Research themes and projects

The Baltic herring research project aims to gain comprehensive insights into the intricate reproductive biology of the Baltic herring (Clupea harengus membras) within the northern Baltic ecosystem. The roots of the  research project are in a study examining the effects of increased ship traffic in the  Archipelago Sea (SW Finland) to local herring stocks. Since this first study, the project has continued and expanded to map out herring spawning areas in the inner archipelago and to investigate ecological and evolutionary factors influencing herring spawning events and reproductive success.

During the project’s almost four-decade long history, environmental conditions at the spawning area have changed considerably, e.g. due to eutrophication, rising sea temperature and decreased salinity, which gives us means to study how the herring adapt to changing environmental conditions and anthropogenic pressures.

Ongoing projects

The variation and significance of iodine in the Baltic Sea ecosystem

Iodine and Its Importance:

Iodine is an essential trace element crucial for maintaining health. It plays a pivotal role in the production of thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3). These hormones are indispensable throughout an individual’s life, as they are vital for normal growth, development, and metabolism.

Project Objectives:

The primary aim of this research project is to:

Objective 1: Study Temporal and Spatial Variation of Iodine Investigate the temporal variations, both long-term and seasonal, as well as the spatial distribution of iodine within the brackish ecosystem of the Baltic Sea.

Objective 2: Assess the Impact on Key Species Examine how variations in iodine availability, including instances of iodine deficiency, influence key species residing in the Baltic Sea ecosystem. This includes species such as the Baltic herring and bladderwrack (Fucus vesiculosus).

The project is conducted in close collaboration with Ruuskanen Group (Dep. of Biological and Environmental Science, JYU) and Anttila Lab (Dep. of Biology, UTU). Nationally our collaborators also include the Geology division (UTU), Laboratory of Industrial Physics (Department of Physics and Astronomy,UTU), Pyhäjärvi-Institute, and the Finnish Food Authority. Internationally we also collaborate with Dr. Patrick Polte and his colleagues at the Johann Heinrich von Thünen-Institute (Rostock, Germany) and with Prof. Karin Limburg from the department of Environmental and Forest Biology at SUNY-ESF (New York, USA).

The project started in summer 2020. The work has been supported by Sakari Alhopuro Foundation (PI Katja Mäkinen, 2020-2023) and Turku University Foundation (to K. Mäkinen).

Researchers and contact persons: Katja Mäkinen, Marjut Rajasilta

Related publications:

Herring in the food web of the Bothnian Sea. Background to the variation in the condition and weight loss of herring in 2020–2022

Project Objective:

The central objective of this project is to assess the food sources of Baltic herring within the Bothnian Sea. This investigation aims to provide insights into why the proportion of malnourished individuals in the area exhibited an increase during the period spanning 2020 to 2022.

Project Coordination and Collaboration:

The project is a collaborative effort with coordination led by the National Resources Finland (LUKE). Additionally, the herring project at UTU actively participates in this two-year initiative.

Funding and Duration:

The project is financially supported by the Ministry of Agriculture and Forestry. It commenced in February 2023 and is scheduled for completion by the end of 2024.

Researchers and contact persons at UTU: Katja Mäkinen, Marjut Rajasilta & Johannes Sahlsten

Stable isotope ratios, otolith microchemistry and biological parameters as indicators of natal origin, migration patterns and home range of the Baltic herring in the northern Baltic Sea

The doctoral thesis aims to address the following key objectives:

Objective 1: Population Identification Methods Develop methods (e.g. stable isotopes and otolith microchemistry) for accurately identifying Baltic herring populations and stocks in the northern Baltic Sea.

Objective 2: Hormonal Factors and Migration Investigate the influence of hormonal factors, particularly thyroid hormones, on herring migrations.

Objective 3: Bridging Research Gaps Enhance our understanding of the alignment between herring research conducted in open sea environments and research conducted in herring spawning grounds.


Currently, distinguishing the origins of herring caught in open sea feeding areas is challenging. Different stocks intermingle, and potential migrations between various feeding grounds further complicate identification. Similarly, the overwintering locations of spring-spawning herring remain uncertain.


Understanding herring migration patterns holds significant importance, as environmental conditions at feeding grounds directly impact their reproductive success. Additionally, this knowledge can inform the planning and management of marine protected areas, recognizing the vital role of fish spawning grounds in the ecosystem.

