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: