Habitat and Biology
Depth zone: Shelf (50 m - 200 m). Vertical distribution: Pelagic.
The year-class strength of the Gulf of Riga herring is strongly influenced by the severity of winter, which determines the water temperature and the abundance of zooplankton in spring. A series of mild winters since 1989 has been favourable for the reproduction of Gulf of Riga herring and resulted in a series of rich year classes for the period 1989–2010; the year classes were below average only in 1996, 2003, 2006, and 2010 after cold winters. Due to favourable reproduction conditions the SSB has been high since the beginning of the 1990s. The mean weight-at-age started to decrease in the mid-1980s and in 1997 reached the lowest values, especially in the older age groups. Afterwards the mean weight-at-age increased and since 2000 it has fluctuated without a clear trend, being still much lower than in the 1980s.Environmental influence on the stock
The Gulf of Riga is a semi-enclosed ecosystem of the Baltic Sea characterized by low salinity that restricts the occurrence of marine species. The predation mortality by cod is likely to be low because cod is found in the Gulf of Riga only in periods when the cod stock size is very high (last time in the early 1980s).
Jurisdictional distribution: Shared between nations
Considered a single stock: Yes
The herring fishery in the Gulf of Riga is performed by Estonia and Latvia, using both trawls and trapnets. In the recent years the share of trapnets has been slightly above 30% and has been rather stable. Herring catches in the Gulf of Riga include the local Gulf of Riga herring and the open-sea herring, which enters the Gulf of Riga for spawning.
Effects of the fisheries on the ecosystem
||Total herring landings in Gulf of Riga (2011) are 29.6 kt (69.8 % trawls and 30.2% trapnets). No discards or unallocated removals have been taking place. All landings are for human consumption.
Pelagic trawl is the main fishing gear used in the trawl fishery. The bycatch of sprat is low (about 10% in recent years), and bycatch of other species is insignificant. The bycatches of other species in herring trapnets are also very low. Discarding in the herring fishery is not allowed and has not been observed by on-board sampling.
ICES advises on the basis of the MSY approach that catches in 2013 should be no more than 23 200 tonnes.
Overall Assessment Results
|Figure 22.214.171.124 Herring in Subdivision 28.1 (Gulf of Riga). Summary of stock assessment (predicted recruitment values are shaded). Top right: SSB and F for the time-series used in the assessment. |
Assessment ModelQuality consideration
The sampling of trawl and trapnet catches was performed by Estonia and Latvia on a regular basis; there are no gaps in fisheries coverage. The amount of unallocated catches has been gradually decreasing in the recent years and in 2011 it was considered that there are no unallocated catches of the Gulf of Riga herring.
|Figure 126.96.36.199 Herring in Subdivision 28.1 (Gulf of Riga). Historical assessment results (final-year recruitment estimates included). |
||Age-based analytical assessment (XSA).
||One acoustic survey index (BIAS);
One commercial cpue index (trapnets).
|Discards and bycatch
||No indications of discarding of target or non-target species.
||The latest benchmark was performed in 2008.
|Working group report
Management unit: YesManagement plans
No specific management objectives are known to ICES.
Biological State and Trend
Exploitation rate: Above target
Abundance level: Above trigger
|Figure 188.8.131.52 |
The estimated SSB in 2011 is 95 900 tonnes, well above the MSY Btrigger
biomass of 60 000 t. Following high recruitment, SSB increased in the late 1980s and is currently estimated to be above the long-term average. The year classes of 2005, 2007, and 2009 are strong, while the 2006 and 2010 year classes are poor.
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 Subdivision 28.1 (Gulf of Riga). Report of the ICES Advisory Committee, 2012. ICES Advice, May 2012. http://www.ices.dk/committe/acom/comwork/report/2012/2012/Her-riga.pdf