A detailed study was made by Mr Y. THEREZIEN “Etude en vue du développement de la pêche au Lac Itasy” 1964 p. 92. This study is based on the investigations of the years 1963–64. According to this study the fish catch was about 1200–1500 metric tons per year. The lake is about 3500 ha. In that case the per ha catch would be 340–430 kg per ha per year. According to the recent observations, the catch declined since then sharply. The present catch is estimated not more than 200–300 tons per year (50–80 kg per ha) or even less. Many signs are showing that the lake is heavily over-fished and because the undersize, young fish is captured in spite of the official prohibition, there is no enough fish to grow and exploit the ressources given in the lake. Only the narrow shore area and the inundated shallow parts are exceptions.
The expert made the investigations between 5–9 November, just at the beginning of the rainy season, and 18–21 December, when the lake already inundated a great part of the shallow shore area, otherwise dry land during the dry season.
It was investigated the qualitative planoton of the water column (open water), the oxygen situation and temperature in different depth. It was observed the planoton content of 1 liter water taken in different depth. Qualitative samples were taken from the bottom up to two meters depth. The shore area was also investigated concerning the planoton and other livings' population. The fish catch techniques and the composition of the catch also were observed.
The greatest part of the fish catch contains of exotic, introduced fish species. The only indigenous fish, the Anguilla mossambica and Anguilla nebulosa labiata which represent some importance in the catch. The other three local species Gobius macrorhynchus, Eleotris legendrei and a local Cichlid, Ptychochromis betsileanus are very rare. They have no economic importance.
The greatest part (more than 90%) of the catch throughout the year, consists of different introduced Tilapia species ;
Tilapia macrochir
Tilapia mossambica
Tilapia melanopleura (rendalli)
Tilapia zillii
Tilapia nilotica
and different hybrids of the mentioned species.
The Tilapia species are caught mostly around the shore area and on the shallower parts of the lake up to 3 m depth by different gill nets, pulled gill nets, or the fish are freightened into the layed gill net, cast net, traps, or from the shore and in the shallow water by hook and rood. Practically everybody, the population around the lake is angling in leisure time or in whole day. Professional fishermen use gill nets of different type, castnet, traps.
The Tilapia catch is mostly small, and young 30–50 g individuals of T. macrochir, T. mossambica, and hybrids. The biggest Tilapia are mostly T. nilotica caught not too often. Young T. nilotica is also often in the commercial catch.
The common carp represents also an important part in the commercial catch. Although bigger carps above 1 kg are rare, sometimes they catch 4–6 kg specimens. The most of common carp is caught in their 4–6 months age (weight 80–200 g) in the southern part of the lake around the estuary of MALIANDRANO and NANDAMBA rivers. The daily catch contains some hundred of such young common carp in December.
The Black bass (Micropterus salmoides) is caught also time to time. Most of them are juveniles not reaching the one year age. Many 50–150 g, September born black bass, were caught in December, by hook and castnet.
Because there, does not exist a regular checking of the catch, it is difficult to estimate the composition and quantity of the catch.
Water temperature in November was 24 C° on the bottom and 24 C° in 2 m depth and 25.5 C° in the surface. In December 24.5 in the bottom and 25.5 up to 26 C° in the surface (measuring time 10 A.M.). The dissolved oxygen was estimated only. No sign of oxygen deficiency could be traced above the bottom.
The plancton is rich especially of the phytoplancton and smaller zooplancton as rotifers, small cladocera (Cerodaphnia ?) and Cyclops. Diaptomus and Diaphanosoma are about 40–60 per liter together. The accumulation of the zooplancton was in day time about in 3 m depth. Migration of the zooplancton was observed toward the surface before sunset.
Some Chironomids should live in the bottom of the open water because in the surface many “exuvia” (chitin slough of the pupae) could be found.
The plancton was more dense in December as in November.
Qualitative bottom samples were taken only about from 2 m depth because the limitation of the equipment. Here molluses, very few Chironcmids and worms, and above the bottom. Chaeoborus type insect larvae could be found. The bottom soil here is full of rotten and rotting plant materials. The rotting plant content becomes more rich near the shore.
The shore area was investigated quite round the lake. The shore bottom is rich in rotting or already decomposed plant materials. It has often the smell of fermenting cellulose. Gas (methan) accumulation in the bottom mud everywhere can be observed except the stony shores. The rotting plant material and gas accumulation is especially great under the floating islets of water plants (mostly Gramineae). Floating islets are found round the lake in December except the stony shores and Cyperus growth. By low water these islets are settled on the bottom.
