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Discussion by O. Jaag
The progressive deterioration of numerous European and extra-European freshwater lakes is due to
the increase of microscopic and macroscopic vegetation and animal life as a result of increasing
eutrophication, brought about by discharges of sewage and industrial waste waters.
It was considered important when Tilapia aurea was discovered as a freshwater fish capable of
developing in lakes, eating plankton and water weeds, and so lowering the eutrophication level in
freshwater and in reservoirs.
Water blooms in Switzerland were found to be due to the red colored blue-green alga Oscillatoria
rubescens, which occurred for the first time in Lake Murten in 1825. The transparency of the water
was lowered, the water turned red, and the population of Coregones were replaced by Cyprinids (fish
of inferior quality). During summer time the algal filaments stay at depths of 8 to 12 meters but are
transported to the upper layers by the currents until a complete overturn due to changes in density of
the water occurs as a result of changing temperature. At the end of the vegetation period in autumn
dead algal cells and filaments move from the upper layers to the bottom of the lake where the mass of
decayed organic matter becomes oxidized. Diminution of the oxygen content takes place and the
water becomes anaerobic. Simultaneously water weeds appeared, increasing the anaerobic conditions
in the lakes. Since its first appearance Oscillatoria rubescens invaded many lakes in several Western and
Central European countries. In practically all of these lakes the same catastrophical development took
place so that one spoke of "ill" or "dead" lakes.
In the meantime other unexpected algal invasions took place in quite a few European lakes, the
most important ones being: another blue-green alga Aphanizomenon flosaquae, the diatoms Tabellaria
fenestrata, Rhizosolemia longiseta and many others. Some of them disappeared later while others
remained in the invaded lakes. We attribute this continuous increase in algal bloom and water weeds
production to the increase of polluting substances among which phosphorus and nitrogen compounds
are the most important. We therefore try to reduce plant growth by lowering the phosphorus and
nitrogen compounds. We thereby hope to avoid the over-production of plant and consequently animal
life in our lakes precipitating phosphorus compounds.
Of course, we should like to have plankton and waterweeds eaten by Tilapia or other plantnourishing fish, but unfortunately Tilapia, grass carp and silver carp do not develop in the relatively
cold water of our lakes. Thus, our aim is to find other plant eating fish in order to get rid of the
surplus of plant nutrient matter.
In Switzerland the waste waters of about half the population and its industry are treated in
purification plants. We think that the improvement now being observed in the chemical and biological
conditions of our lakes can be attributed to this thorough primary, secondary and in most cases even
tertiary treatment of sewage and industrial waters. The results obtained in H. Leventer's interesting
study should be compared with similar investigations carried out in countries like Poland for
instance where the water of lakes, ponds and artificial reservoirs reach somewhat lower temperatures
than in the Israeli water bodies. Under those conditions artificial breeding of Tilapia, grass and silver
carp is needed. Therefore, our main task to overcome the deterioration of lakes in cooler climates
may consist of discovering plant eating fish in order to prevent excessive eutrophication.
P. Laurent, France.
In order to see if the author's results would be capable of extrapolation under favourable climatic
conditions, it is important to establish the relationship between the biomass of fish and the volume
of water.
H. Leventer
In carp fish ponds in Israel, 1500-2000 of Tilapia aurea per hectare are used as a supplementary
fish. This density secures after two years' growth fish of 400-600 gram. In our reservoirs an optimal
density is 5000 fish of 50-300 gram per hectare. Their growth depends on the'quantity of food at the
bottom. Examination of fish population by fishing methods permits us to know if there is an overpopulation or an underpopulation.
J. McKendrick, Rhodesia
Lake Mcllwaine in Rhodesia has an algal bloom for 12 months each year. Although there are large
numbers of Tilapia, the blooms persist. Do the fish only help in a small reservoir and would they help
in Lake Kinneret?
H. Leventer
Tilapia fish is not fed on phytoplankton. Living blue-green algae cannot be digested by them, but
when these algae begin to die down they may became digestible by fish. Not all species of Tilapia are
able to be fed on detritus or on broken-down organic matter. The species T. zillii and T. melanopleura
are only submerged plant feeders. In Lake Kinneret T. aurea growths are quite good. There is no high
Discussion by O. Jaag
The progressive deterioration of numerous European and extra-European freshwater lakes is due to
the increase of microscopic and macroscopic vegetation and animal life as a result of increasing
eutrophication, brought about by discharges of sewage and industrial waste waters.