PhD researcher: Johannes Sahlstén, MSc, University of Turku

Supervisors: Marjut Rajasilta (UTU), Katja Mäkinen (UTU) and Jari Raitaniemi (Natural Resources Institute Finland)

The research has been supported by the Olvi Foundation (Working grant to J. Sahlsten, 2023)

Research themes and interests

Monitoring of the spawning population

Long-Term Monitoring of Reproductive Characteristics:

Since the 1980s, this project has been dedicated to monitoring the reproductive characteristics of the spawning herring population within the Archipelago Sea. Over the years, the project has accumulated extensive and valuable long-term datasets pertaining to the herring population. These datasets are not only vital for advancing our scientific understanding but also serve as essential references for stakeholders, offering critical insights into the health and sustainability of Baltic herring stocks, which are pivotal for both local communities and the broader marine ecosystem.

Data Collection Methods:

The project employs two primary methods for data collection:

  1. Herring Sampling: Regular sampling of herring occurs at their spawning grounds. This process involves the collection of approximately 1000 adult herring per year between April and July. The collected data includes standard length, weight, sex, maturity stage, gonad weight, and age.
  2. Spawning Bed Monitoring: Complementary to herring sampling, the project has also conducted monitoring of the spawning beds using scuba diving techniques.

Comprehensive Data Collection:

In addition to standard measurements, the project maintains records of atypical observations. These include the identification and tracking of Corynosoma-parasite occurrences, gonad abnormalities, and otolith abnormalities.

In addition to traditional biological traits such as fish size, condition, and age, the research places emphasis on understanding biochemical and hormonal changes. These include alterations in lipids, fatty acids, and thyroid hormones. The objective is to discern how these changes relate to reproductive success and the population’s overall resilience.

Sample Archives for Retrospective Study:

To further enhance its research capabilities, the project systematically collects, stores, and maintains herring tissue and otolith samples. These archives encompass samples dating back to the 1980s. This extensive repository of samples is an invaluable resource, facilitating retrospective studies that provide deeper insights into the evolutionary adaptations, environmental changes, and long-term trends that have shaped the Baltic herring population and its surrounding ecosystem.

Contact persons: Marjut Rajasilta, Katja Mäkinen

Related publications:

Climate change and the Baltic herring

Over nearly four decades, the project has witnessed significant transformations in environmental conditions within the northern Baltic Sea. These shifts are primarily attributed to climate change and various human-induced activities.

Research Objectives:

The current research endeavors to achieve several key objectives:

Objective 1: Assessing Impact of Environmental Changes Examine the key changes that have transpired within the herring spawning population in response to these evolving environmental conditions. Identify the primary environmental factors influencing these changes and explore the mechanisms of adaptation employed by the population.

Objective 2: Reproductive Resilience Assessment Investigate the reproductive resilience of the herring population, specifically its capacity to maintain the necessary reproductive success for long-term stability.

Environmental and Zooplankton Monitoring:

Leveraging long-term environmental and zooplankton monitoring conducted at the Archipelago Research Institute (UTU), the project aims to:

  • Investigate shifts in zooplankton composition within the nursery and feeding grounds of the Baltic herring over recent decades.
  • Examine the relationship between prey availability, quality, and factors such as larval survival and growth.

Contact persons: Katja Mäkinen & Marjut Rajasilta

Related publications::

Understanding Ecological Dynamics and Reproductive Fitness of Baltic Herring in Variable Environmental Conditions

This research theme aims to investigate the intricate relationships between abiotic environmental factors, female traits, and egg quality in Baltic herring  under variable environmental conditions in the Baltic Sea. By analyzing long-term data, we seek to unravel the associations between population density, prey availability, lipid content, growth, and reproductive performance, providing valuable insights into the ecological dynamics and reproductive fitness of this vital species. Through comprehensive studies on fatty acid composition, lipid content, somatic condition, and ovarian weight, we aim to contribute to a deeper understanding of the species’ biology and its responses to changing environmental scenarios.

