The objective of this work was to study the survival and growth of juveniles of the oyster Crassostrea gasar reared in ponds at different depths. Rearing trials were carried out for 42 days (6 weeks) in two ponds (12 m 2 x 1.10 m deep), one of which was fertilized with poultry droppings and the other with no fertilization. For each pond, 03 depths (30 cm, 60 cm and 90 cm) were tested in duplicate. At each depth, two iron mesh bags (40 cm x 20 cm x 10 cm) were used. Oysters with an average size of 40.50 mm and average weight of 15.36 g were cultivated at a density of 30 individuals per bag. Phytoplankton density, pH, temperature, transparency and salinity of the water were daily measured between 7:00 and 8:00 am. Oysters were counted weekly. Individual length and weight were measured to evaluate their survival and growth. The values of physico-chemical parameters measured during this study met the requirements of tropical water oysters. After 6 weeks of rearing, the best survival rates were observed at 60 cm in the ponds (86.67% and 81.67% in the fertilized and unfertilized ponds, respectively) while the lowest survival (46.67% and 41.67 in the fertilized and unfertilized ponds, respectively) was recorded at 90 cm depth. The highest mean final sizes (40.584 mm and 40.568 mm in the fertilized and unfertilized ponds respectively) were recorded at 60 cm depth while the lowest values (40.557 mm and 40.553 mm in the same order) were recorded at 90 cm depth. The average final weight ranged from1 5.32 g (at 90 cm depth) to 15.38 g (at 60 cm) while it overlapped between 15.3 g (90 cm depth) to 15.35 g (30 cm depth) in the unfertilized pond. The average daily gain was 0.0004 g/day and 0.0005 g/day respectively for the 30 cm and 60 cm depths of the fertilized pond. The best zootechnical performance was recorded between 30 cm and 60 cm depth in the fertilized pond. Fertilized pond culture highlight future prospects for the promotion of Crassostrea gasar oyster culture in controlled environment.

In order to domesticate the freshwater oyster Etheria elliptica, a 42-day freshwater pond rearing trial was conducted in two ponds of equal size (12 m 2 x 1.10 m deep), one of which was fertilized with poultry droppings and the other one was a control (unfertilized). Three depths (30 cm, 60 cm, 90 cm from the bottom of the pond) were tested with one repetition. For each depth, 02 sieves (40 cm diameter and 15 cm depth) were used. A total of 180 oysters with an average size of 37.56±1.95 mm and an initial average weight of 30.35±0.72 g were used with a density of 15 oysters per colander. pH, temperature, transparency, and phytoplankton density were measured daily between 7:00 am and 8:00 am in June-July 2018. Each week, the colanders were checked. Oysters were counted, measured and weighed. The physico-chemical parameters measured in the ponds were consistent with the requirements of tropical water oysters. At the end of the experiments, the survival rate (50±35.36%) was identical for all depths in both ponds. The best growth in size was recorded in the fertilized pond. The sizes increased 2.06 mm and 3.8 mm at 90 cm and 30 cm depth respectively). In contrast, the smallest increases were obtained in the control pond, 0.93 mm and 1 mm at 90 cm and 30 cm depth respectively. In the fertilized pond, the average daily gain ranged from 0.017 g/d (at 90 cm depth) to 0.027 g/d (at 30 cm) while it was between 0.005 g/d (90 cm) and 0.017 g/d (30 cm) in the control pond. As for the specific growth rate, it varied from 0.014%.d -1 (90 cm) to 0.047%.d -1 (30 cm) in the control pond while it was between 0.047%.d -1 (90 cm) and 0.078%.d -1 (30 cm) in the fertilized pond. The best zootechnical performance was recorded between 30 cm and 60 cm in both ponds. Therefore, depths between 30 cm and 60 cm seem to be suitable for rearing Etheria elliptica in ponds. Also, fertilization of the rearing ponds with poultry droppings would improve the growth of the reared oysters.

No abstract

La palourde Amiantis umbonella présente dans les zones intertidales sablonneuses/vaseuses en Asie constitue une source de protéines bon marché de ces régions. Malgré ces potentialités économiques, elle demeure peu étudiée à Djibouti où elle est fait l’objet d’une production. Cette étude a pour but d’étudier l’effet de la densité d’élevage sur la croissance de la palourde Amiantis umbonella. Les expériences ont été conduites sur les sites de Douda avec des densités de 50 individu/m2 et de 100 individu/m2 et Obock avec 50 individu/m2. La taille et le poids des spécimens ont été mesurés toutes les deux semaines, pendant 6 mois de mars à aout 2018. La température moyenne était de 36°C. Les paramètres de croissance des palourdes ont été estimés avec l’équation de Von Bertalanffy. A la densité de 50 individus/m2, les constantes de croissance K étaient de 0,13 cm/mois et 0,15 cm/mois respectivement à Douda et Obock tandis qu’elle était de 0,11cm/mois pour une densité de 100 individus/m2à Douda. Ainsi, il ressort que la taille commerciale (40 mm) a été obtenue plutôt après 8 semaines à Douda à la densité 50 individus/m2 et Obock contre 14 semaines d’élevage à la densité de 100 individus/m2à Douda. Ces résultats révèlent une amélioration de la croissance de la palourde Amiantis umbonella à la densité de 50 individus/m2 dans les conditions d’élevage à Douda et Obock.