It was considered important when Tilapia aurea was discovered as a freshwater fish capable of
developing in lakes, eating plankton and water weeds, and so lowering the eutrophication level in
freshwater and in reservoirs.
Water blooms in Switzerland were found to be due to the red colored blue-green alga Oscillatoria
rubescens, which occurred for the first time in Lake Murten in 1825. The transparency of the water
was lowered, the water turned red, and the population of Coregones were replaced by Cyprinids (fish
of inferior quality). During summer time the algal filaments stay at depths of 8 to 12 meters but are
transported to the upper layers by the currents until a complete overturn due to changes in density of
the water occurs as a result of changing temperature. At the end of the vegetation period in autumn
dead algal cells and filaments move from the upper layers to the bottom of the lake where the mass of
decayed organic matter becomes oxidized. Diminution of the oxygen content takes place and the
water becomes anaerobic. Simultaneously water weeds appeared, increasing the anaerobic conditions
in the lakes. Since its first appearance Oscillatoria rubescens invaded many lakes in several Western and
Central European countries. In practically all of these lakes the same catastrophical development took
place so that one spoke of "ill" or "dead" lakes.
In the meantime other unexpected algal invasions took place in quite a few European lakes, the
most important ones being: another blue-green alga Aphanizomenon flosaquae, the diatoms Tabellaria
fenestrata, Rhizosolemia longiseta and many others. Some of them disappeared later while others
remained in the invaded lakes. We attribute this continuous increase in algal bloom and water weeds
production to the increase of polluting substances among which phosphorus and nitrogen compounds
are the most important. We therefore try to reduce plant growth by lowering the phosphorus and
nitrogen compounds. We thereby hope to avoid the over-production of plant and consequently animal
life in our lakes precipitating phosphorus compounds.
Of course, we should like to have plankton and waterweeds eaten by Tilapia or other plantnourishing fish, but unfortunately Tilapia, grass carp and silver carp do not develop in the relatively
cold water of our lakes. Thus, our aim is to find other plant eating fish in order to get rid of the
surplus of plant nutrient matter.
In Switzerland the waste waters of about half the population and its industry are treated in
purification plants. We think that the improvement now being observed in the chemical and biological
conditions of our lakes can be attributed to this thorough primary, secondary and in most cases even
tertiary treatment of sewage and industrial waters. The results obtained in H. Leventer's interesting
study should be compared with similar investigations carried out in countries like Poland for
instance where the water of lakes, ponds and artificial reservoirs reach somewhat lower temperatures
than in the Israeli water bodies. Under those conditions artificial breeding of Tilapia, grass and silver
carp is needed. Therefore, our main task to overcome the deterioration of lakes in cooler climates
may consist of discovering plant eating fish in order to prevent excessive eutrophication.
P. Laurent, France.
In order to see if the author's results would be capable of extrapolation under favourable climatic
conditions, it is important to establish the relationship between the biomass of fish and the volume
of water.
H. Leventer
In carp fish ponds in Israel, 1500-2000 of Tilapia aurea per hectare are used as a supplementary
fish. This density secures after two years' growth fish of 400-600 gram. In our reservoirs an optimal
density is 5000 fish of 50-300 gram per hectare. Their growth depends on the'quantity of food at the
bottom. Examination of fish population by fishing methods permits us to know if there is an overpopulation or an underpopulation.
J. McKendrick, Rhodesia
Lake Mcllwaine in Rhodesia has an algal bloom for 12 months each year. Although there are large
numbers of Tilapia, the blooms persist. Do the fish only help in a small reservoir and would they help
in Lake Kinneret?
H. Leventer
Tilapia fish is not fed on phytoplankton. Living blue-green algae cannot be digested by them, but
when these algae begin to die down they may became digestible by fish. Not all species of Tilapia are
able to be fed on detritus or on broken-down organic matter. The species T. zillii and T. melanopleura
are only submerged plant feeders. In Lake Kinneret T. aurea growths are quite good. There is no high