Spawning areas

Characteristics of Baltic Herring Spawning

Majority of the Baltic stock reproduces in spring, but the spawning time forms a continuum, both spatially and temporally. In the southern Baltic, herring spawning starts at the end of February and ends in early May. In the Archipelago Sea, the first spawning shoals generally arrive at the spawning grounds in the beginning of May, and spawning continues to the middle of July or even to August. As individual fish release their gametes at one time, long spawning period is most likely a consequence of different maturation rates of fish in the herring stock.

In the Baltic Sea, herring spawns along the coastal zone, which extends from the eastern side of the Danish sounds to the northernmost parts of the Bothnian Bay. The most important and well-known spawning grounds are the Greifswalder Bodden in the south, the Gulf of Riga, the Stockholm Archipelago, and the Archipelago Sea in the southwestern Finland (see map).

Major spawning areas of the herring in the southern, central and northern Baltic Sea. AS=Archipelago Sea; SA=Stockholm Archipelago; GOR=Gulf of Riga; GB= Greifswalder Bodden, B=Bothnian Bay. Map on the right: Airisto inlet in the Archipelago Sea; blue circles indicate monitored spawning beds.
Major spawning areas of the herring in the southern, central and northern Baltic Sea. AS=Archipelago Sea; SA=Stockholm Archipelago; GOR=Gulf of Riga; GB= Greifswalder Bodden, B=Bothnian Bay. Map on the right: Airisto inlet in the Archipelago Sea; blue circles indicate monitored spawning beds.

Spawning Herring

In all parts of the Baltic Sea, herring spawns in shallow water (0-10 m) among aquatic vegetation. Baltic herring is not selective in the choice of a spawning substrate, but eggs can be found on a variety of vascular plants and algae that grow in the littoral zone. However, the presence of vegetation on the seabed is a prerequisite for spawning, among some other environmental factors. Being adhesive, herring eggs attach to aquatic plants growing on the bottom and normally keep tightly attached to the plant surface until hatching.

The bottom area having a vegetation cover has substantially decreased on the coastal areas of the Baltic Sea, due to the weakening of underwater light conditions. Eutrophication and high particle load prevent light from penetrating into such depths where submersed vegetation normally grows and plants either die or move to more shallow waters, where light is still available. This process leads to a gradual shrinking of potential and actual spawning beds of the herring. How this affects the herring stock is not known, but most likely the destruction of the spawning grounds results in poor recruitment and decreasing size of the stock in the long time run.

We have monitored the spawning beds of the herring in our study area, the Airisto Inlet, since the 1980’s. The previous general idea that Baltic herring spawn everywhere, where suitable environment is available, has proven to be false. Long-term monitoring indicates that herring spawn selectively, only in specific locations from year to year. The reason for the selective behavior is not known, but the use of traditional spawning beds is a typical feature for the herring also elsewhere, as well as for many other fish species, e.g. salmonids.

The spawning behavior has developed in the course of evolution and become a permanent character of the species, due to its benefits to the reproductive success. However, in the present Baltic Sea the traditional use of the spawning beds has become less profitable. Following their instincts, herring now spawn in areas, where environmental conditions are no more optimal for the development of eggs. For example, egg mortality in the Airisto Inlet is nowadays high due to increased eutrophication and high load of sediment which contain heavy metals and toxic substances, such as TBT. Majority of eggs die before hatching and, consequently, the larval abundance is low in the area at present. A new threat for the reproduction of herring is the detachment of eggs from the spawning substrate before hatching, which has been observed in SCUBA diving surveys of the spawning beds in Airisto. For an unknown reason, eggs are detached from their substrate, their development terminates and they are drifted away by waves and currents. It even seems that the major part of the spawn may be lost soon after spawning.

Related Publications

  • Rajasilta, M., Eklund, J., Kääriä, J. & Ranta-aho, K. 1989. The deposition and mortality of the eggs of the Baltic herring, Clupea harengus membras L., on different substrates in the south-west archipelago of Finland. – J. Fish. Biol. 34: 417-427.
  • Rajasilta, M., Eklund, J., Kääriä, J. & Ranta-aho, K. 1989. The deposition and mortality of the eggs of the Baltic herring, Clupea harengus membras L., on different substrates in the south-west archipelago of Finland. – J. Fish. Biol. 34: 417-427.
  • Rajasilta, M., Eklund, J. Hänninen, J., Kurkilahti, M., Kääriä, J. Rannikko, P. & Soikkeli, M. 1993. Spawning of herring (Clupea harengus membras L.) in the Archipelago Sea. – ICES J. mar. Sci. 50: 233-246.
  • Kääriä, J., Rajasilta, M., Kurkilahti, M. & Soikkeli, M. 1997. Spawning bed selection by the Baltic herring (Clupea harengus membras) in the Archipelago Sea of SW Finland. – ICES J. mar. Sci. 54: 917-923.
  • Griffin, F.J., Pillai, M. C., Vines, C. A., Kääriä, J., Hibbard-Robbins, T., Yanagimachi, R. & Cherr, G. N. 1998. Effects of salinity on sperm motility, fertilization, and development in the Pacific herring, Clupea pallasi. Biol. Bull. 194: 25-35.
  • Vahteri, P. & Vuorinen, I. 2001. The crash in Baltic herring (Clupea harengus membras L.) reproduction – a case study in the northern Airisto spawning grounds, Archipelago Sea S-W Finland. Baltic Sea Science Congress 2001, Abstract Volume. Stockholm University, Stockholm, Sweden.