Habitat and Biology
Depth zone: Shelf (50 m - 200 m). Vertical distribution: Pelagic.
Sprat biomass is strongly dependent on the overlap with the cod stock through predator–prey interactions. Sprat biomass was low in the 1980s when the cod stock was high. A decline in cod biomass and favorable conditions for sprat recruitment led to the development of sprat to a record high in the 1990s. High stock size resulted in a marked decline in sprat mean weights (density-dependent effects). After the 1990s the sprat stock size increased mainly in the northern areas (Subdivisions 27–29 and 32), where cod decreased the most, exacerbating the decrease in mean weights especially in these areas. The decline of the stock in numbers may to some extent be compensated by an expected increase in weights-at-age because of density-dependent effects on growth.Environmental influence on the stock
Since the 1990s, trends in Baltic sprat have been driven mainly by reduced predation by cod and variable, but high recruitment. Recently, a strong increase of cod has occurred in the southern Baltic (especially in Subdivision 25 and, to a minor extent, in Subdivision 26), whereas no significant increase has been noticed in the northern areas (Subdivisions 27–32). The increase of cod in Subdivision 25 will have a strong effect on sprat in this area, but very limited effect on the whole Baltic sprat population which is currently out of reach for cod, at least in some seasons.
Jurisdictional distribution: Shared between nations
Considered a single stock: Yes
The mesh size (minimum of 16 mm) and TAC are the main regulatory measures adopted for the Baltic sprat fishery. Landings usually do not exceed the TAC, and in 2010 the EU TAC was not taken. Discarding of herring and sprat in the Baltic has been prohibited in the EU fisheries since 2010.
Effects of the fisheries on the ecosystem
|Catch distribution ||Total landings (2011) are 268 kt. Most of the catch is taken by pelagic trawlers, discards are negligible. |
Because sprat and herring are the major prey for cod, the mixed pelagic fishery can indirectly affect the cod stock. On the other hand, a smaller stock size of sprat would release its pressure on the consumption of cod eggs that in some areas and periods may be substantial.
ICES advises on the basis of the MSY approach that catches in 2013 should be no more than 278 000 tonnes and furthermore that a spatial management plan needs to be developed.
Overall Assessment Results
|Figure 18.104.22.168 Sprat in Subdivisions 22–32 (Baltic Sea). Summary of stock assessment. Top right: SSB and F for the time-series used in the assessment. |
Assessment ModelQuality consideration
The assessment shows a historical retrospective pattern, with a tendency to underestimate the SSB and overestimate the F. In some fisheries the species composition of catches in the mixed industrial fishery is imprecise which may add additional uncertainty to the assessment.
Collection of co stomach contents data would improve the data basis for application in multispecies stock assessment models.
|Figure 22.214.171.124 Sprat in Subdivisions 22–32 (Baltic Sea). Historical assessment results (final-year recruitment estimates included). |
|Assessment type ||Age-based analytical assessment (XSA). |
|Input data ||Three survey indices from two acoustic surveys (BASS: May; BIAS: October, BIAS for age 0). |
|Discards and bycatch ||Discards are not included and are considered to be negligible. |
|Indicators ||None. |
|Other information ||The latest benchmark was performed in 2005; the next one is planned for 2013. |
|Working group report ||WGBFAS |
Management unit: YesManagement plans
The International Baltic Sea Fishery Commission (IBSFC) long-term management plan for the sprat stock was terminated in 2006 and has not been replaced.
Biological State and Trend
SSB has declined from a historical high in the late 1990s, and the SSB in 2011 was estimated at close to the long-term average. The fishing mortality in 2011 declined to 0.29, which is the lowest estimated for the past ten years. None of the recent three year classes (2009–2011) are strong; the 2009 year class is estimated to be weak, the 2010 close to average, and the 2011 year class is predicted to be close to the average. SSB and recruitment in 2012 are predicted values.
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. Sprat in Subdivisions 22–32 (Baltic Sea). Report of the ICES Advisory Committee, 2012. ICES Advice, May 2012. http://www.ices.dk/committe/acom/comwork/report/2012/2012/spr-2232.pdf