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The origin of the
problem:
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Copper in sea-water, either in the form of
bivalent cupric ion, copper hydroxide, carbonate complexes, oxides or as organic
copper compounds, is a toxicant to marine life. In aquaculture, the
antifouling products used to impregnate the cage netting contain 18-25%
cuprous oxides.
The use of antifouling “paints” has increased
in line with the proliferation of the large circular fish cages, which are
necessary for expanding production and moving gradually to more wave exposed marine
sites. However, their netting is heavy and frequent net changes are laborious
and time consuming. Copper and copper oxides from the impregnated nets
gradually dissolve in the sea-water prohibiting the establishment of algae
and bivalves that foul the netting, thus prolonging the continuous use of the
nets.
After about a year in the sea, the
impregnated nets are washed on shore in net washer drums. The effluent from
these often flows into the sea, either untreated or subsequent to some degree
of sedimentation. Commonly, fresh net impregnation takes place near the shore
creating spillage, which also finds its way into the sea with rainfall. As a
result the copper content of the bottom sediments and of the sea-water is
elevated and adversely affects fish health.
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Susceptible farmed
species, age/size:
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All cultured species are susceptible; their
sensitivity being size/age dependent. Smaller fish sufer the more accute
consequences.
Empirical, personal observations over the
recent years have concluded that sea bass (Dicentrarchus
labrax) is much more sensitive than sea bream (Sparus auratus) to the presence of elevated copper levels in the
water.
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Main lesions and
diagnosis:
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Sub-lethal copper toxicity in fish relates to
cell membrane permeability and osmoregulation, enzyme synthesis and function.
 
The skin darkens, thickens and appears
fragile and ulcerated, in particular around the mouth. The fins are eroded.
There is congestion of the meninges, which is evident through the thin and
semi-transparent skull (meningitis ensues, particularly on young bass).
The gill epithelium is inflamed, thickened
and degenerate; excessive mucus is secreted, gill function and resistance
against parasite and bacteria establishment are compromised.
The liver and spleen appear pale and
degenerate.
The red blood cells are intoxicated and their
shape is disturbed. Typically, a large proportion of erythrocytes on fresh or
fixed Giemsa stained blood smears appear elongated, spindle shaped or bent.
Immature erythrocytes proliferate due to increased blood destruction and the
affected fish become progressively anemic.
Such compromised fish are slow growers and become
easy targets for common parasites. Gill trematodiases and bacterial gill
disease unexpectedly become serious causes of fish losses. In pump-ashore
nurseries young fry turn anorectic and lethargic, darken, emaciate and may
die in numbers.
Toxicity depends on copper bioavailability,
which is influenced in turn by water temperature and water movement
re-suspending in the water column sediments rich in copper. Young age classes
of fish are more sensitive, sea-bass being more susceptible than sea bream.
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Sea water testing:
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During the past twenty months, twenty five
samples of surface
sea-water among cages, tested with Atomic Absorption Spectroscopy, revealed total copper content from 0.43 to
23.2 ìg/lt. (ppb), mean value
of 5.53 ppb and standard deviation 5.61.
Half of the samples
contained copper above 3ppb. Fourteen samples of sediments under the cages
returned copper content values ranging between 2.85 and 221.20 mg/kg (ppm)
with a mean value of 47.41 ppm and standard deviation 63.66. The U.S. Environmental
Protection Agency considers copper at 2.9 ìg/lt. sea-water as the “Criterion Maximum
Concentration”.
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Economic Implications:
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Expected
to be significant
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Frequency of occurrence:
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Frequent
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Consequences:
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Costs associated with copper toxicity
may comprise:
1.
Increased mortality, especially among juveniles and fry.
2. Chronic stress and high propensity to infectious diseases (indirect
mortality).
3. Growth retardation.
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Environmental issues:
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Increased copper levels in the water are
toxic for all biota. Copper-based antifouling contamination of the marine
environment must be prevented in view to sustaining coastal aquaculture.
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Regulations:
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No
specific regulations are in place concerning upper limits for total copper
concentration in bottom sedimends or in sea-water. Regulations exist only as
regards copper in potable water, hence there is some confusion.
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Selected Bibliography:
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Eleftheriadou, M. and
Skoulos, M. (2004) Nutrient and trace metal distribution in the Gulf of Astakos,
Aetoloakarnania, Greece. Global Nest: the Int. J. 5(3):117-124
Mourad, M. and Wahby, O.
(1999) Physiological and histological changes in Tilapia zillii
exposed to sublethal concentrations of the effluent of the Egyptian copper
works. Acta Ichthyologica et Piscatoria, 29(2):73-80
United States Environmental
Protection Agency, Office of Water Regulations and Standards, Washington, DC
20460 (1986) Quality Criteria for Water (EPA 440/5-86-001) pp. 150
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Acknowledgements
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The Chemical Engineer Mr
Paul Nissianakis, who has calibrated the AAS methodology for copper in
sea-water and bottom sediments, according to the internationally accepted
standards and has provided the copper measurements.
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