Age structure and growth of the spawning population
Age of the Baltic herring is usually estimated either from whole otoliths or from cut and stained otoliths. For the herring spawning in the Archipelago Sea, 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-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 and that they can change in the course of the time with the changing environmental conditions. During the study period, Baltic salinity first clearly decreased and then stabilized to the current level, and the mean winter months’ water temperature in the overwintering areas of the herring fluctuated.
According to our time-series data, the mean length of the population has gradually decreased from 21 cm to 16 cm, while the mean age has stayed the same. 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, the amount of lipids has decreased significantly both in the ovarian tissue and also in the whole fish, when measured as the sum of muscle and ovarian lipids.
The changes in lipid content have been found to be related 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 the formation of the lipid reserves of the herring is an obvious one, but the available information on the abundance of the herring’s prey species is scarce or even contradictory in the study area. Thus, more information is needed especially on the temporal and local variation of macroscopic prey such as amphipods and mysid shrimps, which the herring feed on during autumn when the accumulation of lipids take place.
Our results have also indicated that the gradual change in the herring population toward a smaller body size may be an indication of an adaptive process favouring slow-growing individuals, which are able to increase their fitness due to better egg quality. As fish tend to scale their energy budget to the body size, this perspective should also be taken into account as an alternative explanation in further studies, since it means that small body size could be the phenotypic trait selected for in the variable environmental conditions of the Baltic Sea.
Short-term Changes in Spawning Population
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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