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

Baltic flounder (Platichthys solemdali) is a key species in many coastal areas of the Baltic Sea. It is the only endemic fish species of the Baltic Sea. Baltic flounder spawns in shallow coastal areas and on offshore banks, with eggs developing on the sea floor. Successful spawning may be expected at salinities down to around 5-7 (Nissling et al. 2002). ‘‘Potential spawning areas’ were initially delineated by a species distribution model (Orio et al. 2017) developed based on years 1993-1997 to consider a period with relatively better oxygen conditions, but applied with more recent data (2011-2014). The area was further delineated to encompass only areas shallower than 30 m in order to represent the demersal spawning habitat. ‘High probability spawning areas’ were identified as the sub-section encompassing salinity > 6. It should be noted that flounders in the Baltic Sea were recently separated into two species, and that spawning areas of the European flounder (Platichthys flesus) are described separately. The two data layers do not overlap and can be combined to obtain a map on spawning areas for both flounder species taken together.

Stocks: ICES identifies two stocks of Baltic flounder: ICES subdivisions 26, 28 (East of Gotland and Gulf of Gdansk), and 27, 29-32 (Northern Central Baltic Sea and Northern Baltic Sea).

EFH Type: Spawning areas

Approach: Species distribution modelling combined with identification of environmental salinity window and depth conditions for spawning, supplemented with additional information from monitoring in Estonian waters.

Variables and thresholds: Depth < 30 m, Salinity > 6

Quality: The data layer is based on species distribution modelling focusing on mature flounder at the spawning stage and should be considered a rough estimation.

The data layers on environmental variables are based on modelling. Other variables than those tested in the model may also be influential.

The studies from which the thresholds values for environmental variables have been obtained are based on publications conducted before the separation of Baltic flounder from European flounder but have taken the specific characteristics of the separate spawning ecotypes into account.

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

References:

- Momigliano, P, GP Denys, H Jokinen, and J Merilä (2018) Platichthys solemdali sp. nov. (Actinopterygii, Pleuronectiformes): a new flounder species from the Baltic Sea. Frontiers in Marine Science 5:225

- Nissling, A, L Westin, and O Hjerne (2002) Reproductive success in relation to salinity for three flatfish species, dab (Limanda limanda), plaice (Pleuronectes platessa), and flounder (Pleuronectes flesus), in the brackish water Baltic Sea. ICES Journal of Marine Science 59:93-108

- Orio, A, U Bergström, M Casini, M Erlandsson, R Eschbaum, K Hüssy, A Lehmann, L Ložys, D Ustups, and A-B Florin (2017a) Characterizing and predicting the distribution of Baltic Sea flounder (Platichthys flesus) during the spawning season. Journal of Sea Research 126:46-55

- Seifert, T, F Tauber, and B Kayser (2001) A high resolution spherical grid topography of the Baltic Sea -2nd edition. Baltic sea Science Congress, Stockholm 25-29 November 2001, Poster #147