Research themes and projects

odine 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.

The primary aim of this research project is to:

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.

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:

• Mäkinen, K., Rajasilta, M., Ruuskanen, S., Karpela, T., Lauerma, A., Sahlsten, J. 2023. Effects of incubation temperature and maternal phenotype on Baltic herring (Clupea harengus membras) eggs and larvae: An experimental study. Canadian Journal of Fisheries and Aquatic Sciences. Accepted. Doi: 10.1139/cjfas-2023-0032
• Mäkinen, K., Rajasilta, M., Mäkilä, E., Jokinen, S., Hänninen, J. (2022) Varying frequency of vateritic otoliths in the Baltic herring Clupea harengus membras. Journal of Fish Biology (Accepted). Doi: 10.1111/jfb.15127.
• Rajasilta, M., Mäkinen, K., Ruuskanen, S. Hänninen, J., Laine, P. (2021) Long-term data reveal the associations of the egg quality with abiotic factors and female traits in the Baltic herring under variable environmental conditions. Frontiers in Marine Science; Marine Fisheries, Aquaculture and Living Resources. Doi: https://doi.org/10.3389/fmars.2021.698480.

The  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.

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.

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

Contact person at LUKE: Jari Raitaniemi

The doctoral thesis aims to address the following key objectives:

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.

Hormonal factors and migration: Investigate the influence of hormonal factors, particularly thyroid hormones, on herring migrations.

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 mix, 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)

Monitoring reports on the environmental and fisheries impact of dredging disposal activities in the Port of Turku, alongside our research indicate a significant decline in herring offspring production in Airisto over the past 30 years.

For about 25 years, monitoring reports have estimated that roughly 97-99% of herring eggs are washed away from their spawning substrates before they can develop to the hatching stage (e.g., Vahteri & Savoila, 2018). This phenomenon is linked to eutrophication; however, apart from observations reported in mandatory monitoring, the extent of egg washout/detachment and its underlying causes have not been extensively studied.

The surface water quality in the marine area off Turku is classified as moderate. On herring spawning grounds, water quality and environmental conditions are influenced by factors such as nutrient loading, shipping traffic, and the disposal of dredging waste into the sea. Increased levels of phosphorus and nitrogen contribute to water turbidity, algal blooms, and hypoxia in bottom areas. Additionally, surface waters have warmed significantly since the 1990s. Eutrophication-induced turbidity and warming of the seawater have altered the vegetation on spawning grounds, causing herring to spawn closer to the water surface (0-3 m). This change increases the likelihood of egg washout due to waves and currents.

The pilot study starting in 2024 aims to investigate which environmental factors are contributing to the decline in herring spawning success in the inner archipelago. The phenomenon will be examined through fieldwork and experimental aquarium studies. As part of the project, a summary of relevant reports and scientific literature on this issue will also be compiled.

Contact person: Katja Mäkinen

A related master’s thesis has been supported by the Finnish Nature Conservation Foundation’s Baltic Sea Fund (personal grant to BSc Saara Miinala).

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.

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 and more recently an ROV-camera.

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:

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:

• Mäkinen, K., Rajasilta, M., Mäkilä, E., Jokinen, S., Hänninen, J. (2022) Varying frequency of vateritic otoliths in the Baltic herring Clupea harengus membras. Journal of Fish Biology (Accepted). Doi: 10.1111/jfb.15127.
• Rajasilta, M., Elfving, M., Hänninen, J., Laine, P., Vuorinen, I., & Paranko, J. (2016). Morphological abnormalities in gonads of the Baltic herring (Clupea harengus membras): Description of types and prevalence in the northern Baltic Sea. Ambio, 45(2), 205-214.
• Rajasilta, M. Eklund, J., Laine, P., Jönsson, N. & Lorenz, T. 1999. Intensive monitoring of spawning populations of the Baltic herring (Clupea harengus membras L.). Final report of the study EU 96-068.74 p + annexes.
• Rajasilta, M., Eklund, J. Hänninen, J., Kurkilahti, M., Kääriä, J. Rannikko, P. & Soikkeli, M. 1993. Spawning of herring (Clupea harengus membras L.) in the Archipelago Sea. – ICES J. mar. Sci. 50: 233-246.

