Several authors have dealt with the study of mangrove forests in Madagascar, including Perrier de la Bathie in 1953, Gachet in 1959, Kiener in 1965 and 1971. In his study on the possibilities for the development of Malagasy mangroves (see previous note ), This last researcher insists in particular on the fauna which interests most, from the point of view of its exploitation, that is to say fish and crustaceans. In particular, it distinguishes areas that are submerged in everyday life and those that are only submerged during high tides or shallow waters.
The former are characterized by deep vases, rich in alluvial matter and detritus, the latter being semi-muddy and semi-sandy. There are also submerged areas at high equinoctial tides with sands enriched with a small proportion of alluvial deposits, saline soils whose sands are generally sterile and stands sparse, and finally land.
Overall, the fauna mainly consists of euryhaline species and dominant marine affinities, and A. Kiener retains mainly two major groups, the upper crustaceans and the fish. Among the decapods, “important both by their trophic role (even for man) and their spectacular abundance”, dominate two groups. Shrimp include six families and a dozen species of which the Penaeus (Penaeidae) and the small Acetes or Tsivakihiny (Sergestidae) are the most abundant. Crabs have three very numerous families: Ocypodidae with terrestrial ocypods and Uca amphibians, Graspsidae with terrestrial Sesarma and Portunidae, which include the large mangrove crab (Sylla serrata), very common in marine channels, In the mangroves, and on the immersed sandy mud banks.
As with the rest of the fauna, there is a certain zonation for fish, due both to the ecological conditions and to the physiology of the more or less euryhaline species. A. Kiener takes the example of the southern Mahavavy mangrove and Kinkony Lake. From
Twenty eight species
Totally euryhalines which travel indifferently between the sea and the fresh waters, six can reproduce in all the environments, the twenty two others having to go to breed at sea, like the muges. The author also speaks of the “frequent and curious cases” of the strong uplift of sharks and sawfish. Thirteen of the fifty-six families surveyed in the study areas are of economic importance.
The influence of tides and salinity is crucial for the movement of many fish, especially in the west, to invade rivers and mangroves with the rising tide, and then return partly to sea with the falling tide. In this zone, the transition between seawater and freshwater is almost always done in a very gradual way. “The abundance of calcium ion seems to favor the penetration of continental waters by certain marine species. The eastern coast, on the contrary, often has a clear biological barrier, characterized by a rather brutal passage between the rivers and the Indian Ocean.
The presence of a broad continental shelf along the western coast also favors the development of a fish stock which constitutes a basic stock from which numerous incursions of fish into the neighboring inland waters occur. This presence also explains the difference in intensity between the West Coast and East Coast fisheries where there is a bar that makes fishing operations at sea less comfortable.
For a long time, mangroves around the world have been used for fishing, but in America as well as in Africa, including Madagascar, they are generally only developed in a very extensive way. In the Far East, they are exploited very intensively for mainly demographic reasons because of a large population.
A.Kiener then gives a quick overview of some achievements already obtained in other countries, such as valli-cultura in Italy, notably on the banks of the Po.