The Baltic Sea is a large semi-enclosed sea
that receives salt water from the North Sea and Kattegat through
the Danish Straits and fresh water from surrounding rivers and
precipitation. In the deeper areas of the Baltic Sea south of
the Aaland Island there is a permanent salinity stratification
with a low salinity surface layer (app. 7 psu) and a deep saline
layer (11-17 psu). The halocline separating the surface layer
from the saline layer forms an effective barrier to thermal convection.
As a result, the volume and hydrographic conditions (i.e. salinity,
temperature and oxygen content) of the deep saline water depend
on the magnitude and intensity of inflows from the North Sea,
the distance from the Danish Straits and the oxygen consumption
rate.
The fresh surface water exits the basins through
the shallow straits in the western Baltic, i.e. west of the Bornholm
Island, while oxygenated, saline water flows in at depth. The
inflowing water is mainly collected by three deep basins in the
eastern Baltic, reaching first the Bornholm Basin where the water
after a transition time, may pass to the eastern Gdansk Deep and
the north-eastern Gotland Deep. The depth of halocline increases
towards East and North in accordance with the increasing volume
of fresh water relative to saline water.
The inflow of saline water through the Danish
Belts is a major determinant of the oxygen and saline conditions
in the deep basins of the eastern Baltic (Bornholm, Gdansk and
Gotland Deeps). This inflow is wind driven and occurs with steady
and strong northwesterly winds over the North Sea and the Skagerrak.
In 1976 there was a strong inflow but between 1981 and 1993 no
strong inflows were observed. In 1993 and in 1994 there were two
moderate inflows but no inflows since then. The later part of
the period without inflow from the mid 1980s to the beginning
of the 1990s corresponds to years with poor cod recruitment. After
the inflow in 1993-94 the oxygen conditions have improved and
the bottom salinity has increased in the southern and central
part of the Baltic. However, the recruitment has not returned
to the high levels observed from the mid 1970s to the early 1980s,
indicating that other factors are also important (such as the
spawning stock biomass, which is low, and the high proportion
of first time spawners giving low quality eggs). Sprat predation
on cod eggs may also be important at least in the western part
of the Baltic.
The distribution of fish in the Baltic Sea
is salinity dependent. The marine fish, e.g. cod, need a minimum
salinity while the fresh water species also found in the Baltic
cannot tolerate a high salinity. Therefore the fresh water species
are found in the coastal areas and in the northern parts of the
Baltic Sea, e.g. in the Gulf of Bothnia while the marine species
dominates in the Baltic Proper and the Western Baltic. The individual
adult can tolerate lower salinities but requires higher salinity
for successful reproduction. Many of the species have floating
eggs and for these species the egg will sink to the bottom and
die at lower salinities. Also the water mass associated with neutral
bouyancy must have a minimum oxygen content to allow the eggs
to survive around at 2 ml/l. The table below presents the minimum
salinities required for reproduction for a number of species
| Species | Floating eggs | Minimum salinity for reproduction (psu) |
| Cod | ||
| Herring | ||
| Sprat | ||
| Plaice | ||
| Flounder | ||
| Dab | ||
| Turbot |
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