Spawning population

The spawning population of Baltic herring in the Archipelago Sea

Since 1984, we have monitored the spawning population of Baltic herring in the Airisto area of the Archipelago Sea. More than 50 000 spawning herring have been examined. The data show that the population has changed considerably: spawning herring are now smaller and have lower lipid reserves than in the 1980s.

How is the age of Baltic herring determined?

Estimating the age of Baltic herring is primarily done using otoliths (ear bones), which record growth rings similar to tree rings. Until 1997, whole otoliths were used for age reading in the Archipelago Sea population. However, due to the slow growth of herring, distinguishing the annual rings became challenging. Since 1998, a new method was adopted: each otolith is fixed onto a polycarbonate chip, embedded in epoxy resin, and then ground to expose the nucleus. This technique significantly improves the visibility of the growth rings, allowing for more accurate age estimation.

A row of herring otoliths on a polycarbonate plate.

 

Hand-ground and stained otolith.

How old do Baltic herring get?

Baltic herring usually reach sexual maturity at the age of 3, although some individuals mature at the age of 2. Herring can live for 10–15 years. In the Airisto spawning area, some individuals have been aged at over 20 years (max. 30 years). The proportion of older fish increased at the end of the 1990s. A similar increase was observed in the stock assessment of the Bothnian Sea, and it has been interpreted as a sign of increased lifespan rather than an effect of fishing. However, the result should be interpreted with caution, because ageing older, slow-growing herring is difficult and the change coincided with the change in the age-reading method.

Who spawns in the Archipelago Sea?

The spawning population consists of fish of many ages. About 90 % of spawners are 2–9 years old. Spawning shoals include fish of different sizes, and the smallest mature herring are about 10–11 cm long. Several spawning waves may arrive at the same site during the season, but they do not differ in fish size or age (Rajasilta et al. 1993).

Age structure of the spawning population in 1984 (n=647) and 2016 (n=747).

How has the spawning population changed?

Size. The mean length of spawning herring has decreased from about 21 cm in the 1980s to about 16 cm today. In the mid-1980s, about half of the spawners were over 20 cm long, whereas today such fish are rare. The decline is not explained by a younger age structure but by slower growth: fish of the same age are now smaller than before. Similar changes in growth have been observed throughout the Baltic Sea.

Condition. The condition of spawning herring (weight relative to length) weakened in the 2000s but has since recovered close to the level of the 1980s.

Lipid reserves. The lipid content of Airisto herring decreased strongly in 1987–2014. The decrease was best explained by salinity and the size of the herring stock (Rajasilta et al. 2019). Lipids are essential for egg quality and larval survival. Egg quality in the Airisto population was associated with female size and with salinity and winter temperature (Rajasilta et al. 2021).

Reproduction. Smaller females produce fewer eggs, because fecundity, egg size and other reproductive traits increase with female size and age (Rajasilta et al. 2021). In poor feeding conditions, herring females have been shown to allocate resources from growth to reproduction (Rajasilta et al. 2015).

Growth of herring in the Archipelago Sea in different years from age 2 onward

 

Mean length (cm), weight (g) and age of spawning herring population in the Archipelago Sea in 1984-2026.

How does food affect herring condition?

Lipid content in herring is closely linked to environmental factors and food availability. While female size alone does not determine ovarian lipid concentrations, higher lipid reserves are often observed in years with abundant prey species, such as mysid shrimps and amphipods. These prey are particularly important in autumn when herring accumulate energy reserves for spawning.

There is thus a need for more comprehensive data on the abundance of key prey species, especially during the critical feeding period before winter. This information could help explain variations in lipid content and provide insights into how changing prey availability affects herring condition and reproductive success.

How does spawning time affect females?

In the Archipelago Sea, spawning now takes place mainly from late April to May. In the 1980s and 1990s, it could continue until July or even August. The timing of spawning affects reproductive traits. In 1990, females spawning later in the season were slightly smaller and had lower gonad weights than those spawning earlier. By contrast, in 2024–2026, samples covered mainly April to early June, and no consistent seasonal trend was observed. Recent females are smaller and have lighter gonads than in 1990, but the comparison is also affected by differences between sampling sites. Seasonal variation and sampling site must therefore be taken into account when samples from different times or areas are compared.

Mean length and gonad weight of ripe females (maturity stages 5–6) by sampling date in 1990 and 2024–2026. Point size shows the number of females; vertical lines show 95 % confidence intervals.

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