A large (130 pages of text an 42 pages of graphs) study paper was issued about the fishery development in the lake Alaotra, compiled by I. THEREZIEN (Etude en vue du développement de la pêche au Lac Alaotra, 1964). Later, M. VINCKE provided newer data about the lake fishery in his report (Essai d'estimation de la production piscicole des eaux continentales Malgaches 1970–71 - p. 21–27). Hydrobiologically this interesting lake was not yet investigated.
The fishery situation has been changed since the mentioned investigations enormously which is illuminated by A. COLLART table : “Evolution des effectifs de pêche 1969–1979, Lac Alaotra”.
| Elements | 1979 | 1969 | évolution |
| Pêcheurs | 2512 | 1300 | +100% |
| Pirogues | 2222 | 742 | +300% |
| Filets senne nbre/m | 100/2234 | 102 | stable |
| Filets maillants " | 458/38950 | 65000 | -50% |
| Eperviers | 819 | 77 | +1058% |
| Nasses | 14493 | 7636 | +100% |
| Lignes | 16588 | ? | ? |
The table shows that the development trend is toward the overexploitation of the lake.
The open water surface of the lake is about 22000 ha which is in connection with about 55000 ha marsh area covered mostly by Cyperus growth. The huge marshy area provides very little living possibility for fishes and even less possibility for fishing activities due to the dense plant growth. According these data less than 10 ha has to provide catch for one fisherman (+ family).
The expert could visit the lake only one time between 22–29 January 1980. The measurements and observations were made only for orientation to find answer for some important questions of the future of the lake fishery.
Five local fish species were objects of the previous (before 1920) fishing activities.
Rheocles alaotrensis,
Electris legendrei,
Paratilapia polleni (this was the main objects of the catch),
Anguilla mossambica,
Anguilla marmorata.
There are no estimations how much was the catch of these fishes before the introduction of economically important fish species which provide at the present more than 99% of the catch.
The introductions were made in different years,
Carassius auratus 1922
Cyprinus carpio 1923
Tilapia rendalli 1955
Tilapia macrochir 1958
Tilapia nilotica 1959
Tilapia mossambica 1959
Micropterus salmonides 1961
Gambusia holbroocki 1948 (This fish also captured in great quantity
to make sun-dried fish for consumption).
According to the previous investigations the fish catch contained mostly from Tilapia species (in 1970, 79%) and 20% was common carp.
During the expert stay the catch was mostly common carp and only very few (about 2–5%) Tilapia was offered fresh in the markets.
The black bass is very rare throughout the year (may be due to the high turbidity and soft bottom of the lake).
It was told as well, that the Tilapia nilotica is very meagre in the lake, the cause of this is not known.
Small Tilapia are captured plenty, by hook, by children and women who are angling around the lake whenever they have time.
According to the market observations the captured carps are about 50–70% 4–5 months of age (juveniles), the rest is around 1 year of age. Older than one year carps are rare (only a few) in the market. That shows that the carp stock is overfished ; but the effectivity of the natural propagation keeps abrest yet with the intensive exploitation at the present. But it is not known when will come the time that the overfishing results the decrease of the propagating stock so far, that the capture will steeply decline.
The colour of the lake. The colour and turbidity of the water determine many production-biological factors of the lake in connection with the fish production. The colour of the lake is the different shades of brown. Around the Gyperus covered area, the South-West part of the lake the water colour is dark brown. The Cyperus plant association is the main supplyer of the brown water deriving most probably from the tannin content of the huge amount of decomposing plant materials of this area. The basic dark brown colour is shaded by the white-greyish, sometimes reddish colloid content of the inflowing rivers which make the water clear-brown or red-dish-brown.
The colour combination of the water is influencing the transparency (Secchi disk visibility) of the water as well. The Secchi disk visibility is very different ranging from 5–10 cm before the inflow of Sahamaloto river (300 m from the estuary) 20–30 cm in most part of the lake and 40–50 cm near to the Cyperus area (data from the same day). The transparency of the lake is influenced as well by the plancton and detritus content. The transparency is higher in calm days about 10 cm as in windy ones.
The temperature of the lake water is depending on the daily average temperature of the area and the strengh of the wind. The shallow lake is totally mixed up by a normal wind making the bottom and surface water temperature the same. It was measured 24.5 C° bottom (2.5 m) and 24.5 C° surface temperature at 23 Janv., 25.0 C° bottom (2.8 m) and 25.5 C° surface at 25 January, and 25.5 C° bottom (2.5 m) and 29 C° after two days calm weather at 28 January. All temperature measurements were taken around 11 A.M.
pH of the water was not measured because the lack of equipment.
The oxygen content of the bottom water (20 cm above the bottom) was estimated in each day in different places of the lake (near to the Cyperus growth, deepest part of the lake, etc…) ; everywhere the oxygen content seemed to be sufficient in spite of that near the Cyperus growth great accumulation of gas was observed in the mud (most probably methane and CO2 due to the rotting plant rests there).
