MAP Technical Reports Series No. 106 UNEP

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of red tide produced by these silicoflagellates were described in Japan in 1955. After 1983,
numerous cases of silicoflagellate red tides were described in Europe, not only at Kiel (Jochem,
1987), in the Kattegat (Aertebjerg and Borum, 1984), in Ireland (Doyle et al., 1984; Gowen, 1984)
but also in the Mediterranean in the north-east of the Adriatic (Gulf of Trieste) (Fanuko, 1989).
The damage is attributable either to O 2 deficiency or to irritation of branchial apparatus
by silicous structures, because a toxic substance has not been demonstrated to be present.
In 1983 it was discovered that Dictyocha speculum occurs also in the naked stage
(characteristic of cell proliferation) and this stage is thought to be responsible for a fish kill in
south western Denmark.
6.1.5 General tests on the toxicity of phytoplankton
To investigate the presence of biotoxins acting on marine animals when discoloured
water red tide or mucilaginous aggregates occurs, various laboratory tests can be performed.
(a) Qualitative and quantitative determination of the phytoplankton
The qualitative and quantitative determination of the phytoplankton can be performed
using Millipore filters (Margalef, 1969) and Utermöhl's method (1958). In the qualitative analysis
of the phytoplankton particular attention must be paid to the species of the classes that produce
ichthyotoxins: Dinophyceae, Prymnesiophyceae, Raphidophiceae, Dictyochophyceae.
(b) Acute toxicity tests on fish using samples of water and phytoplankton extracts
(i) Tests on affected fish from areas of water or sea discoloured by phytoplankton blooms
Affected fish can be brought to the laboratory from their natural environment (brackish
or sea water) in the water in which they were caught and then transferred and divided
into equal numbers in suitable 10 litre continually aerated tanks containing: a) the
discoloured water under test, and b) artificial control water. If all the fish kept under the
two conditions recover and continue to live for 96 hrs it can be concluded that the
disease or mortality in the natural environment was due to simple O 2 deficiency. If only
the fish in tank a) die even with significant oxygenation, then biotoxins can be present.
If some fish die in both tank a) and tank b) (control), it means that irreversible lesions
(and branchial clogging) had occurred. In this type of screening it can also be useful to
make preliminary tests for NH 3 and H 2S in the water, as their presence accompanies
conditions of anoxia.
(ii) Effect of the acute toxicity of discoloured water on healthy fish in the laboratory
Because of its euryhaline characteristics, Mugil cephalus can be very useful in
examining sea water samples of varying salinity. In this case also, the times of any
deaths in the fish in the "discoloured water" tanks are observed. The absence of
mortality over a 96 hrs period suggests that no water soluble biotoxins are present,
whereas a high mortality suggests their presence. With a high mortality rate the type
of phytoplankton must be considered, and investigations are started on the extraction
or isolation of the biotoxins.