Region-wide distribution models of aquatic species in the Baltic Sea
This paper presents a unified, harmonized dataset of 630 high-resolution species distribution models for fish, macrophytes, and invertebrates across the entire Baltic Sea, designed to overcome existing data fragmentation and support effective transboundary conservation and spatial planning.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The Baltic Sea is a unique body of water, a semi-enclosed basin in Northern Europe where freshwater from rivers mixes with saltwater from the ocean. This brackish environment supports a distinct collection of life, from fish and underwater plants to the tiny creatures living in the mud on the sea floor. However, this delicate ecosystem faces mounting pressure. Overfishing, pollution, and rising temperatures are degrading habitats and causing populations of key species to decline. To stop this damage and restore the sea, managers need to know exactly where these species live. They need detailed maps that show the full range of every fish, plant, and invertebrate across the entire region, not just in isolated pockets. Without this complete picture, efforts to protect the sea are like trying to navigate a dark room without a light; you might stumble upon a problem, but you cannot plan a safe path forward.
For years, creating such a comprehensive view has been difficult. The Baltic Sea is shared by nine different countries, each with its own monitoring programs, methods, and data formats. A survey conducted in Sweden might look very different from one in Finland, making it hard to compare results or stitch them together into a single, coherent map. Existing data often covered only specific national areas or focused on just a few well-known species, leaving large gaps in our understanding of the region's biodiversity. To solve this, a team of researchers from across the Baltic nations came together to build a unified dataset. They created a single, harmonized set of maps for 630 different species, covering the entire Baltic Sea, including the Danish Straits and the Kattegat. This dataset represents the first time such a vast array of marine life has been mapped with a consistent method across all nine countries.
The researchers began by gathering observation data from a wide variety of sources, including scientific surveys conducted between 2000 and 2024. They collected records for fish, underwater plants known as macrophytes, and invertebrates like crabs and worms. Because the methods used to find these animals differ so much—fish are often caught in nets, plants are spotted by divers, and small invertebrates are scooped up from the mud—the team had to process each group separately. For fish, they focused on data from scientific net surveys, which provide a reliable way to know if a species is present or absent in a specific area. For underwater plants, they combined data from full dive surveys, where every plant is counted, with other types of records. For the creatures living in the sediment, they carefully constructed "absence" data, ensuring that they only counted a species as missing if the sampling method used was capable of finding it. This careful filtering prevented the maps from showing false gaps where a species might simply have been missed by a less effective tool.
Once the data was cleaned and organized, the team used computer models to predict where each species lives across the entire sea. They did not rely on a single type of computer program but instead combined the results of three different modeling approaches. This "ensemble" method acts like a panel of experts, where the final map is a weighted average of the different models, giving more influence to the ones that performed best. The models were trained using environmental layers such as water depth, temperature, salinity, and the type of bottom substrate, ranging from soft mud to hard rock. The resulting maps cover the whole region at a resolution of 250 meters, a scale fine enough to be useful for local planning as well as broad regional strategies.
The output is not just a simple picture of where a species might be. The dataset includes seven different types of maps for each of the 630 species. Some maps show the probability of a species being present, while others show a clear "yes or no" prediction. Crucially, the team also generated maps that show how confident the model is in its own predictions. These confidence maps highlight areas where the data is strong and the prediction is reliable, as well as areas where the model is less certain. This allows managers to see not just where a species lives, but where they can trust the information enough to make decisions. For example, if a species is predicted to be in an area but the confidence is low, planners might know they need more field surveys before taking action.
To ensure the maps were accurate and useful, the researchers subjected them to a rigorous review process. First, internal experts from the research institutions checked the models for obvious errors, such as predicting that a plant that needs sunlight was living in the deep, dark ocean. They flagged any species where the model seemed to be overestimating or underestimating the range. Then, an external panel of experts from national agencies across the Baltic region reviewed the maps. These experts, who know the local ecology intimately, evaluated how well the maps matched their own knowledge of where the species actually live. Based on these reviews, every species was given a score. Most of the 630 species received high marks, indicating that the maps are reliable tools for conservation.
The final dataset covers 101 fish species, 189 types of underwater plants, and 340 invertebrate species. This comprehensive collection provides a foundation for the next generation of marine management in the Baltic Sea. It supports international goals to protect 30 percent of the sea by 2030 and helps countries coordinate their efforts to restore habitats and manage fisheries. By providing a single, consistent view of the region's biodiversity, this work removes the barriers that have previously made transboundary planning difficult. The maps are now available to anyone, from scientists to policymakers, allowing them to see the full scope of life in the Baltic Sea and to plan for its future with clarity and confidence.
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