Habitat and Biology
Depth zone: Shelf (50 m - 200 m). Vertical distribution: Pelagic.
Herring biomass is dependent on the cod stock through predator–prey interactions, and on sprat through competition. Regional differences in growth rate result in a high proportion of small individuals in the north (Subdivisions 28.2, 29, and 32) and large individuals in the south (Subdivisions 25 and 26). The strong increase in sprat stock size since the early 1990s in the northern areas (Subdivisions 27–29 and 32) exacerbated the inter-specific competition and the decrease in herring weight-at-age especially in these northern areas. Herring mean weights have stabilized since the late 1990s, but remain low.Environmental influence on the stock
The decline in SSB of Central Baltic herring was partly caused by a reduction in mean weights-at-age. Growth rate tends to change due to salinity variations, changes in zooplankton (prey) community, and competition with the Baltic sprat, i.e. density-dependent effect.
Recently, a strong increase of cod has occurred in the southern Baltic (mainly in Subdivision 25 and, to a lesser degree in Subdivision 26), whereas in the northern areas (Subdivisions 27–32) no significant increase has been noticed. The increase of cod in Subdivision 25 might have a significant effect on herring in this area, but very limited effect on the whole central Baltic herring population.
Jurisdictional distribution: Shared between nations
Considered a single stock: Yes
The pelagic fisheries take a mixture of herring and sprat and this causes uncertainties in the catch of each species. The extent to which species misreporting has occurred is not well known. Since 2006 the restrictions on unsorted landings, including EU member states obligation to ensure adequate sampling, may have improved the accuracy of estimating proportions of sprat and herring in the catches.
Effects of the fisheries on the ecosystem
|Catch distribution ||Total landings (2011) are 117 kt. Discards are considered to be low. |
As both herring and sprat are the major prey of cod, the mixed pelagic fishery can indirectly affect the cod stock.
ICES advises on the basis of the transition to the MSY approach that catches in 2013 should be no more than 117 000 tonnes.
Overall Assessment Results
|Figure 188.8.131.52 Herring in Subdivisions 25–29 and 32 (excluding Gulf of Riga herring). Summary of stock assessment (SSB and recruitment in 2012 predicted). Top right: SSB and F for the time-series used in the assessment. |
Assessment ModelQuality consideration
There are uncertainties related to mixed landings of herring and sprat. It would be beneficial to have a higher sampling coverage of the species composition of the small-mesh industrial fisheries targeting sprat in Subdivisions 27–29 and 32 to decrease the potential uncertainty. The overall biological sampling (length and age data) seems to be sufficient. However, for Germany it is difficult to monitor the national fishing activities since a larger part of the herring/sprat catches are landed in foreign ports.
|Figure 184.108.40.206 Herring in Subdivisions 25 to 29 and 32, excluding the Gulf of Riga. Historical performance of the assessments. Fpa and FMSY are indicated as horizontal lines in the middle panel. |
|Assessment type ||Age-based analytical assessment (XSA). |
|Input data ||One acoustic survey index (BIAS) and catch-at-age data. |
|Discards and bycatch ||Discards are not included, but are considered to be low. The bycatch of sprat and juvenile cod is unknown. |
|Indicators ||None. |
|Other information ||The latest benchmark was performed in 2004. A new benchmark is planned for 2013. |
|Working group report ||WGBFAS |
Management unit: YesManagement plans
No specific management objectives are known to ICES.
Biological State and Trend
Exploitation rate: Above target
Abundance level: Undefined
SSB in 2011 (628 000 t) was 70% of the long-term (1974–2011) average. Fishing mortality has been above Fpa
since the beginning of the 1980s. The last stronger year classes were the 2002 and 2007 year classes. Both year classes are, however, just above the long-term average.
Source of information
The above excerpts are from the first two pages of the ICES advice, the supporting information to this advice can be read in full at the following reference:
ICES. Herring in Subdivisions 25–29 and 32 (excluding Gulf of Riga herring). Report of the ICES Advisory Committee, 2012. ICES Advice, May 2012. http://www.ices.dk/committe/acom/comwork/report/2012/2012/Her-2532-Ex-Go.pdf