All days of the investigation time, plancton samples were collected mostly from 20–60 cm depth. The quantitative assesment of the plancton was made by the filtering through a 60 microns mesh plancton net 10 litre pond water. It was found that the zooplancton is richer about 3–4 times as in the lake Itasy in December. 5–6 Cladocera species were present in the plancton. The Diaptomids are dominant among the Copepods and not the Cyclopids. The great number of juveniles and the number of eggs and embryos in the egg pouches showed that the zooplancton population was in high production stage. The Rottoaria fauna is also rich especially near the Cyperus growth in the dark brown water. In the open water there was a rich detritus amount in the plancton which could be traced as food for the zooplancton feeders.
The phytoplancton is not so rich as in the lake Itasy, most probably due to the less transparency of the water. Clusters of blue-green algae which specific weight is less than of the water were the most common algae. Near the Cyperus growth Desmideaceae could be found in great number.
There was not full scale fishing activity on the lake during the time of investigation due to the high water level and other agriculture activity of the fishermen. The fishing activity was limited mostly on the shore area where fish-traps (nasses) and castnets were used. Mostly common carp, crucian carp (carassius) and few Tilapia were captured. The markets in Ambatosoratra (where each day market inspection was made by the expert) and Ambatondrazaka (prefecture centre) were saturated of fish in spite of the low fishing activity, and many kg. fresh fish remain unsold there. Smoked and dried fish (mostly Tilapia and small size carps) was offered also on the markets.
The fresh fish transport toward further areas or to the capital was hampered by the bad road conditions in January (the season of heavy rains), therefore could happen the saturation of the local markets. It was offered about 100–150 kg carp daily in the Ambatosoratra market, and in average 400 kg was in the Ambatondrazaka market.
It was observed that the captured common carps of 12–15 cm standard length belong to the age group no older than 4–5 months (age was estimated according to the gonad development). The 16–20 cm size common carps were mixed one year old and young ones. For this statement it was supposed that the common carp becomes sexually ripe within its first year age, as it is the case in subtropics and tropics. Above 20 cm standard lenght all common carp was sexually ripe and may be more than one year age.
The carp stock of the lake derived from the introduction of mirror carp strains. But typical mirror carp is hardly to find among the present stock. Most of them became irregular scaly carp (their body entirely or mostly covered with irregular scales). This fact is strenghtening the presumption that the scaly carp would be better to stock into the lake, to refresh the present population.
It is not known which carps of what body form were introduced in the lake, most probably farm cultured improved strains of more rounded form. The profil index (standard length divised with the body height) varied between 2.8 and 3.3 that shows there are mostly elongated type of carp now in the lake.
The investigated samples were in good body condition and the guts of the carps were in most cases full with food rests. Besides insects larvae and plancton remnants there were plenty seeds of different water plants (Polygonum and Gramineae) further detritus in the gut content.
It could be also surely stated that the sexually ripe common carps were in spawning activity during the time of investigation. It was found among the freshly captured carps (captured with traps) ovulating individuals with flowing eggs. Others were found already spent. The males released freely the milt. Other carps were found with developed eggs in the ovary in stage before the ovulation. It was captured with plancton net one few day old carp fry as well.
It can be stated that the common carp spawn in the lake not only in September-October but also in January as well. This observation is supported by the experience that the well nurtured common carp became able to propagate in each 3–4 months period among subtropical and tropical conditions.
It could not be concluded that the Sahabe river would be a suitable spawning place for river spawner fish species.
a) It seems that the common carp population is overfished in the lake because more than the half of the captured ones are juveniles.
It may come the time when the natural propagation of common carp in the lake could not keep abrest with the intense capture resulting the steep decline of the catch.
b) The originally introduced mirror carp strain became irregular scaly carp in the lake showing that the scaly body is more suitable to the lake environment.
c) The common carp spawns in January as well, so there is a second spawning season for this fish (The main spawning season is in September-October).
d) The plancton of the lake seems to be richer as of the lake Itasy, but it is not exploited by column feeder fish, because does not exist such fish in the lake.
e) The detritus content in the water column is very high due to the shallowness of the lake deriving from the rotting plant materials (organic detritus). The organic detritus content of the bottom is also very high in the Southern part of the lake which was investigated.
To avoid catastrophic over fishing of common carp in the lake it is recommended to make a decision not to allow marketing common carp less than 16 cm standard length (about 20 cm total length). That restriction would result that the carps would have the chance to spawn at least one time in the lake.
The other possibility would be to ban the fishing activity on carp in September and January.
To refresh the common carp stock in the lake it is recommended to introduce improved scaly carp strain there.
It is recommended to introduce plancton feeder (column feeder) fish species into the lake as silver carp and bighead carp (Hypophthalmiohthys molitrix, and Aristichthys nobilis). They are phytoplancton feeder and micro zooplancton feeder respectively and will surely not interfere with the other already settled fish species. This introduction will be a solution to exploit better the open water and provide better catch for the increasing number of fishermen.
It can be considered also the introduction of the mostly detritus feeder rohu (Labeo rohita) into the lake. The detritus is also a food source which not adequately exploited by an economically important fish species.
After the experimental introduction of the mentioned fish species which are all river spawners (they spawn only in the current of greater rivers) could be find out weather they can maintain their stock by natural propagation in the inflowing rivers (first of all in the Sahabe river). In the case when they could not propagate naturally there, a special hatchery should be constructed to propagate these fishes artificially for the lake.
It has to be considered to establish organisation (as co-operatives) for the fishermen to regulate and increase the quality of the catch through such organisation.
