Research project Hidden biodiversity in a connected sea: mechanisms maintaining fine-scale population differentiation in Baltic Sea herring
Biological diversity within species is essential for adaptation and resilience, yet the processes that maintain this diversity are not fully understood. This project uses Atlantic herring in the Baltic Sea as a model to investigate how genetically and ecologically distinct populations persist despite extensive mixing and gene flow.
Atlantic herring (Clupea harengus) is one of the Baltic Sea's most important fish species and an ideal model for studying biodiversity within species. Although herring form large schools and mix extensively in offshore feeding areas, populations differ in traits such as growth, body size, feeding ecology and spawning season. Research has shown that much of this variation has a genetic basis, but the mechanisms that maintain these differences remain unclear.
The project focuses on herring populations in the central Baltic Sea, where fish from different populations intermingle while feeding but return to specific spawning areas to reproduce. By combining genomic, ecological and environmental data with studies of herring larvae at fine spatial and temporal scales, the research will examine how population differences are maintained and how ecological and evolutionary processes contribute to resilience in a changing environment.
Why do populations of the same species differ from each other, even when they live in the same sea and can freely mix? This project addresses this question using Atlantic herring in the Baltic Sea.
Herring may appear to be a uniform species, but they are surprisingly diverse. Different groups vary in when and where they spawn, how fast they grow, and how they interact with their environment. This diversity is important because it helps populations cope with environmental change. However, it remains unclear how such differences persist, given that herring migrate freely between coastal spawning areas and offshore feeding grounds, allowing populations to mix through exchange of individuals, leading to gene flow.
This diversity has been known for long and thought to be driven mainly by environmental conditions. However, recent advances in DNA technology show that some of these differences also have a genetic basis. But how genetic, environmental factors and behavior interact to shape herring diversity remains poorly understood.
In this project, we focus for the first time on herring at their earliest life stage, the newly hatched larvae, and at fine spatial scales within key spawning regions. We combine advanced DNA analyses with chemical measurements of earstones (otoliths) and detailed phenotypic characterization. Otoliths are laid down during the egg stage, grow throughout life, store elements in relation to the chemical composition of the surrounding water and are physiologically inert. Otoliths can be seen as the fish's "black box", which can be analysed retrospectively. These approaches allow us to identify where and when fish spawn, how larvae differ, and how populations are structured at a much finer scale than previously done.
By integration these data, we will map hidden population structure link it to environmental conditions such as temperature, salinity, and plankton blooms. This will enable us to identify the key environmental pressures that drive adaptation and maintain diversity within the species.
Ultimately, this project will reveal how biodiversity is maintained in a highly connected marine system, and how this diversity supports population resilience in a changing climate. This will provide new knowledge essential for understanding, managing and protecting marine diversity.