overlap strikingly with genes known to play a role in growth and reproduction. The study also found that a known chromosomal inversion, a structural change in the genome commonly relevant to environmental adaptation, followed a directional selection pattern. This confirms that the "shrinking" of cod has a genetic basis and that human activities have left a measurable mark on their DNA.
Strong directional selection for slow growth due to fishing pressure
To reach these conclusions, the researchers used an unusual archive: the tiny ear stones (otoliths) of 152 cod, caught in the Bornholm Basin between 1996 and 2019. Much like tree rings, otoliths record annual growth, making them valuable biological timekeepers. These samples are part of GEOMAR's Baltic Sea Integrative Long-Term Data Series, which has been collecting annual data since 1996. This dataset enabled the scientists to conduct a genetic time-travel exercise stretching back to the period before the collapse of the Eastern Baltic cod population.
Using a combination of chemical otolith analysis and high-resolution DNA sequencing, the researchers investigated how cod growth and genetic composition have changed over 25 years under fishing pressure.
Their central finding was that the genomes of fast- and slow-growing individuals differ systematically, and that the fast growers have nearly disappeared. Cod that grow slowly but reach reproductive maturity at a smaller size have had a survival advantage under high fishing pressure.
"When the largest individuals are consistently removed from the population over many years, smaller, faster-maturing fish gain an evolutionary advantage," explains Prof. Dr Thorsten Reusch, Head of the Marine Ecology Research Division at GEOMAR and Dr Han's PhD supervisor. "What we are observing is evolution in action, driven by human activity. This is scientifically fascinating, but ecologically deeply concerning."
Smaller and less diverse populations recover more slowly
The evolutionary consequences are severe. Genetic variants associated with faster growth and later maturation may already be lost, and the surviving cod now reach maturity at smaller sizes and produce fewer offspring. It also means a loss of adaptive potential with implications for the population under future environmental changes.
"Evolutionary change unfolds over many generations," says Reusch. "Recovery takes far longer than decline, and it may not even be possible. This is evident in our 2025 length data from the current ALKOR cruise: despite the fishing ban, there's no sign of a rebound in body size."
The study underscores a clear message: management and protection measures must consider generational timescales. "Our results demonstrate the profound impact of human activities on wild populations, even at the level of their DNA," says Dr Han. "They also highlight that sustainable fisheries are not only an economic issue, but also a matter of conserving biodiversity, including genetic resources."
About: Eastern Baltic Cod
The Eastern Baltic cod population is native to the central Baltic Sea and belongs to the species Atlantic cod (Gadus morhua). It diverged 7,000 to 8,000 years ago when the Baltic Sea formed due to post-glacial sea level changes. Today, it differs biologically and genetically from other Atlantic cod populations, such as Western Baltic and North Sea cod. It is adapted to the Baltic's unique conditions: low salinity, high carbon dioxide, widespread oxygen depletion, and extreme seasonal temperature fluctuations.
Since the mid-1990s, the stock's condition has deteriorated significantly. Spawning stock biomass (fish over 35 cm) has declined sharply, and two major spawning areas have been lost due to deteriorating environmental conditions. The Bornholm Basin is now the last remaining spawning ground. In recent years, the size at maturity and condition of the fish have dropped to record lows, with L50 values (the length at which 50% of the population is mature) falling below 20 centimetres. The collapse of the stock prompted a complete fishing ban in 2019, which remains in place — yet recovery has so far failed to materialise.
