Baltic cod reproduction in the Gotland Basin: causes of annual variability Maris PLIKSHS 1) , Hans-Harald HINRICHSEN 2) , Didzis. ELFERTS 1,4) , Ivo SICS 1) , Georgs KORNILOVS 1) and Friedrich W. KÖSTER 3) 1) Institute of Food Safety, Animal Health and Environment “BIOR”, Riga, Latvia 2) Leibniz-Institute of Marine Sciences (IFM-GEOMAR), Kiel, Germany 3) Technical University of Denmark, National Institute of Aquatic Resources, Charlottenlund, Denmark 4) University of Latvia, Faculty of Biology, Riga, Latvia Background: The Gotland spawning ground is one of three main spawning areas of the Baltic cod. The concept of ”reproduction volume“ was developed to quantify Baltic cod reproduction habitat. This habitat is defined by water volume below the halocline with salinity >PSS 11 and O 2 >2mL L -1 . Most of the years there is no such volume in Gotland Basin and therefore successful reproduction is impossible. As the survived eggs and cod larvae is recorded in years when physical environment is outside the limits necessary for successful development on early stages of ontogeny, to characterise this conditions we suggest the “unsuitable reproduction volume” (volume with salinity <PSS 11 and oxygen saturation <2mL L -1 ). The main aim of the study is to examine whether variations in suitable and unsuitable reproduction volume quantitative estimates can explain the fluctuations in cod recruitment. The results may be valuable to understand the cod reproduction peculiarities in the Gotland Basin. Methods: The reproduction and unsuitable reproduction volumes in the Gotland Basin were estimated based on single point observations at three oceanographic monitoring stations using the contouring software Balthypsograph. To test the spatial hydrological heterogeneity in the Gotland Basin we used 15 observations during 1969-1995 on four stations in each of two transects. Influence of environment parameters on the abundance of recruits was explored by multiple regression analysis. Results: Only Southern Gotland basin had sufficient reproduction conditions till 1981. In later decades reproduction potential of the Gotland Basin has decreased. Latest major inflows in 1993 and 2003 in the Central Gotland basin formed reproduction volume that due to egg neutral buoyancy features was not utilised. Significant relationship between recruit abundance and suitable and unsuitable reproduction volumes was found only for the Southern Gotland basin (p<0.03). Conclusions: We demonstrated that single point estimates sufficiently quantify the reproduction conditions and thus can be used for estimation of the reproduction volumes in the Gotland Basin. Single strong inflows after long stagnation period in eastern spawning grounds cannot improve the environment where cod egg development occurs. However, it is too premature to re-define the concept of reproduction volume because it is applicable only to the Gotland spawning ground of cod. Key words: Baltic Sea, Gotland Basin, Eastern Baltic cod, recruitment, reproduction volume, unsuitable reproduction volume. INTRODUCTION Cod is a temperate marine fish that spawns in saline water layers of the continental shelf and produces pelagic eggs. Conditions in the Baltic, with a surface salinity of PSS 7-8 and bottom salinity up to PSS 10-17, are marginal for marine species. This generally confines cod spawning to four deep areas: the Bornholm Basin, the Gdansk Deep, the Gotland Basin and to a less extent the Slupsk Furrow (Bagge et al.1984). As a result, cod eggs in the Baltic are concentrated in deep waters below a permanent halocline (Grauman 1984; Wieland and Jarre- Teichman 1997; Wlodarczyk and Horbowa 1997; Makarchouk and Hinrichsen 1998). Due to the irregularity of water exchange between the Kattegat and the Baltic Sea and to limited vertical mixing, the hydrological conditions in the cod spawning and their egg concentration areas significantly vary. Periods between the inflows are characterised by oxygen depletion and decrease of salinity. Therefore, the volume of water that provides minimum