The diversity, composition and distribution of the Baltic fish fauna is influenced by the brackish-water character of the Baltic Sea, the two-layered water mass and the variable environmental conditions. The fish fauna include marine species (e.g. cod, flatfish, sprat and herring), anadromous (e.g. Atlantic salmon and Sea Trout) and catadromous species (e.g. European eel) as well as fresh water species (e.g. pike and perch). The fish have immigrated at different times by different ways. However, the distribution pattern of the various species reflects their original habitat and salinity tolerance. Thus, the number of marine species is highest in areas near the Danish Straits and diminishes eastwards and northwards, while the number of fresh water species increases when salinity decreases.

Besides the salinity tolerance, the spreading of marine fish is limited by wintering and reproduction conditions, the latter affecting especially fish having floating eggs. Most species living in the Baltic Sea have adapted to the environment in various ways and differ from fish of the same species living e.g. in the North Sea or in fresh water.

In marine fish the most important adaptations are: 1) a general decrease in body size and growth rate towards East and North, 2) and increase in egg diameter of the species having floating eggs and in fecundity and 3) changes in morphological characters and the existence of a large number of populations or sub-populations, especially in species closely connected with the coast.

Marked fluctuations are characteristic of the Baltic Sea ecosystem. The hydrography of the Sea is largely regulated by the sporadic inflows of saline North Sea water and the intermediate stagnation periods. In spite of all protection measures taken by all countries around the Baltic Sea, the sea itself reflects the changes very slowly since the renewal of the water takes 25-30 years.

In the Baltic Sea eutrophication is still the main environmental problem although the symptoms of eutrophication (increased plant biomass, oxygen deficiency in the bottom water) have decreased in some coastal areas where the reduction of nutrient inputs has been substantial. During the 1980s the winter concentrations of nitrate showed an overall increase in the surface layer of all sub-regions of the Baltic Sea and remained unchanged on a high level until mid-1990s. In the Gulf of Riga, nitrate values dropped sharply in the 1990s due to reduced land-based inputs. The winter concentrations of phosphate showed an overall decrease in the surface layer of almost all sub-regions of the Baltic Sea. In some coastal areas, the decrease of phosphate is closely linked to the construction of sewage treatment plants.

There are in total about 100 fish species living in the Baltic Sea Region. Tables 1 and 2 contain those of some commercial importance such as cod, herring, sprat, salmon, sea trout, garfish, eel, the flatfishes - flounder, plaice, dab, turbot and brill and brackish and fresh water species -white fish, perch, pike, pike perch, vendace, vimba, bream, eelpout, roach, smelt and three spined stickleback. HELCOM has adopted a list of endangered species; there are a few fish species on that list notably sturgeon.

It should also be noted that the biomass of clupeids is positively related to the availability of soluable Phosphorus and then contribute to the eutrophication.

In the Baltic Sea, about 30 phytoplankton species have been proved to be harmful. Toxic events and poisoning caused by blue-green algae have been documented since the early 1960s. There are indications that the frequency and the spatial coverage of harmful blooms in the Baltic Sea have increased.

The information available on macrophytobenthos strongly suggests that general changes have taken place during the recent decades along the coasts of virtually the whole of the Baltic Sea Area. The depth distribution of perennial macrophytes, attached to the seabed, has decreased, and short-lived filamentous or thin-bodied epiphytic or drifting algae have become increasingly prevalent in recent time. These changes are most easily explained by a higher input of nutrients during this period.

Some improvement of the environmental situation has also been observed, such as improved sanitary conditions in coastal waters. In the open sea decreased concentrations of PCB, DDT, HCH, HCB, and lead in biota have been observed. Since 1991 the levels of artificial radioactivity, particularly for 137Cs, in the Baltic Sea have shown a general decreasing trend in sea water and biota. The same decreasing trend applies to the transfer of artificial radionuclides to man and the subsequent doses.