Essential fish habitat (EFH) map on Potential spawning areas for cod was prepared in PanBalticScope project (co-founded by the European Maritime and Fisheries Fund of the European Union) http://www.panbalticscope.eu/

Cod (Gadus morhua) is represented by three stocks in the Baltic Sea; Eastern Baltic, Western Baltic and Kattegat cod, which is reflected in the map. “Potential spawning areas” were initially delimitated based on Hüssy (2011). In addition, the Gdansk deep as delineated by Bagge et al. (1994) was included as it sometimes contributes to reproduction of Eastern Baltic cod (Hinrichsen et al. 2016). The Gotland basin has ceased to contribute to the reproduction of cod (Hinrichsen et al. 2016). These definitions were applied in the HOLAS II project (HELCOM 2018a) based on approval by all HELCOM Contracting Parties in a review process (there referred to as ‘occasional successful spawning’ and ‘successful spawning’). Following HELCOM (2018b) additional potential spawning areas were identified by environmental thresholds for egg development and survival based on salinity and oxygen conditions (Hinrichsen et al. 2016) during 2011-2016. Separate thresholds were used for Eastern Baltic, Western Baltic and Kattegat cod. Areas denoted “high probability spawning areas” correspond to where the initial delineations (Hüssy 2011, Bagge et al. 1994) achieve the environmental threshold values.

Stocks: Kattegat cod: ICES subdivision 21, Western Baltic cod: ICES subdivisions 22-24 Eastern Baltic cod: ICES subdivisions 24 + 25-32

EFH type: Potential spawning areas

Approach: Literature review combined with identification of environmental window for spawning based on: salinity and oxygen for Eastern Baltic cod, and on: salinity and depth for Western Baltic Cod and Kattegat cod

Variables and thresholds: Eastern Baltic cod: Salinity > 11, Oxygen > 1.5 ml/L (annual average)

Western Baltic cod and Kattegat cod: Salinity > 18, Depth >20 m

Quality: The Arkona deep is functional for spawning of both the Eastern and the Western Baltic cod and in effect, the definition of the Arcona Basin as a high probability areas in the Arkona basin reflect the result for Eastern Baltic cod.

The effective distribution of cod spawning areas is highly dependent on the prevailing hydrological regime, and the presence of spawning also depends on seasonally variable hydrographical conditions, such as temperature, salinity and oxygen. Seasonal differences lead to a progressive spawning season towards the east, typically starting in Kattegat and the Sound in January/February and ending in July/August in the Bornholm area. Fluctuations in temperature can delay the spawning season up to two months.

It is difficult to collect egg samples to verify cod spawning, as cod eggs may drift in deep areas, and instead the level of ichthyoplankton is a main source for estimation of good environmental conditions for cod spawning. Modelling based on ichthyoplankton should be validated by comparison with distribution of running adults, to resolve the potential influence of prevailing current speed. The proposed delineations are also influenced by research on the maturity of adults and histology of gonads.

The adult and juvenile cod are distributed far outside of the spawning areas depicted in the map.

Attribute information: Raster value representing no spawning (0), potential spawning area (0.5) and high probability spawning area (1).

References

- Bagge, O, F Thurow, E Steffensen, and J Bay (1994) The Baltic cod. Dana 10:1-28

- HELCOM (2018a) State of the Baltic Sea - Second HELCOM holistic assessment 2011-2016. Baltic Sea Environment Proceedings 155

- HELCOM (2018b) Outcome of the regional expert workshop on essential fish habitats, organized by Pan Baltic Scope project and HELCOM (HELCOM Pan Baltic Scope EFH WS 1-2018)

- Hüssy, K (2011) Review of western Baltic cod (Gadus morhua) recruitment dynamics. ICES Journal of Marine Science 68:1459-1471

- Hüssy, K, HH Hinrichsen, and B Huwer (2012) Hydrographic influence on the spawning habitat suitability of western Baltic cod (Gadus morhua). ICES Journal of Marine Science, doi:10.1093/icesjms/fss136

- Hinrichsen, HH, A Lehmann, C Petereit, A Nissling, D Ustups, U Bergström, and K. Hüssy (2016) Spawning areas of eastern Baltic cod revisited. Using hydrodynamic modelling to reveal spawning habitat suitability, egg survival probability, and connectivity patterns. Progress in Oceanography 143:13-25

SwAM (2019). Swedish Agency for Marine and Water Management. Symphony Metadata March 2019.w https://www.havochvatten.se/download/18.67e0eb431695d86393371d86/1552566811384/bilaga-1-symphony-metadata.zip