Mémoire de fin d’étude pour l’obtention du Diplôme de Master en Sciences Marines et Halieutique

Abstract

The bay of Toliara in Madagascar is mainly covered by sedimentary bottoms, with a strong
presence of sea grass beds. It is home to diverse communities of benthic macrofauna.
Traditional fishing is a major socio-economic activity. However, knowledge of invertebrate
populations and the functioning of these ecosystems associated with soft substrates remains
incomplete. This study therefore aims to: 1/ to improve the knowledge of non-reef coastal areas
and in particular the benthos of unconsolidated substrates; 2/ to define existing biotic
interactions between the different communities in order to provide new elements in the
knowledge of the biological and ecological mechanisms and processes involved in the dynamics
of the coastal environment. To this end, an analysis of the benthic communities fished by
"draotsy" net trawl was carried out, based on density, biomass, size and geographical location
variables. This trawling gear, classified as destructive by regulations, is among the most used
by fishermen in the Toliara lagoon, particularly in coastal meadows. Sampling was carried out
monthly from fishermen's catches during neap tides in Ankilibe for seven months. 43 species
of invertebrates were recorded, mainly crustaceans, stomatopods and cephalopods. A quarter of
the species were present in more than 80% of the samples, and dominated by the Portunidae
family. A significant difference in density was observed between families. A seasonal effect
was observed in the monthly density values of macroinvertebrates. Macrofauna communities
vary according to the habitat exploited. This variation is related to the nature of the bottom, in
particular the density of the vegetation cover. The study showed that the recovery rate of
meadows is low, dominated by Thalassia hemprechii and Halodula. Mixed beds are richer in
terms of macrofauna density.