Background. The Gotland Basin spawning ground is one of three main$backslash$nspawning areas of Baltic cod, Gadus morhua Linnaeus, 1758. The threshold$backslash$nwater parameters for cod development are the salinity exceeding 11% and$backslash$nthe oxygen level above 2 mL . L-1. Such conditions are only present when$backslash$nthe 11% isohaline is above the 2 mL . L-1 isooxygen. In such situation$backslash$nthe water volume between the isolines is called the
suitable$backslash$nreproduction volume'́. When the position of the isolines is reversed,$backslash$nthe salinity and the oxygen level of the water layer demarcated by them$backslash$nare below the required thresholds and as such the water is unsuitable$backslash$nfor the cod development. We refer to it as the unsuitable reproduction$backslash$nvolumé. The main aim of the presently reported study was to examine$backslash$nwhether variations in suitable and unsuitable reproduction habitat$backslash$nestimates could explain the fluctuations in cod recruitment.$backslash$nMaterial and methods. The suitable and unsuitable reproduction volumes$backslash$nin the Gotland Basin were estimated based on single point observations$backslash$nat three oceanographic monitoring stations using the contouring software$backslash$nBalthypsograph. To test the spatial hydrological heterogeneity in the$backslash$nGotland Basin we used 15 observations during 1969-1995 on four stations$backslash$nin each of two transects. The oceanographic monitoring and demersal$backslash$ntrawl research survey data from 1974-2012 have been used in analyses.$backslash$nResults. The sufficient reproduction conditions in the southern Gotland$backslash$nBasin persisted only until 1981. In later decades the reproduction$backslash$npotential of the Gotland Basin has decreased. The latest major North Sea$backslash$nwater inflows in 1993 and 2003 in the Central Gotland Basin formed the$backslash$nsuitable reproduction volume below the layers where cod eggs are$backslash$nfloating. A significant relation between the recruit abundance and$backslash$nsuitable and the unsuitable reproduction volumes was found only for the$backslash$nSouthern Gotland Basin (P textless 0.03).$backslash$nConclusions. We demonstrated that single-point estimates sufficiently$backslash$nquantify the reproduction conditions in the Gotland Basin and thus can$backslash$nbe used for estimation of the reproduction volumes. The unsuitable$backslash$nreproduction volume concept can be used as an ecological indicator for$backslash$negg survival probability in the Gotland Basin. However, it is too$backslash$npremature to re-define the concept of the suitable reproduction volume$backslash$nbecause it is applicable only to the Gotland spawning ground.