The herring in the Baltic Sea is divided into several genetically distinct populations that sometimes interbreed
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Scientific Frontline: Extended "At a Glance" Summary: Genetic Mapping of Baltic Sea Herring
The Core Concept: Baltic Sea herring are subdivided into distinct genetic populations adapted to local variations in salinity and temperature. These distinct genetic clusters occasionally interbreed, demonstrating a high degree of previously unrecognized genetic diversity within the species.
Key Distinction/Mechanism: While genetics establishes an optimal spawning window (primarily spring or autumn), environmental factors such as water temperature and nutritional status trigger the actual spawning event, likely mediated by hormonal communication within the school. This behavioral adaptability allows individual herring to successfully spawn alongside a surrounding population even if they are genetically predisposed to a different season.
Major Frameworks/Components:
- Genetic Clustering: The subcategorization of widely distributed spring-spawning herring into discrete Northern, Central, and Southern genetic clusters.
- Population Hybridization: The confirmed identification of successful interbreeding between genetically distinct spring-spawning and autumn-spawning herring.
- Extreme Local Adaptation: The discovery of specialized groups, such as the "wild rose herring," which spawn in mid-July and possess extreme genetic adaptations suited for warmer water conditions.
- Phenotypic Plasticity: The capacity of the species to modify spawning behavior based on immediate environmental and social cues, allowing adaptation beyond strict genetic timing.
Branch of Science: Population Genetics, Ichthyology, Evolutionary Biology, and Marine Biology.
Future Application: This research provides a genetic baseline for the Swedish Agency for Marine and Water Management’s monitoring programs to track key marine species over time. It also establishes a framework for implementing targeted industrial fishing restrictions to protect genetically distinct herring subpopulations.
Why It Matters: Current regulatory frameworks manage Baltic herring as only two broad populations (the Baltic Proper and the Gulf of Bothnia), which fails to protect localized genetic subgroups. Conserving these distinct gene variants—particularly those adapted to warmer temperatures—is critical to ensuring the survival of the species as global sea temperatures rise.
Herring from different parts of the Baltic Sea belong to distinct populations genetically adapted to local differences in salinity and temperature. These populations can also mix with each other, according to researchers from Uppsala University, Stockholm University, and the Swedish University of Agricultural Sciences. These results have important implications for the management of Baltic herring.
Spring- and autumn-spawning herring in the Baltic Sea, as well as in the Atlantic Ocean, are genetically distinct. This is well known.
"Despite their striking genetic differences, we were able to identify hybrids between the spring- and autumn-spawning populations, thanks to the very large sample size in our present study," says Leif Andersson, professor at the Department of Medical Biochemistry and Microbiology at Uppsala University, who led the study together with Professor Linda Laikre at the Department of Zoology, Stockholm University.
Adaptation in Spawning Behavior
In other words, there are herring that, when they end up in a population spawning at the "wrong" time of year, have been able to adapt their behavior and spawn at the same time as the other herring in the surrounding population.
"Our interpretation is that genetics sets an optimal window for spawning, primarily spring or autumn, but water temperature and nutritional status influence when spawning happens. This would imply that there is communication within the school, possibly due to hormones that set the spawning time for the school," explains Andersson.
Spring- and Autumn-Spawning
Baltic herring are not only split into spring- and autumn-spawning populations. Thanks to this new study, the researchers discovered that the spring-spawning herring, which are widely distributed in the Baltic Sea, are further subdivided into northern, central, and southern clusters. There are also additional groupings within the major clusters of spring-spawning herring. Laikre points to a striking example from the Stockholm archipelago, the so-called "wild rose herring":
"We noticed that the genetic constitution of this population was more extreme than that of the populations from the southern cluster. The explanation was that these herring were spawning in mid-July when the water is much warmer than in the spring. A population like this with adaptation to spawning in warmer waters may harbor gene variants of critical importance for future adaptation to a warming sea," she says.
Important for Management
The locals call the fish "wild rose herring" because it spawns when the wild roses are in bloom.
The researchers believe their results have very important implications for the management of Baltic herring.
"Our findings showing that herring are subdivided into different clusters and groups are of great importance for management, since herring along Sweden's east coast are currently managed as two large populations, one in the Baltic Proper and one in the Gulf of Bothnia. The current management does not correspond to the genetic groupings we see," says Lovisa Wennerström from the Swedish University of Agricultural Sciences.
"We would like to see much more restrictive industrial fishing for fish meal production to reduce the risk that important local populations and the genetic diversity they harbor get lost. Further, our results will constitute a basis for the Swedish Agency for Marine and Water Management's monitoring program that aims at tracking genetic changes over time in key species such as herring," says Laikre.
Published in journal: Proceedings of the National Academy of Sciences
Title: The population structure in the Baltic herring reflects natural selection and local adaptation
Authors: Jake Goodall, Mats E. Pettersson, Anastasia Andersson, Iris Dahlin, Nils Ryman, Gunnar Ståhl, Lovisa Wennerström, and Leif Andersson
Source/Credit: Uppsala University
Edited by: Scientific Frontline
Reference Number: gen061526_01
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