Baltic herring are not one population but many, and current fishing rules ignore the boundaries

Proceedings of the National Academy of Sciences, 2026

In the Stockholm archipelago, there is a herring that spawns when the wild roses bloom. The locals call it "wild rose herring," and it turns out to be genetically distinct from the spring-spawning herring that populate nearby waters. Its genome carries adaptations for reproducing in the warmer temperatures of mid-July, gene variants that could prove critical for the species' survival as the Baltic Sea warms.

This is one finding among many in a sweeping genetic survey published in the Proceedings of the National Academy of Sciences by researchers from Uppsala University, Stockholm University, and the Swedish University of Agricultural Sciences. The study analyzed more than 4,500 herring collected from 150 spawning locations along Sweden's east coast, producing the most detailed genetic map of Baltic herring populations to date.

More populations than anyone managed for

The basic split between spring-spawning and autumn-spawning herring has been known for some time. These groups are genetically distinct and adapted to different seasonal conditions. But the new study reveals far more structure within the spring-spawning population than was previously recognized.

Spring-spawning herring in the Baltic Sea, the study shows, are subdivided into at least three major clusters: northern, central, and southern. Within these clusters, additional groupings exist. The genetic boundaries between these populations correspond to local differences in water temperature and salinity, two of the Baltic's most variable environmental features.

This matters because current fishery management treats herring along Sweden's east coast as just two large populations: one in the Baltic Proper and one in the Gulf of Bothnia. The genetic reality is considerably more complex. Lovisa Wennerstrom from the Swedish University of Agricultural Sciences put it directly: the current management does not correspond to the genetic groupings the study reveals.

Hybrids that change their schedule

Perhaps the most surprising finding is that spring- and autumn-spawning herring, despite their clear genetic separation, sometimes hybridize. Thanks to the study's unusually large sample size, researchers were able to identify individuals with hybrid genomes within both spawning populations.

Lead author Leif Andersson of Uppsala University offered an interpretation: genetics sets an optimal spawning window, primarily spring or autumn, but water temperature and nutritional status influence exact timing. A herring that ends up in a school spawning at the "wrong" season may adapt its behavior to match. Andersson suggested that hormonal communication within the school may coordinate spawning timing, overriding individual genetic predispositions.

This flexibility has implications for how populations respond to environmental change. If some individuals can shift their spawning behavior, populations may have more adaptive capacity than their genetic boundaries would suggest. But that flexibility has limits, and the genetic structure remains the foundation.

The wild rose herring and future adaptation

The wild rose herring stands out as a case study in local adaptation. Its mid-July spawning time places it in substantially warmer water than typical spring spawners encounter. Genetically, its profile is more extreme than even the broader southern cluster to which it belongs.

Linda Laikre of Stockholm University highlighted the conservation significance: a population adapted to spawning in warmer waters may harbor gene variants critical for future adaptation as the Baltic Sea's temperature rises. If industrial fishing or habitat loss eliminates this population, those genetic resources are lost permanently.

This is not a hypothetical concern. Herring are the most abundant fish in the Baltic Sea, serving as a critical link between plankton production and predatory fish, seabirds, marine mammals, and human fisheries. But industrial fishing for fish meal production operates at scales that can deplete local populations, particularly when management boundaries do not reflect genetic population structure.

What needs to change

The researchers call for substantially more restrictive industrial fishing to protect local populations and the genetic diversity they carry. They also note that their results will serve as a baseline for the Swedish Agency for Marine and Water Management's monitoring program, which aims to track genetic changes in key species over time.

The study cannot determine how much damage current fishing practices have already done to genetic diversity in Baltic herring. That would require comparing present-day genetic profiles to historical samples. But it does establish that the diversity worth protecting is far greater than current management recognizes, and that the mismatch between genetic reality and regulatory boundaries creates ongoing risk.