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Ecosystem considerations.Interaction of pollution and fish stocks. As a brackish-water sea, the Baltic has an extremely low number of species compared with other sea areas , and a unique mix of marine, brackish-water and freshwater species. In the north of the Bothnian Bay salinity is very low and only a few marine species and a number of tolerant freshwater species are able to survive. The number of marine species decreases from the boarder with Kattegat along a gradient to the northernmost Baltic, while the number of freshwater species decreases in the opposite direction. There are many gaps in our understanding of marine biodiversity and at present we do not know how many species given habitats support or whether there are species with a genetic make-up unique to specific sea areas. The principal threats to the biological diversity are eutrophication, toxic pollutants and the use of marine resources and waters through activities such as fishing, aquaculture, shipping, construction and extraction of mineral resources. The spread of non-native organisms and climate change could have serious consequences in the future. In order to facilitate future development of ecologically sound and productive marine ecosystems there is a need to study various biological interactions between species in specific reproduction and feeding areas. The need to reduce pollution with negative impacts on fish stocks There may be impacts on fish stocks resulting from the effects of contaminants on individual fish, particularly the effects of organic contaminants on the development of fish eggs and larvae. There is inadequate research on this topic and especially on how contaminant-related mortality of fish eggs and larvae, observed in experimental situations, may impact on populations of wild fish. In addition, contaminants may play a role in the aetiology of the M-74 syndrome in Baltic salmon, resulting in mortality of fry. The role and type of contaminants is still under investigation, along with the mechanisms of their action. Another type of impact relates to excessive nutrient inputs, which may cause increased phytoplankton primary production. Phytoplankton that are not consumed usually sink to near-bottom waters and ultimately the sea bed, where they decompose, using oxygen. In the Baltic Sea, in deep areas and in many areas under stratified or stagnant conditions, this may result in low oxygen concentrations or even anoxia, thus affecting benthic prey of demersal fish species (and occasionally the fish themselves). Low oxygen conditions that occur in water layers where cod eggs are buoyant affect the survival and development of these eggs. Impact of eutrophication Eutrophication caused by anthropogenic inputs of nutrients has had extensive effects on the Baltic marine ecosystem. Anoxia, caused by a combination of eutrophication and hydrographic/climatic conditions, occurs frequently in the western Baltic and in the deep waters of the central and northern Baltic, and has wiped out benthic communities over large areas of the seabed. In the case of cod, oxygen concentrations > 2 ml/l at the depths where eggs are neutrally buoyant are a necessary prerequisite for obtaining good recruitment, and anoxia in the bottom layer combined with low salinity at the spawning grounds reduces recruitment. The occurrence of oxygen in the bottom layer in the eastern and northern parts of the Baltic Sea is dependent on major inflows of oxygenated water from the North Sea. Impact of pollutants Toxic pollutants have had a great impact on animals at the tops of food webs, such as fish, seabirds and seals. Populations of seals crashed to extremely low levels due to combined effects of persistent organic pollutants and hunting. Effluents from pulp mills have harmed fish populations and altered patterns of genetic variation in mussel populations. Levels of toxic pollutants are now generally declining in marine biota, but levels are still high in sediments and Baltic seals still show signs of chronic disease. Changes in the food web and toxic pollutants probably contribute to the extensive mortality of salmon fry caused by the M74 syndrome in the Baltic Sea. Impact of human activities on fish stocks The utilization of coastal and offshore waters for activities such as fishing, aquaculture, shipping, recreation, electric or engineering projects, dredging, dumping, extraction of gravel and sand, and oil prospecting has a large combined effect on the biodiversity of species and habitats. Shallow-water sites, which are of particular importance as spawning and nursery grounds, are at risk from a combination of factors such as coastal development, tourism and eutrophication. Some waters of the Baltic Sea are of international importance as overwintering areas for seabirds. Hydroelectric schemes built on rivers flowing into the Baltic have reduced the number of rivers in which salmon spawn from around 60 to approximately 30. The genetic diversity of wild salmon (which according to ICES now constitute less than 10% of total salmon populations) has been eroded by a combination of damming, M74 and fishing based on massive stockings of smolt from the more endurable hatchery reared populations. About 30 non-native organisms have become established and most of the alien species are found close to shore but a few have become common in deep waters. Global warming Global warming due to the greenhouse effect could alter inputs of salt water, fresh water, oxygen, nutrients and pollutants with potentially large consequences for marine ecosystems and species. Change in currents would also influence the recruitment of organisms to coastal and offshore waters. Changes in the input of fresh water from rivers would have particularly large effects in the Bothnian Bay. Impact of Fisheries on the ecosystem; the need to protect species and habitats Fisheries have both direct and indirect effect on the Baltic Sea ecosystem. Foremost are the direct effects caused by the removal of fish and shellfish for landing. Other direct effects include, inter alia, mortality on non-target fish and shellfish and other animals. There is a threat to the biodiversity and a loss of habitats and biotops in certain parts of the Baltic Sea area. The protection of habitats and species in marine and coastal areas is essential for the functioning of the ecosystem of the Baltic Sea. The ecosystem should be used in a manner consistent with sustainable development, ensuring the conservation of biological diversity and the sustainable use of its components. This implies that adverse impacts of fishing activities on species and habitats in and around the Baltic Sea should be minimized by appropriate measures. In areas where fishing methods and practices are not consistent with the principle of responsible and sustainable fisheries, these methods or practices should be restricted or prohibited by the competent authorities. Bottom-trawling affects the bottom habitat while gill nets are a potential threat to marine mammals and sea birds. In cases where a disproportionately harmful ecological impact on species and habitats has occurred, appropriate measures for the restoration of biological diversity and habitats should be implemented. This includes, inter alia, the possible establishment of temporary or permanent closed areas. A precautionary approach should be applied to all human activities involving non indigenous stocks, alien species and genetically modified organisms. Further research should be encouraged in relation to:
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