Research themes and projects

The Baltic herring research project focuses on the reproductive biology of the Baltic herring (Clupea harengus membras) and processes and factors affecting clupeid fish in the Baltic Sea.

The roots of the Baltic herring research project are in a study examining the effects of increased ship traffic in the Archipelago Sea (northern Baltic Sea) 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 study factors influencing the spawning biology of herring.

During the project’s almost four-decade long history, environmental conditions at the spawning area have changed considerably, e.g. due to eutrophication, increased water temperature and decreased salinity, which gives us means to study how herring reproduces and survives in the changing conditions of the Baltic Sea.

Ongoing funded projects

The variation and significance of iodine in the Baltic Sea ecosystem

Iodine is an essential trace element for health as it is needed in the production of thyroid hormones thyroxine (T4) and triiodothyronine (T3), which are required throughout life for e.g. normal growth, development and metabolism.

The aim of the project is to study the temporal (long-term and seasonal) and spatial variation of iodine in the brackish ecosystem of the Baltic Sea and how its availability (e.g. iodine deficiency) affects key species of the Baltic Sea, for example 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) and Turku University Foundation (to K. Mäkinen).

Researchers and contact persons: Katja Mäkinen, Marjut Rajasilta

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Research themes and interests

Monitoring of the spawning population

The reproductive characteristics of the spawning herring population in the Archipelago Sea have been monitored since 1980s with the result that the project nowadays maintains and utilizes several long-term datasets of the population.

The data has been collected by sampling  herring at their spawning grounds as well as by monitoring the spawning beds by scuba diving.

In the monitoring program, standard length, weight, sex, maturity stage, gonad weight and age are annually measured from adult herring, collected at their spawning grounds between April-July (ca. 1000 fish per year). In addition, atypical observations such as the occurrence of Corynosoma-parasites, gonad and otolith abnormalities are recorded.

The project also collects, stores and maintains herring tissue and otolith samples, the oldest stored samples being from the 1980s. The sample archives can be used to study retrospectively e.g. changes in nutrition, energy stores and chemical composition.

Contact persons: Marjut Rajasilta, Katja Mäkinen

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Climate change and the Baltic herring

During the project’s almost four-decade long history, environmental conditions at the northern Baltic Sea have notably changed as a result of climate change and other anthropogenic activities.

As a result, a major aim of our current research is to study what key changes have occurred in the spawning population as a result of these changes, what are the main influencing environmental factors, and what are the mechanisms of adaption. With our work, we also aim to provide information on the reproductive resilience of the population (i.e. the capacity of the population to maintain the reproductive success needed to result in long-term stability).

In addition to biological traits such as fish size, condition and age, we’re also interested in the biochemical and hormonal changes (e.g. lipids, fatty acids and thyroid hormones) and how changes in these traits are related to the reproductive success and resilience of the species. 

With the help of long-term environmental and zooplankton monitoring, conducted in the Archipelago Research Institute (UTU), we’re also able to study how the zooplankton composition in the nursery and feeding grounds of the Baltic herring have changed within the past decades, and how prey availability and quality is related for example to larval survival and growth.

Contact persons: Katja Mäkinen & Marjut Rajasilta

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Energy reserves of the Baltic herring

Lipids and their components fatty acids are, together with proteins, the major organic constituents of fish, and play crucial roles as sources of metabolic energy for e.g. growth, reproduction, and movement.

We have studied long-term, seasonal and spatial variation of herring quality characteristics (i.e., muscle and ovarian lipid content and fatty acid composition) in order to understand how the energy reserves of herring are formed and utilized in the variable environment of the Baltic Sea. This helps us to understand factors affecting herring reproduction and growth but our aim is also to produce useful information for  consumers and fish industry.

The research is conducted in collaboration with the department of Life Technologies (Food Chemistry and Food Development; Adj. Prof. Jukka-Pekka Suomela).

Contact persons: Marjut Rajasilta, Katja Mäkinen

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Sampling methodology: trap net project

The herring dataset utilized in our research has been largely collected in close collaboration with local fishermen, from who we have annually received samples of spawning herring. However, in the recent years, traditional method of fishing spawning herring has largely been abandoned in the Airisto area and the trap net catch of herring has decreased as a consequence of this.

Photo: Johannes Sahlsten

Since 2019, the herring samples used in our studies have been collected with two trap nets, built especially for research purposes. The trap nets were acquired as part of a project funded by the Southwestern Finland ELY-centre and European Maritime and Fisheries Fund (EMFF), Archipelago Sea Fisheries Action Group (Saaristomeren kalatalouden toimintaryhmä).

The aim of the project was to transfer knowledge and skills from professional fishermen to students.
The trap net project (2019-2020) was conducted in collaboration with local fishermen and Livia college.

Contact persons: Jari Hänninen, Johannes Sahlsten

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