Contact persons: Katja Mäkinen & Marjut Rajasilta

Related publications::

• Rajasilta, M., Mäkinen, K., Ruuskanen, S. Hänninen, J., Laine, P. (2021) Long-term data reveal the associations of the egg quality with abiotic factors and female traits in the Baltic herring under variable environmental conditions. Frontiers in Marine Science; Marine Fisheries, Aquaculture and Living Resources. Doi: https://doi.org/10.3389/fmars.2021.698480.
• Rajasilta, M., Hänninen, J., Laaksonen, L., Laine, P., Suomela, J. P., Vuorinen, I., & Mäkinen, K. (2018). Influence of environmental conditions, population density, and prey type on the lipid content in Baltic herring (Clupea harengus membras) from the northern Baltic Sea. Canadian Journal of Fisheries and Aquatic Sciences, (999), 1-10.
• Rajasilta, M., Eklund, J., Hänninen, J., Vuorinen, I., & Laine, P. (2015). Female Baltic herring Clupea harengus allocate resources from growth to reproduction in poor feeding conditions. Journal of Fish Biology, 86(2), 575-591.
• Mäkinen K, Vuorinen I, Hänninen J (2017) Climate-induced hydrography change favors small-bodied zooplankton in a coastal ecosystem. Hydrobiologia. Doi: 10.1007/s10750-016-3046-6
• Rajasilta, M., Hänninen, J., & Vuorinen, I. (2014). Decreasing salinity improves the feeding conditions of the Baltic herring (Clupea harengus membras) during spring in the Bothnian Sea, northern Baltic. ICES Journal of Marine Science, 71(5), 1148-1152.

The 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 identify 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.

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

Contact persons: Marjut Rajasilta, Katja Mäkinen

Related publications:

• Rajasilta, M., Mäkinen, K., Ruuskanen, S. Hänninen, J., Laine, P. (2021) Long-term data reveal the associations of the egg quality with abiotic factors and female traits in the Baltic herring under variable environmental conditions. Frontiers in Marine Science; Marine Fisheries, Aquaculture and Living Resources. Doi: https://doi.org/10.3389/fmars.2021.698480.
• Rajasilta, M., Hänninen, J., Laaksonen, L., Laine, P., Suomela, J. P., Vuorinen, I., & Mäkinen, K. (2018). Influence of environmental conditions, population density, and prey type on the lipid content in Baltic herring (Clupea harengus membras) from the northern Baltic Sea. Canadian Journal of Fisheries and Aquatic Sciences, (999), 1-10.
• Mäkinen, K., Elfving, M., Hänninen, J., Laaksonen, L., Rajasilta, M., Vuorinen, I., & Suomela, J. P. (2017). Fatty acid composition and lipid content in the copepod Limnocalanus macrurus during summer in the southern Bothnian Sea. Helgoland Marine Research, 71(1), 11.
• Rajasilta, M., Laine, P., & Paranko, J. 2010. Current growth, fat reserves and somatic condition of juvenile Baltic herring (Clupea harengus membras) reared in different salinities. Helgoland marine research, 65(1), 59.
• Rajasilta, M. , P. Laine & J. Hänninen. 2001. Ovarian weight of the Baltic herring (Clupea harengus membras) in relation to spawning time in the Archipelago Sea, Northern Baltic. – ICES J.Mar.Sci. 58: 106-113.

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.

A storehouse of chemical information:

In addition to growth rings, 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.

Collaboration:

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.

Contact person: Katja Mäkinen

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

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.

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).

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 person: Jari Hänninen

Project report:

• Jari Hänninen et al. (2021): Rysäkalastuksen tekniikka ja ammatillinen tietotaito silakan tutkimuksen käyttöön – pilottitutkimus 2019-2021. Loppuraportti: hankenumero 93977. [Trap net fishing technology and vocational know-how for the use of herring research – pilot study 2019-2020. Final research report no 93977]. SEILI Archipelago Research Institute Publications 8.