Age structure and growth of the spawning population
Age of the Baltic Sea herring is usually estimated either from whole otoliths or from cut and stained otoliths. For the Archipelago Sea herring, whole otoliths were used in 1984-1997, but since then, the method was changed due to the slow growth rate which made the annual rings difficult to distinguish. From 1998 onwards, the otoliths have been prepared for age readings according to the procedure developed by the Natural Resources Institute Finland. In the method, each otolith is fixed onto a polycarbonate chip (nuclei on the edge of the chip); embedded in epoxy glue, ground by hand to the nucleus using a grinding paper and stained with neutral red. In this way, the annual rings become easier to see and their number can be counted.
Baltic herring attains sexual maturity at the age of 3 years, but some individuals mature and spawn already at age of 2 years. The life span of the herring is long: fish live normally 10-15 years and even fish older than 20 years have been found in the spawning shoals in the Airisto spawning area.
The spawning population consists of fish of different ages, but typically 3-6 years old fish form the dominant group of spawners. Respectively, the spawning shoals are mixtures of fish of different sizes. The smallest reproducing herrings found in the Airisto spawning area have a length of 10-11 cm.
Long- and Short-term Changes in Spawning Population
We have monitored the spawning herring population by annual samplings since 1984 in the Airisto area. The time series based on various population parameters (e.g. fish length, weight and age; gonad weight, fat content of muscle and ovary) indicates that the characteristics of the herring population are not stable, but they change in the course of the time. For instance, in the study period 1984-2001, the mean length of the population gradually decreased from 21 cm to 16 cm, while the mean age slightly increased. The change is due to the decline of herring growth rate, which has been observed also in other parts of the Baltic Sea.
In addition to annual and long-term variation, the population characteristics vary also within the spawning season. On the average, herring spawns between May and July; i.e. the first spawning shoals arrive to the spawning grounds in early May and the last ones in the middle of July. In some years, spawning may start already in April and last until beginning of August. As demonstrated by the data from the year 1990, e.g. gonad weight (and fecundity) of herring females may be quite different, depending on the timing of spawning. Seasonal variation should be taken into account in fish sampling, especially when samples from different areas and/or different times are compared.
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- 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.