Rapid evolution in the Baltic Sea: How herring became fish-eaters

The Baltic Sea, one of the world’s youngest bodies of water, has long been known to host Atlantic and Baltic herring – plankton-eating fish critical to northern marine ecosystems. 

Yet new research, published in the journal Nature Communications, has uncovered a surprising adaptation in this environment: genetically distinct, fish-eating Baltic herring (Sardinops sagax caerulea). 

Conducted by scientists at Uppsala University in Sweden, the study challenges traditional assumptions about how Baltic herring evolved and found new ecological niches in brackish waters formed at the end of the last Ice Age.

Herring in the Baltic Sea

Herring serve a crucial function in the Atlantic Ocean and the Baltic Sea, transferring energy from plankton up the food chain to predatory fish, seabirds, marine mammals, and humans. 

Previous studies by the Uppsala group had already established that herring populations adapt genetically to variations in climate, salinity, and spawning seasons. 

However, the newly discovered fish-eating Baltic herring adds an intriguing twist: these larger individuals exhibit distinct biological and genetic characteristics that set them apart from the traditional plankton-eating varieties.

The Baltic herring enigma

Renowned Swedish taxonomist Carl Linnaeus once defined the Baltic herring as a subspecies of the Atlantic herring, specifically adapted to the Baltic’s brackish waters. Over the centuries, scientists have consistently noted how Baltic herring are generally smaller and contain less fat than their Atlantic relatives. 

But when a local fisherman near Uppsala spoke of a large herring “that always spawns just before midsummer and which is as big as the Atlantic herring,” questions arose as to whether a previously unidentified herring population existed within the Baltic.

The study was led by Leif Andersson, a professor at the Department of Medical Biochemistry and Microbiology at Uppsala University.

“When I learned that the locals are aware of a specific population of very large Baltic herring that always spawns in the same area year after year, I decided to sample and explore their genetic constitution,” said Professor Andersson. 

“Now we know that this is a genetically unique population that must have evolved over hundreds, if not thousands, of years in the Baltic Sea.” 

Switching to a fish-based diet

By collecting samples of these large herring, researchers noticed striking differences from the smaller, plankton-eating Baltic herring. 

Physical examinations, growth measurements, and fat-content analyses revealed that the larger herring contain higher levels of fat and show signs of damaged gill rakers – features aligning with a switch from filtering plankton to eating fish such as the common stickleback, which has sharp defensive spines.

According to the scientists, the relatively low dioxin content makes this fish-eating Baltic herring interesting for human consumption. With a faster growth rate and lower pollutant load, the fish-eating herring may offer both ecological and nutritional advantages.

Uncovering two genetic subpopulations

Once it became clear that these larger Baltic herring were genetically distinct, the scientists expanded their focus, comparing samples of big herring from different Baltic locations. Whole genome sequencing confirmed that these fish fall into two separate subpopulations.

“Our genetic analysis demonstrates that there are at least two distinct subpopulations of fish-eating herring in the Baltic Sea; one occurs north of Stockholm, and the other occurs south of Stockholm,” said lead author Jake Goodall, a researcher at Uppsala University. 

By identifying multiple genetic groups, the study reveals even greater diversity within the Baltic herring gene pool than previously thought.

Why fish-eating herring evolved in the Baltic

The Baltic Sea, with salinity levels far lower than the Atlantic’s, has existed for only around 8,000 years. Such an environment is inhospitable to many marine species, and only a select few have successfully established populations there. 

The researchers propose that fish-eating herring have emerged due to a scarcity of competing predators, making it advantageous for some herring to exploit fish as a food source.

“We hypothesize that fish-eating Baltic herring have evolved due to a lack of competition from other predatory fish, for instance, mackerel and tuna, which do not occur where we find fish-eating herring,” said Andersson.

“Thus, these herring take advantage of an underutilised food resource in the Baltic Sea.”

The ability to feed on smaller fish rather than plankton likely opened an ecological niche that larger herring could occupy, capitalizing on prey that other species could not exploit.

Balancing ecosystems and human needs

The emergence of fish-eating Baltic herring underscores the Baltic Sea’s capacity to foster evolutionary innovations. As a keystone species, herring regulate energy flow in their ecosystem. 

The discovery of these large, fish-feeding populations highlights the adaptability of herring to different diets, environmental pressures, and competition. It also suggests that such herring may provide a safer, fattier food option for consumers, given their comparatively lower dioxin levels.

These insights could inform fishery management and conservation strategies, ensuring that as the Baltic continues to evolve, both ecosystem integrity and human consumption interests remain aligned.

Rapid evolutionary change of Baltic herring 

Because the Baltic Sea is geologically recent, this discovery showcases how quickly species can adapt to novel conditions. 

The fish-eating herring’s genetic distinctiveness, alongside significant physical and dietary differences, underlines the dynamic nature of evolution in brackish environments. 

For scientists and policymakers, these findings serve as a reminder that even seemingly uniform species can harbor hidden diversity essential for resilience in changing waters.

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