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The herring living in the Baltic Sea is a low-fat and slow-growing subspecies of the herring family, and it can be caught in our sea regions all the year round. From the fisheries’ point of view the species caught is basicly determined by the catch area: the bigger and fattier herring of the North Atlantic is not caught in the territorial waters of Finland. In practice, the dividing point is the southern end of Sweden: east of the Kalmar region the fish caught is the membras -subspecies of Clupea harengus, that is Baltic herring. Herring move in shoals which come to the coastal areas for spawning and then return to their feeding grounds in the open sea. Herring normally reach their sexual maturity at the age of three years and are then 14 – 15 cm long. On the basis of their spawning season the stock is divided into two groups, the spring-spawning and the autumn-spawning herring. Most of the herring spawn in the spring from May to June, when the temperature of sea water reaches about +5ºC, which means that spawning starts first in the archipelago and in the southern waters. A smaller autumn-spawning stock spawns further out and in warmer waters of about 12-16ºC, from August to September. The spawning of Baltic
herring is most successful in areas with high salinity. The less
saline eastern parts of the Gulf of Finland and the Bothnian
Bay are not the best suited areas for the spawning of herring.
Herring spawn everywhere in the Baltic Sea, a behaviour different
from that of the Atlantic herring who gather in huge shoals in
specific spawning areas. The Baltic herring are spread evenly
on wide areas. Thus harvesting them does not easily endanger
the stock, contrary to the situation of the Atlantic herring.
The fluctuation of the size of year classes, which is typical
of the Baltic herring, is due to the natural conditions and the
amount of animal plankton they feed on.
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