BIOLOGY AND TRADITIONAL FISHING OF PENAEI D SHRIMPS AT THE MOUTH OF ARIVER IN A SEMIARID AREA (SOUTHWEST MADAGASCAR)

Traditional fishing using beach seines was studied over a period of on e year at the mouth of the Onilahy river. The catches/efforts were rec orded on a daily basis, biological sampling and the measurement of hyd rological parameters being carried out twice a month. The moderate var iations of salinity and temperature (14 < salinity < 35; 25 degrees C < temperature < 30 degrees C) which were observed in the estuary can b e explained by the semiarid climate of the area.Four species of penaei d shrimps are to be found on the Onilahy: Metapenaeus monoceros, Panae us indicus, Penaeus monodon and Penaeus japonicus, which were mainly c aught at the subadult/adult stage with a Carapace length between 10 an d 50 mm. The presence of these important sizes at the mouth of the riv er, associated with the fact that no mature female shrimps were observ ed in the catches, means that part of the population tends to extend i ts stay in the estuary. Growth parameters were estimated for M. monoce ros and P. indicus, and the growth in the estuary appears to be faster than that observed at sea, especially for P. indicus. In contrast wit h what is observed in wet tropical areas, the decline in salinity is l imited in semiarid climates and is thus not a major factor in the emig ration of the shrimps towards the open sea. On the other hand, there i s a rather precise link between emigration towards the open sea and fa lling temperature: important sizes disappear from the river mouth duri ng the transition between autumn and winter. In the estuary of the Oni lahy, a temporal partitioning of resources for the four species of pen aeid, leading to a reduction of interspecific competition, can be unde rscored. The traditional fishing yield in the estuary was estimated at 220 metric tons of shrimps per annum, with a catch per unit effort of 15 to 22 kg/seine/trip.