Spawning areas

Baltic herring spawning in the Archipelago Sea

Baltic herring spawn in shallow coastal waters, where the eggs attach to submerged vegetation. Our group has studied herring spawning grounds in the Archipelago Sea in southwest Finland by diving since 1981. The results show that herring return to the same spawning sites year after year, although the condition of many of these sites has deteriorated.

When do Baltic herring spawn?

Baltic herring spawn mainly in spring, but the timing varies across the Baltic Sea. In the southern Baltic, spawning begins as early as late February and continues until early May. In the Archipelago Sea, spawning starts typically in late April or early May, depending on the preceding winter. In 1987–1997, spawning began between 14 April and 2 May after mild winters, and in the first week of May after severe winters (Rajasilta & Aneer 2006).

In the 1980s, spawning in the Archipelago Sea could continue until July or even August. Today, spawning takes place mainly in May, and late spawning has decreased considerably. Each herring releases all its eggs in a single spawning event, so the long spawning season reflects differences in maturation between individuals. Several successive waves of spawners may arrive at the same site during the season, but these waves do not differ in fish size or age (Rajasilta et al. 1993).

Where do Baltic herring spawn?

Baltic herring spawn along the coasts of the Baltic Sea, from the Danish straits to the northern Bothnian Bay. Important spawning areas include Greifswald Bay, the Gulf of Riga, the Stockholm Archipelago and the Archipelago Sea. The Archipelago Sea is one of the most important spawning areas on the Finnish coast. Many of the spawning sites in the Airisto inlet have been in use for decades, according to monitoring and fisheries records.

Major spawning areas of the herring in the southern, central and northern Baltic Sea. Map on the left: Airisto inlet in the Archipelago Sea; blue circles indicate the approximate locations of regularly monitored spawning beds.

Spawning grounds and egg survival

Herring spawn in shallow water among submerged vegetation. The eggs are sticky and remain attached to the plants until hatching. Herring are not highly selective about the plant species, but vegetation cover is essential for egg attachment. No spawning has been found on soft bottoms (Kääriä et al. 1988).

The spawning substrate affects egg survival. In our studies, less than 5 % of the eggs attached to Cladophora and Potamogeton died, whereas mortality on red algae (Furcellaria, Phyllophora) exceeded 50 % (Rajasilta et al. 1989).

Challenges in spawning grounds

Eutrophication and sediment load reduce the amount of light reaching the bottom, which leads to a loss of submerged vegetation. In the Airisto inlet, the spawning sites have remained largely the same since 1981, but the area of suitable spawning bottoms has decreased and the vegetation zone has moved closer to the shore. At the beginning of our monitoring, spawning was observed at depths of 0–10 metres. Today it occurs mainly at depths of 0–3 metres. The loss of spawning grounds due to eutrophication and siltation was already considered one of the reasons for the decline of the spawning-time trap net fishery in the 1980s (Kääriä et al. 1988). Diving surveys have also shown that eggs may detach from the vegetation before hatching, particularly in the inner parts of the spawning areas. Detached eggs stop developing and are lost to waves and currents.

Why do herring return to the same spawning sites?

Like many other herring species, Baltic herring show strong spawning site fidelity: the same sites are used year after year, even when their condition has deteriorated. This behaviour has probably evolved because these sites once offered good conditions for egg survival. Herring spawning in a particular area may also be locally adapted, so the degradation of local spawning grounds may reduce the genetic diversity of the stock (e.g. Han et al. 2020). The renewal of the herring stock therefore depends strongly on local conditions in coastal spawning areas.

Related Publications

  • Han, F., Jamsandekar, M., Pettersson, M.E. et al. (2020). Ecological adaptation in Atlantic herring is associated with large shifts in allele frequencies at hundreds of loci. eLife 9: e61076.
  • Kääriä, J., Rajasilta, M., Kurkilahti, M., & Soikkeli, M. (1997). Spawning bed selection by the Baltic herring in the Archipelago Sea. ICES Journal of Marine Science, 54: 917-923.
  • Rajasilta, M., Eklund, J., Hänninen, J., Kurkilahti, M., Kääriä, J., Rannikko, P., & Soikkeli, M. (1993). Spawning of herring in the Archipelago Sea. ICES Journal of Marine Science, 50: 233-246.
  • Rajasilta, M., Eklund, J., Kääriä, J., & Ranta-aho, K. (1989). The deposition and mortality of herring eggs on different substrates in the southwest archipelago of Finland. Journal of Fish Biology, 34: 417-427.
  • Vahteri, P. & Vuorinen, I. (2001). The crash in Baltic herring reproduction: A case study in the northern Airisto spawning grounds, Archipelago Sea, SW Finland. Baltic Sea Science Congress, Abstract Volume.
  • Griffin, F.J., Pillai, M.C., Vines, C.A., Kääriä, J., et al. (1998). Effects of salinity on sperm motility, fertilization, and development in Pacific herring. Biological Bulletin, 194: 25-35.