The most important food source is in the shore area a Crustacea (Decapoda) Caridina houva about 1.5–2.5 cm size. It is also exploited by the fishermen and sold in dried form. The most Caridina could be found on rotting grass (sweet grass) about 30–50 cm depth. It was found further under the water lily (Eichornia crassipes) and less around the Cyperus roots. It seems to be a planticol, detritus feeder animal. Besides Caridina few dragonfly larvae, water bugs and beetles and snails could be found in the shore area. The plancton in the shore water is hardly traceble because the dense population of young Tilapias and Gambusia.
The inundated meadows in the flat bays of the lake are so densely populated with young Tilapia and different age groups of Gambusia, that no other animal could be found here. It was like an aquarium full of fish. There, not a single Caridina was found in spite of the systematical search. The paddy fields around the lake inundated from the lake are also very poor in plancton and other animals except fish.
It seems that the natural propagation of the Tilapia species is enough successful. Plenty of different size of young Tilapia could be observed everywhere in the shore area, and especially in the inundated meadows. More precise investigation could find out weather the propagation of Tilapia nilotica is adequately successful or not. In December most of the captured T. nilotica was meagre.
The professional fishermen are catching the Tilapias from the size of about 50–60 g. But the anglers catching fish for subsistance take out even the 10 g or less Tilapias.
There are some “dangerous” capturing technique which have to be prohibited. Such as is the spearing the male T. nilotica luring it with a line bound living other Tilapia to the surface. That is sure that the nest which was guarded by the speared male fish is annihilated with the offsprings. The same effect has the castnet used along the shore area in the shallow water
The propagation of common carp is successing here only with moderate result as it follows from the propagation habit of this fish. (It scatters and abandones its eggs and most of those and the larvae are eaten by the Gambusia teeming in the shallow water).
The common carp seems to be more frequent in the southern part of the lake around the estuaries of inflowing rivers. Most probably they have the spawning places there as well.
The spawning occurs in September and first half of October according to the fishermen. The smallest size of carps taking part in the propagation are above half kg. Young carps are caught from December with gillnet and castnet. The carps are scared into the layed gillnet.
It seems that the common carp has enough natural food here. The observed animals were in good body condition. More information could be got by gut content investigations and age determinations.
The propagation of black bass is also successful in the lake. May be the breeder stock (sexually matured specimens) is sparse. The anglers and professional fishermen capture many young black bass about 50–150 g deriving from the September propagation.
The black bass seems to be here a shore fish which does not invade into the deep water area. The food supply of the young and older black bass seems to be satisfactory ; the observed fish have good body condition and many had full stomach.
a) The lake Itasy is over-fished. Even a pair of professional fishermen (they work mostly together in pairs) could not catch more than 2–5 kg in a day with 6–8 hours long hard work. Excellent catches are very very rare.
b) Because the size (and age) restrictions are not followed, most of the big or medium growing fish (common carp, black bass, Tilapia nilotica) are captured in their early ages as small fish. That means the food sources available for the species in question are not exploited.
c) Only the Tilapia species could cope with that fishing intensity concerning effectiveness of propagation, although the quality of the catch, due to the most under-size fish, is poor.
d) Concerning the three main living places (water column, bottom, shore area) of the lake, only the shore area is densely populated and well exploited. There, does not exist such a fish or fishes which would be able to exploit the water column economically (Column feeder fish species). There are lacking as well the bottom feeder fish. (Only the Anguilla species could be counted as such fish, but their number is trifling as compared to the bottom area).
e) It would be very dangerous to introduce into the lake such fish as Heterotis niloticus and snake-head fish, which could not be controlled and even captured, because the floating islets and dense population of emerging plants, where these fishes used to live.
It is recommended to find the ways and means to enforce the minimum size restrictions at least among the professional fishermen.
The spearing should be totally eradicated from the fish catching methods.
The mesh of the gillnets used should be controlled and not allow to work with less than 4–5 cm (knot to knot) mesh of gillnet.
It is recommended to ban the common carp catch for two months in each year (December January).
It has to be considered to limit on a reasonable number of professional fishermen.
If anywhere in the country elvers, young larvae of Anguilla species, could be traced in collectable quantity, this opportunity should be exploited and young Anguilla should be stocked into the lake.
It is highly recommended to introduce new column feeder fish species as silver carp and bighead carp.
For the production of young fish for stocking regularly into the lako, it is recommended to establish a suitable size of hatchery and nursery for the economically important fish species preferably near to AMPEFY.
The not utilised meadows in the bays of the lake which are dry during the dry season but inundated with 40–50 cm water during the rainy season, could be converted partly as fish rearing places or semi-intensive fish ponds. This possibility could be included into the lake regulation plans which sometimes will start here.
