The Atlantic cod lays its eggs in the open water. The eggs do not sink, but float within the water column. They lie suspended at the “halocline”. This is an abrupt boundary between a layer of light, lower-density fresh water floating above, and denser, more saline water below. The density of cod eggs is such that they sink in the fresh water but do not fall below into the denser salt water. For their optimal development the roe require salty and oxygen-rich water. In the Baltic Sea these parameters depend greatly on the influx of cool, saline and oxygen-rich North Sea water. If there is no inflow of North Sea water for an extended time, the Baltic Sea salinity decreases due to the input of river water, which also thickens the fresh-water layer above the halocline. The halocline thus deepens. But at greater depth the oxygen content decreases. In some cases the oxygen content is so low that the eggs develop poorly or not at all, especially in the deeper Baltic Basin. The conditions for development are improved again only with the next influx of fresh North Sea water.