Spawning population

Understanding Baltic Herring Age and Growth

Estimating the age of Baltic herring is typically done using otoliths, the earbones of fish. Until 1997, whole otoliths were used for herring from the Archipelago Sea. However, slow growth made it challenging to distinguish annual rings. From 1998 onwards, a new method was adopted. Each otolith is now fixed onto a polycarbonate chip, embedded in epoxy glue, and then ground to reveal the nucleus. This process improves visibility and allows for accurate counting of annual rings.

A row of herring otoliths on the polycarbonate chip
Hand-grinded and stained otolith.

Life Stages and Longevity

Baltic herring typically reaches sexual maturity at age 3, with some individuals maturing at age 2. These fish have a long lifespan, living anywhere from 10 to 15 years, and in recent times, even beyond 20 years in the Airisto spawning area.

Diverse Spawning Population

The spawning population consists of herring of various ages. However, the dominant spawners are typically between 3 to 6 years old. Consequently, spawning shoals consist of fish of different sizes, with the smallest reproducing herring measuring around 10-11 cm.

Age structure of the spawning population in 1984 (n=647) and 2014 (n=319).

Changes Over Time

Since 1984, we’ve been monitoring the spawning herring population in the Airisto area. Our data reveals that the herring population’s characteristics aren’t static and can shift with changing environmental conditions. Factors like decreasing salinity and fluctuating water temperatures have played a role.

Evolution of Size and Lipid Content

Our time-series data indicates a gradual decrease in the average size of the herring population, from 21 cm to 16 cm, while the average age remains stable. This size change is primarily due to a decline in herring growth rate, observed in various parts of the Baltic Sea. Additionally, lipid content in both ovaries and the whole fish has significantly decreased.

Growth of the Archipelago Sea herring in different years from age 2 onward.
Mean length, weight and age of spawning herring population in the Archipelago Sea in 1984-2001.
Mean length (cm), weight (g) and age of spawning herring population in the Archipelago Sea in 1984-2001.

Influences on Lipid Content

Lipid content changes appear linked to salinity and winter temperature. Female length had no association with the ovarian lipid concentration that follows the general pattern found in many other fish species. The role of food resources in herring lipid reserves is evident, but we require more data on prey species abundance, especially of mysid shrimps and amphipods, critical to herring diets during autumn when lipid accumulation occurs.

Adaptive Changes

Our findings indicate a gradual shift toward smaller herring, possibly favoring slow-growing individuals that enhance egg quality. This shift in body size may be an adaptive response to changing environmental conditions.

Seasonal Variation

Herring spawning occurs primarily from May to July, though it can start as early as April and last until August. Seasonal variation, as observed in 1990, impacts herring characteristics like gonad weight and fecundity. This seasonal variation is essential to consider when comparing samples from different times or areas.

Mean length and gonad weight (black squares) of herring females spawning in different times during spring and summer 1990. DOY= day-of-the-year (running number from the 15th of April).
Mean length (cm) and gonad weight (g; black squares) of herring females spawning in different times during spring and summer 1990. DOY= day-of-the-year (running number from the 15th of April).

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 (Accepted)
  • Mäkinen, Katja. 2019. Climate-induced Variability in Northern Baltic Sea Zooplankton – Assessing Driving Forces and Effects on Higher Trophic Levels. Annales Universitatis Turkuensis Ser 358. University of Turku, Turku. PhD dissertation (Pdf).
  • 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., 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 (24 April 2010) DOI:10.1007/s10152-010-0201-8.
  • Eklund, J., Rajasilta, M. & Laine, P. 2001. Baltic herring growth pattern in relation to spawning time. – pp 155-169 In: Herring: Expectations for a new millennium. (Eds. F.Funk, J. Blackburn, D. Hay, A. J. Paul, R. Stephenson, R. Toresen & D. Witherell). University of Alaska Sea Grant, 800 pp.
  • 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.
  • Laine, P. & Rajasilta, M. 1998. Changes in the reproductive properties of Baltic herring females during the spawning season. – Fisheries Research 36: 67-73.
  • Rajasilta, M., Paranko, J. & Laine, P. 1997. Reproductive characteristics of the male herring in the northern Baltic Sea. – J. Fish. Biol. 51: 978-988.
  • Rajasilta, M., Kääriä, J. Laine, P., Pajunen, I. & Soikkeli, M. 1996. Is the spawning of the herring in the Northern Baltic influenced by mild winters? – Proceedings of the 13th Symposium of the Baltic Marine Biologists: 185-191.
  • Rajasilta, M. 1992. Timing of spawning in the Baltic herring (Clupea harengus membras) in the Archipelago Sea, SW Finland: regulatory mechanisms and consequences for offspring production. Annales Universitatis Turkuensis. Ser. A.II. 81. Doctoral thesis; University of Turku.