Pelagic fishes: Herring (Clupea harengus)

 

The most important pelagic fish in the Baltic Sea fisheries are herring and sprat. Their distribution varies with season, food availability, hydrography etc., but in general the concentrations of the clupeids are largest in the zone of intense mixing of the homo- and heterohaline layers. Herring occur throughout the Baltic Sea including the Gulf of Bothnia, Gulf of Finland and Gulf of Riga. Sprat are confined to areas south of the Aaland Islands and are mainly found in the open sea areas.

 

Herring populations include both spring and autumn spawners. Previously autumn spawning herring dominated the herring populations but there was a change in the 1960s and since the spring spawning components have dominated the populations. Herring populations form a continuos chain extending from the North Sea to the northernmost parts of the Baltic Sea. Neighbouring groups mix not only on feeding and wintering grounds, but also on the spawning grounds. Therefore, there is no sharp biological difference between neighbouring groups of spring and autumn spawners. However, the difference between Atlantic and Baltic populations increases towards East and North in the Baltic.

 

In the Western Baltic (Sub-divisions 22-24) the spring-spawning stock of herring (the Rügen herring) dominate. These herring spawn in the Western Baltic Sea, but migrate to feeding areas in the Kattegat, Skagerrak and the eastern part of North Sea where they mix with the North Sea herring.

 

Both spring and autumn herring can be further divided into two population types - the sea and the coastal or Gulf herring. Within each population there are numerous sub-groups.

 

Spring herring spawn in shallow areas at salinities down to 3 promile salinity unit (psu). The autumn herring spawn on off shore banks or slopes. Both populations spawn mainly on the bottom vegetation and fasten the eggs to the substrate.

 

The main food of the herring is plankton and benthos. Depending on the seasonal and spatial composition as well as the abundance of species, the diets may differ. Young stages of fish may also serve as food for larger herring.

 

Herring in the western Baltic (Sub-division 22-24) is assessed as one stock. Herring in the eastern Baltic is assessed as four stocks, i.e. herring in Sub-divisions 25-29 (including the Gulf of Riga) and 32, herring in the Gulf of Riga, herring in Sub-division 30 and herring in Sub-division 31. The herring in the central part of the Baltic sea (Sub-divisions 25-29+32) is much the largest stock. The separation of stocks is a compromise between the large number of stocks/populations identified on biological basis and the ability to allocate of catches to stocks. Also management of the herring stocks are not possible on population by population basis.

 

Analytical assessments are available for the Eastern Baltic herring stocks based on landing statistics and survey data. The scientific parameters used to evaluate the biological status of the stocks include the spawning stock, recruitment, fishing mortality and species interactions. The management advice for herring in Sub-divisions 22-24 and Division IIIa is related to the autumn spawning herring in Division IIIa with which it mixes.

 

State of herring stocks

 

The pelagic stocks in the Baltic are in general exploited at a low or medium levels and the spawning stocks are at or above their respective long-term average levels however the spring spawning herring in Sub-division 22-24 and Division IIIa is exposed to higher mortality levels than the more easternly herring stocks.

 

The absolute levels of stock size for the pelagic stocks is difficult to estimate, whereas the development of stock sizes in relative terms is better described. As the fishing mortality is low compared with the natural mortality, the catch analysis is less reliable than is most cases. There are inconsistencies between years in the results from acoustic surveys and low precision in the estimates of species composition in the mixed fisheries. This has contributed to the variation in stock estimates given in the latest years.

 

State of herring in Sub-division 22-24 (Rügen herring)

 

The migration of older herring to the Kattegat, Skagerrak and eastern North Sea creates difficulties in allocation of catches. Because of uncertain catch statistics and conflicting trends in survey indices no reliable assessment is available for the spring-spawning stock of herring Sub-division 22-24 and Division IIIa.  The available data  indicate that the Spawning Stock Biomass has been relatively stable over the last few years and there are no indications for overfishing.

 

State of herring in Sub-divisions 25-29 (including Gulf of Riga) and 32

 

The exact stock status is uncertain. ICES considers that the Spawning Stock Biomass has continued to decrease and is at a low level.

 

Herring in the Gulf of Riga

 

The stock component is at present considered to be within safe biological limits. Spawning Stock Biomass  and recruitment have been high since 1990.

 

Management of herring fishery

 

Herring is mainly exploited in the open sea by trawls (pelagic single- and pairtrawls) and in coastal water during spawning time both by trapnets, pound-nets and gillnets.

 

IBSFC manages herring in two management areas: the main basin in the Baltic and the so-called "Management Unit III", corresponding to the northern part of Sub-division 29, the Bothnian Sea and the Bothnian Bay. Annual TACs are set at a higher level than expected catches. This allows for higher catch quota to certain Contracting Parties who fully utilise their quota without altering traditional allocation keys between Contracting Parties.

 

IBSFC is working on a Long Term Management Strategy for the Herring Stocks  (Working Group meeting in June 2001). In order to protect the stocks IBSFC has reduced the Herring TACs from 1998 (670 000 tonnes) to 2001 (372 000 tonnes) by 46 %.