Research Objectives include:

  • Evaluate Environmental Influences on Reproductive Success: Investigate the impact of abiotic factors and female traits on the egg quality of Baltic herring, focusing on how varying environmental conditions affect egg development and subsequent reproductive success.
  • Assess Nutritional Dynamics and Their Environmental Drivers: Analyze the lipid content and fatty acid composition in Baltic herring under different environmental conditions to understand how variations in prey availability and other abiotic factors influence nutritional composition, crucial for growth and reproduction.

Collaborative Efforts:

This research is conducted in collaboration with the Department of Life Technologies, particularly in the fields of Food Chemistry and Food Development, under the guidance of Adj. Prof. Jukka-Pekka Suomela.

Contact persons: Marjut Rajasilta, Katja Mäkinen

Related publications:

Otolith microchemistry

Otoliths: Multifunctional Structures:

Otoliths, also known as earbones, serve dual roles within fish physiology by facilitating both hearing and balance. Traditionally, researchers have employed otoliths to estimate fish age, achieved by counting the number of opaque and translucent rings. These age data not only assist in determining growth rates but also provide insights into the age at which maturity is reached.

Beyond Age: A Storehouse of Chemical Information:

In addition to age, otoliths hold valuable chemical data in trace amounts. The elemental composition of discrete layers harbors insights into various aspects of fish biology. For example, it can be harnessed to study migration patterns or identify the natal origin of fish, as chemical uptake often mirrors that of the surrounding water.

Research Focus and Methods:

The research conducted since 2021 centers on the elemental composition of otoliths, employing LA-ICP-MS (Laser Ablation-Inductively Coupled Plasma Mass Spectrometry) analysis. This innovative approach enhances our understanding of fish biology and environmental interactions.

Collaborative Endeavors:

The research represents a collaborative effort with Prof. Limburg’s lab at SUNY-ESF (Syracuse, New York, U.S.). This collaboration strengthens the project’s scope and capabilities.

Otolith of the Baltic herring. Photo: Katja Mäkinen.

Contact person: Katja Mäkinen

Understanding Environmental Drivers of Baltic Herring Growth

Our research aims to comprehend the environmental factors influencing the growth of Baltic herring. Specifically, we investigate the impact of salinity, temperature, prey abundance, and spawning time on the growth patterns of Baltic herring juveniles in the recent decades.

Environmental Influences on Growth: We analyze how salinity and temperature affect growth, providing insights into optimal conditions for juvenile herring growth. Additionally, we explore potential shifts in spawning time linked to changing salinity conditions.

Linking Spawning Time to Growth Advantages: By studying the spawning time, we aim to understand how it influences growth rates, potentially providing growth advantages to herring juveniles. This knowledge enhances our understanding of the early life history of Baltic herring.

Contributing to Sustainable Fisheries Management: Our research contributes to sustainable fisheries management by providing critical insights into Baltic herring growth patterns and their environmental determinants. This knowledge is pivotal for developing informed conservation strategies and ensuring the resilience and sustainability of Baltic herring populations.

Contact persons: Marjut Rajasilta, Katja Mäkinen

Sampling methodology: trap net project

Collaborative Data Collection with Fishermen:

The herring dataset integral to our research has been primarily sourced through close collaboration with local fishermen. These dedicated fishermen have played a pivotal role by consistently providing annual samples of spawning herring.

Shift in Fishing Methods:

In recent years, there has been a significant shift in traditional spawning herring fishing practices in the Airisto area. This shift has resulted in a substantial decline in herring catch using trap nets.

Transition to Research-Oriented Trap Nets:

Since 2019, our research studies have transitioned to using two specialized trap nets designed exclusively for research purposes. These nets have proven invaluable in maintaining the continuity of our herring sample collection.

Project Funding and Objectives:

The acquisition of these trap nets was made possible through a collaborative effort funded by the Southwestern Finland ELY-centre and the European Maritime and Fisheries Fund (EMFF), specifically through the Archipelago Sea Fisheries Action Group (Saaristomeren kalatalouden toimintaryhmä).

Photo: Johannes Sahlsten

Knowledge Transfer and Collaboration:

The overarching aim of this project was to facilitate the transfer of knowledge and skills from experienced professional fishermen to students. The successful execution of the trap net project (2019-2020) was achieved in partnership with local fishermen and Livia College.

Contact persons: Jari Hänninen, Johannes Sahlsten

Project report: