Discussion by: G. Haider
Institute of Sanitary Engineering, Dept. of Biology, University of Stuttgart, Fed. Republic of Germany.
An approved method of mechanically controlling the unwanted plant growth caused by eutrophication is to cut the plants and remove them regularly from the water. Nowadays mechanical plant
control is supplemented by the use of herbicides, e.g. Diuron (HUET 1970). These measures are expensive, hazardous to the use of the water and to fish. A new possibility of biological control of algal
growth is to use the help of mycobacters or viruses (cyanophages) (SHILO 1971). However, these
methods are in the experimental stage. The control of macro- and microphytes by phytophagous fishes
must be also put into this group. The criteria for suitable fish species are as follows: the fishes must be
able to adapt themselves to existing temperatures, not breed too fast and so become a nuisance; they
should also peacefully co-exist with other species. However, the fishes must be able to stand up»to if
attack by enemies. All these requirements cannot be realized by one single fish species. Nevertheless,
there exist some species, especially among cultured phytophagous fishes in the tropics, which
theoretically can maintain plant control. We should mainly think of the grass carp (Ctenopharyngodon
idella), the silver carp (Hypoththalmichthys molitrix), and the herbivorous Tilapia species, for example
T. rendalli (= melanopleura), as well as T. zilli and T. aurea. The Mugil species for brackfish waters
were recommended by PROWSE (1969). Some of these fishes prefer to feed on higher plants, others
choose to feed on algae. Especially detrimental to all herbivorous species are the climatic conditions in
the Federal Republic of Germany. The Tilapia species, common in the tropics, cannot tolerate
temperatures below 13 C. Grass carp do not feed on plants below 20 C, and they reach optimal
feeding temperature at 23 - 24°C (MANN 1968). In our climate, this species can therefore hardly
replace the plant control by mechanical or chemical methods and is therefore of less importance.
Some positive experiments in Bavaria were carried out with the silver carp feeding on algae
(LUCOWICZ 1970). Thus the use of phytophygous fish in temperate zones is limited and much less
important than in the tropics or subtropics.
Contrary to the methods of eutrophication control mentioned before, which are efficient only for
a short period, it would be more successful and more effective to control the causal factors. It is
important to reduce the quantity of fertilizers existing in the waters. The P-compounds should be
eliminated to the maximum extent in the sewage plants (less than 1 mg/1 P). The existing techniques
of today (chemical or biological methods - precipitations by iron or aluminum salts) are not
satisfactory because only the orthophosphates are separated, but not polyphosphates and organic
P-compounds. Operating problems exist with regard to this method of employment (SCHUSTER
1968). No satisfactory techniques are available for the biological P-elimination. The paper by
SEKOULOV (1972) will possibly initiate a new trend. P-elimination by biological precipitation may
become an important technique. During the continuous illumination of algal suspensions the photosynthesis extracts CO2 from the water; this causes the pH value to rise over 9. Another sequence
thereof is that phosphorous is precipitated by the calcium ions in the water.
A effective brake on the increase of eutrophication could be achieved if it is possible to stop the
circulation between the organic sediments and the water body by a cover of clay or similar matter.
However, so far only a few trials have been made by this method. Almost nothing is known of the
micro biological effects and the effect of benthic organisms on the new surface of sediments. This
method therefore requires further investigation.
REFERENCES
1. HUET. M. : Traite de Pisciculture. 4m Ed. Wyngaert Bruxelles 1970, 718 pp.
2. SHILO, M. : Biological agents which cause lysis of blue-green algae. Mitt. Internat. Verein. Limnol.
19,1971,203 - 2 1 3 .
3. PROWSE, G. A. : The role of cultured pond fish in the control of eutrophication in lakes and dams.
Verh. Internat. Verein. Limnol. 17, 1969, 714 - 718.
4. MANN, H. : Untersuchungen über die Futterverwertung beim Grasfisch. Allgem. Fischerei-Ztg. 93,
1968, 1 pp.
5. LUKOWICZ, M.v. : Die Karpfenteichwirtschaft in Bayern. Allgem. Fischerei-Ztg. Festgabe zum
Deutschen Fischerei Tag 1970, 40 - 46.
6. SCHUSTER, G. : Grundlagen der Phosphorelimination durch chemische Fällung. Fortschr, der
Wasserchemie 1968, H. 8, 188 - 203.
7. SEKOULOV, I. : Die Phosphorelimination mit Hilfe von kontinuierlich belichteten Blaualgen.
Stuttgarter Berichte zur Siedlungswasserwirtschaft 50, R. Oldenbourg München 1972, 186 pp.
227
Institute of Sanitary Engineering, Dept. of Biology, University of Stuttgart, Fed. Republic of Germany.
An approved method of mechanically controlling the unwanted plant growth caused by eutrophication is to cut the plants and remove them regularly from the water. Nowadays mechanical plant
control is supplemented by the use of herbicides, e.g. Diuron (HUET 1970). These measures are expensive, hazardous to the use of the water and to fish. A new possibility of biological control of algal
growth is to use the help of mycobacters or viruses (cyanophages) (SHILO 1971). However, these
methods are in the experimental stage. The control of macro- and microphytes by phytophagous fishes
must be also put into this group. The criteria for suitable fish species are as follows: the fishes must be
able to adapt themselves to existing temperatures, not breed too fast and so become a nuisance; they
should also peacefully co-exist with other species. However, the fishes must be able to stand up»to if
attack by enemies. All these requirements cannot be realized by one single fish species. Nevertheless,
there exist some species, especially among cultured phytophagous fishes in the tropics, which
theoretically can maintain plant control. We should mainly think of the grass carp (Ctenopharyngodon
idella), the silver carp (Hypoththalmichthys molitrix), and the herbivorous Tilapia species, for example
T. rendalli (= melanopleura), as well as T. zilli and T. aurea. The Mugil species for brackfish waters
were recommended by PROWSE (1969). Some of these fishes prefer to feed on higher plants, others
choose to feed on algae. Especially detrimental to all herbivorous species are the climatic conditions in
the Federal Republic of Germany. The Tilapia species, common in the tropics, cannot tolerate
temperatures below 13 C. Grass carp do not feed on plants below 20 C, and they reach optimal
feeding temperature at 23 - 24°C (MANN 1968). In our climate, this species can therefore hardly
replace the plant control by mechanical or chemical methods and is therefore of less importance.
Some positive experiments in Bavaria were carried out with the silver carp feeding on algae
(LUCOWICZ 1970). Thus the use of phytophygous fish in temperate zones is limited and much less
important than in the tropics or subtropics.
Contrary to the methods of eutrophication control mentioned before, which are efficient only for
a short period, it would be more successful and more effective to control the causal factors. It is
important to reduce the quantity of fertilizers existing in the waters. The P-compounds should be
eliminated to the maximum extent in the sewage plants (less than 1 mg/1 P). The existing techniques
of today (chemical or biological methods - precipitations by iron or aluminum salts) are not
satisfactory because only the orthophosphates are separated, but not polyphosphates and organic
P-compounds. Operating problems exist with regard to this method of employment (SCHUSTER
1968). No satisfactory techniques are available for the biological P-elimination. The paper by
SEKOULOV (1972) will possibly initiate a new trend. P-elimination by biological precipitation may
become an important technique. During the continuous illumination of algal suspensions the photosynthesis extracts CO2 from the water; this causes the pH value to rise over 9. Another sequence
thereof is that phosphorous is precipitated by the calcium ions in the water.
A effective brake on the increase of eutrophication could be achieved if it is possible to stop the
circulation between the organic sediments and the water body by a cover of clay or similar matter.
However, so far only a few trials have been made by this method. Almost nothing is known of the
micro biological effects and the effect of benthic organisms on the new surface of sediments. This
method therefore requires further investigation.
REFERENCES
1. HUET. M. : Traite de Pisciculture. 4m Ed. Wyngaert Bruxelles 1970, 718 pp.
2. SHILO, M. : Biological agents which cause lysis of blue-green algae. Mitt. Internat. Verein. Limnol.
19,1971,203 - 2 1 3 .
3. PROWSE, G. A. : The role of cultured pond fish in the control of eutrophication in lakes and dams.
Verh. Internat. Verein. Limnol. 17, 1969, 714 - 718.
4. MANN, H. : Untersuchungen über die Futterverwertung beim Grasfisch. Allgem. Fischerei-Ztg. 93,
1968, 1 pp.
5. LUKOWICZ, M.v. : Die Karpfenteichwirtschaft in Bayern. Allgem. Fischerei-Ztg. Festgabe zum
Deutschen Fischerei Tag 1970, 40 - 46.
6. SCHUSTER, G. : Grundlagen der Phosphorelimination durch chemische Fällung. Fortschr, der
Wasserchemie 1968, H. 8, 188 - 203.
7. SEKOULOV, I. : Die Phosphorelimination mit Hilfe von kontinuierlich belichteten Blaualgen.
Stuttgarter Berichte zur Siedlungswasserwirtschaft 50, R. Oldenbourg München 1972, 186 pp.
227