Baltic Rim Economies - Baltic Port List

Expert article 913 Baltic Rim Economies, 21.12.2011 Quarterly Review 5�2011
Dioxin in Baltic salmon and herring – is it a toxicological problem?
By Mikko Nikinmaa
Big Baltic salmon and herring often contain dioxin levels
that exceed the limits set to food items in the European
Union. The permissible level agreed upon is solely a
convention. Setting a limit based on scientific grounds
would be impossible, as for example the acute toxicity of
dioxin in different rat strains varies 10000-fold. Setting an
equal limit for all food items does not take into account that
the consumption of different items varies markedly. In
Finland the milk consumption per day exceeds the
consumption of Baltic herring and salmon per month. Yet
the dioxin limits for both food sources are the same.
After 1970’s the levels of both dioxin and PCBs
(polychlorinated biphenyls, their toxicity is often given as
dioxin equivalents) in the Baltic Sea environment have
decreased, mainly as the result of increasing efficiency of
water cleaning in paper and pulp industry. The decreased
environmental contamination has been seen in Baltic Sea
animals. Whereas seals in 1960’s and 1970’s were quite
often infertile, at present their reproduction is so effective
that they are a major fish consumer in the Baltic. The
estimated population of grey seal in the Baltic Sea is
currently approaching 10000; a five-fold increase from the
population below 2000 in 1970’s.
Despite the fact that both dioxin and PCBs have not
been released in the environment in significant amount
during the past years, they are still found in quite high
concentrations. The major factors contributing to this are
that the compounds are very stable and lipohilic.
Consequently, they accumulate in organisms and
concentrate in top predators such as salmon and seals.
Since salmon and herring are typically quite fatty fish,
lipophilic toxicants, such as dioxin, accumulate in them
easily. Because dioxin and PCBs are very stable they are
included in persistent organic pollutants (POPs).
Owing to the facts that dioxin concentration in Baltic
Sea and its organisms is on the decline, that the
permissible level is based on agreement and not hard
scientific evidence and that the agreed permissible levels
do not take into account the likely differences of intake, one
can conclude that the presently observed dioxin levels in
Baltic herring and salmon are not toxicologically important.
They do not present a threat either to the organisms
themselves or humans that are eating them.
Although one of the factors causing high dioxin levels in
herring and salmon is that they are fatty fish, the regulation
of dioxin levels in fish is poorly known. Dioxin and many
other aromatic hydrocarbons go to the aryl hydrocarbon
receptor (AhR) –dependent biotransformation pathway to
be transformed to excretable forms. Because research on
aryl hydrocarbon receptor started from toxicological angle,
the protein is often called dioxin receptor. However,
although the biotransformation pathway handles organic
man-made toxicants, it did not evolve because of the
recently produced artificial compounds such as dioxin.
Rather, the pathway exists in animals as diverse as the
nematode Caenorhabditis elegans and man. One of the
functions that the AhR-dependent pathway is involved in is
the development of neural system.
The ligands that the AhR-pathway has evolved to
handle are poorly known. In addition to the involvement of
the pathway in the development of neural systems (with
unknown ligands), it may have evolved for the
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biotransformation of toxic compounds in food, possibly of
any coloured compounds (which are often aromatic
molecules), or to treat breakdown products of compounds
like haemo- and other globins or chlorophylls.
In fact, treating toxic food compounds may be the
reason why dioxin remains at elevated levels in salmon and
herring. The foodstuffs eaten by fish and by terrestrial
domestic animals are markedly different. The compounds
contained in cyanobacteria, phytoplankton and zooplankton
are taken in by aquatic animals and will be transferred to
the highest trophic level, top predators. Thus, these
animals will need to be able to treat all the compounds
ingested in the normal food. The compounds reaching the
aquatic, mainly animal-eating, fish, and terrestrial domestic,
mainly plant-eating, animals, are necessarily quite different.
So, if the AhR-pathway plays a role in treating toxic
compounds in food, one can expect that the structure of
aryl hydrocarbon receptors in fish and mammals is
different. Owing to the different structures of the receptors
their ability to treat unnatural ligands such as dioxin can be
markedly different. The possibly important role of the aryl
hydrocarbon receptor in treating compounds contained in
the natural food of aquatic animals is suggested by the fact
that fish have evolved a more versatile AhR system than
any terrestrial vertebrates.
Fish aryl hydrocarbon receptors bind and treat dioxin
more poorly than mammalian ones. Since the ability to
convert dioxin to a more polar compound is necessary for
excretion, dioxin remains in fish but can be excreted in
mammals. As the compound remains unaltered, it
concentrates in fatty fish. The highest levels are reached in
the biggest and oldest fish.
Understanding the reasons behind and possible
consequences of high dioxin levels in fish requires that the
functions of the animals is known in detail. The dioxin
example illustrates that any investigations of environmental
problems needs a functional component to evaluate
alterations in ecosystems. Although environmental effects
are often considered without physiological studies, one
should remember that environmental effects can only take
place, if the function of some organisms in the ecosystem
is affected. Only by combining genetic, physiological and
ecological approaches can environmental responses be
understood. Such understanding is required to predict the
economic consequences of environmental disturbances.
Mikko Nikinmaa
Professor
Department of Biology
University of Turku
Finland
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